Holiday 2012: Purchasing a New Lens

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I’m outgrowing the kit lens that came with my camera. What’s a good starting point for figuring out how to choose my next lens?

There are many variables that go into selecting the best lens for your needs. A good starting point is to establish what your current lens does not do. In other words, is my lens not wide enough, long enough or perhaps  fast enough? Does the lens focus close enough, and at the top of many wish lists, is my lens sharp enough for my needs?

What’s better, zoom lenses or fixed focal length lenses?

It depends on your definition of the word “better.” If you define “better” as meaning “sharper,” generally speaking, fixed focal length (prime) lenses tend to be sharper and offer better contrast levels than the average zoom lenses. But once you start looking at the pricier, higher-performance zooms, the gap becomes smaller, and in many cases is a non-issue altogether. As the old cliché goes, “You get what you pay for,” and this goes for fixed focal length and zoom lenses alike.

If, on the other hand, your definition of “better” is “convenient” or better yet, “more efficient on the job,” that’s another case, and zoom lenses are (in most hands) far quicker to adapt to rapidly changing action, especially when shooting from a fixed position while having to respond quickly to fast-moving subjects. In most cases (and bags), zooms take up less space than prime lenses. Two or three zooms can cover the focal range of six to eight or more fixed focal length lenses. And a camera bag containing two or three zooms is far easier to carry aboard an airplane than a bag packed tightly with fixed prime lenses.

Some photographers claim that because you have to change lenses more often with fixed aperture lenses, there’s a better chance of getting dust on your camera sensor. While this argument holds water in theory, the reality is that many zooms, especially push-pull zooms, create a vacuum effect that sucks dust and moisture into the lens barrel (and ultimately the camera body) every time you zoom from wide to tele or vice versa. This is especially true for less expensive zooms that have fewer weatherproofing seals (if any) than their pricier counterparts, though be advised that dust has been known to find its way into the barrels of the priciest optics.

Valid arguments for fixed focal length lenses include faster wide apertures for shooting in low light. Even under bright light, wide-aperture lenses are easier to focus. Wider apertures, i.e. f/2.0, f/1.4, f/1.2, etc., also enable you to explore the possibilities of selective focusing in which you can isolate your subject from the foreground and background. With the exception of a pair of f/2.0 zooms for Olympus Four Thirds format cameras, the fastest zooms only open up to f/2.8, which limits the ability to employ selective focus in your images. Fixed focal length optics, as a rule, also focus closer than zoom lenses.

 

Is there a difference between a lens made by the camera manufacturer and lenses made by third-party lens manufacturers?

 

Without running side-by-side comparisons between comparable lenses made by original equipment manufacturers (OEM) and third-party manufacturers, it’s difficult to answer this question properly and fairly.

Optically speaking, OEM (Nikon, Canon, Sony, Pentax, Panasonic, Olympus, etc.) and third-party lens manufacturers (Sigma, Tokina, Voigtlander, Tamron, etc.) all produce several tiers of camera lenses ranging from inexpensive “kit” lenses to high performance, pro-quality optics. If you were to compare the optical qualities of “comparable” lenses from both groups, you may or may not be able to discern much, if any, difference between them regarding sharpness, tonality, contrast levels and construction.

Are fixed-aperture lenses better than variable-aperture lenses?

Less expensive zoom lenses usually have slower, variable maximum apertures (e.g. f/3.5 to f/5.6) compared to faster, and almost always, pricier fixed-aperture lenses. The reason for this variation in the lenses’ maximum aperture is that zooms, by nature, are inefficient at transmitting light from the front element to the imaging sensor (or film plane) as you zoom from wide to telephoto. The physical size of the aperture (or f/stop) remains the same, but the amount of light that makes it through from one end to the other greatly diminishes. The net result is that what started out as f/3.5 effectively becomes f/5.6, or perhaps f/6.3, depending on the lens. To manufacture a wide-aperture lens whose maximum aperture remains constantly wide throughout the focal range is more complex, hence, pricier.

It should be noted that the effective “depth of field” of each aperture remains the same, which means that while the effective aperture goes from f/3.5 to f/5.6, the depth of field remains the same as an f/3.5 lens. You lose light, but the laws of optics remain unchanged.

The plus side of variable-aperture lenses is that they are lighter, smaller and less expensive. If you plan on hiking all day there’s much to be said for packing a lighter lens. The downside is, the smaller the aperture, the less responsive your light meter becomes. It's just as accurate, but it might take a wee bit longer to lock an exposure, especially in dimmer light. A more serious issue has to do with your autofocus system, which becomes noticeably slower as the light levels begin to drop off.

This is important because almost all AF systems start to falter at about f/8. What this means is, if you are using a lens that has an effective maximum aperture of f/5.6 and you add a Polarizing filter to it, which eats up about two stops of light, your effective maximum aperture is now f/11. Ditto that light loss if you plan on using a 2x teleconverter. And if you’re thinking of using a Polarizing filter and a 2x teleconverter, your widest aperture will now be an effective f/22. Even if you’re using an image-stabilized lens, you’re going to be hard pressed to handhold your camera and manage to capture sharp images.

The more expensive 70 to 200mm lenses have maximum apertures of f/2.8, which is faster, and remain at f/2.8 throughout the entire zoom range. Add a Polarizer or 2x teleconverter and you are still shooting at an effective aperture of f/5.6, which under low-light conditions might slow the focusing system down a bit, but certainly will not jeopardize the sharpness of the captured imagery.

Another advantage of faster, wider-aperture lenses is their ability to isolate the subject from the foreground and background of the photograph. Commonly known as selective focus, this is one of the creative benefits of shooting with faster lenses. By selecting a wide aperture and faster shutter speed, you eliminate unnecessary foreground and background distractions within the frame, which in turn directs the viewer’s eyes to the subject of your photograph, be it a person, place or thing.

Though conventional wisdom says you have to stop your lens down when shooting on bright, sunny days, higher shutter speeds of up to 1/8000-second enable you to shoot at wider apertures almost anytime and anywhere.

It's important to note that while variable-aperture lenses tend to be less expensive, many of them are also quite sharp. A bit of research can yield inexpensive optics that do not sacrifice image quality.

What are f/stops?

Lens barrel aside, a camera lens is composed of two main components: the clusters of individual glass elements and an iris, which is the circular opening created by a set of curved blades that open and close to allow controlled volumes of light to enter the lens and transmit the image to the camera’s sensor. (The pupil in your eye works the same way, by opening up wider in the dark or smaller in bright light, to allow the necessary volume of light to enter your eye for a “proper exposure.”)

The iris opens and closes at specific increments called f-stops, which work in conjunction with the camera’s shutter to allow the correct amount of light in to create a proper exposure.

The f-stop of a lens is determined by dividing the focal length of the lens by the diameter size of the entrance pupil, i.e., the lens opening. For example, a 100mm with an entrance pupil diameter of 25mm will have a maximum aperture of f/4.0, because 100mm divided by 25mm is 4. So, on a 100mm lens with an entrance-pupil diameter of 25mm, when the calibration is written f/4.0, this means "focal length divided by 4."

Just as shutter speeds double (or halve) the time the shutter remains open as you adjust it up and down, each time you change the f-stop you either double or halve the amount of light entering the lens, and together shutter speed and f-stop create exposure settings (or the ratio of time/amount of light entering the lens).

To best illustrate how shutter speeds and f-stops work together, think of filling up a glass of water. If you only open the faucet slightly (i.e. a small aperture) it takes longer to fill the glass, as compared to opening the faucet to full volume (i.e. the widest aperture), which fills the glass in correspondingly less time.

Shutter speeds and apertures operate in a similar fashion. The wider the aperture (f-stop), the shorter the shutter speed will be, and vice versa. The size of the glass and the amount of water it will hold―i.e. the proper exposure―is a fixed quantity. The only variable is how fast or how slowly you want to fill it.

If you’re shooting in “Program” or “Auto” mode, your camera does this automatically. As you learn more about your camera, you can make adjustments to each of the camera’s exposure modes (Program, Aperture Priority, Shutter Priority and Manual). You can control the f-stop and shutter settings to achieve different visual effects. To learn more about the exposure settings of your camera, you should refer to your camera’s manual.

The “A” setting on your shutter-mode dial does not always mean “Automatic.” Unless your digital camera has separate settings for both modes, the “A” frequently stands for “Aperture Priority”—a mode in which you set the f-stop and the shutter speed is automatically set to a speed that complements the chosen f-stop. “Shutter Priority” is the reverse mode, in which you set the shutter speed and the f-stop is set automatically.

So, lens speed refers to the maximum aperture―or f-stop―to which your camera lens can open up, and the “faster” the f-stop, the easier it is to shoot under low light and freeze fast-moving subjects.

Another lesser-known benefit of fast lenses, as mentioned above, is that the wider the maximum aperture, the more responsively your camera’s autofocus system will perform, especially in lower lighting. Ditto, the camera’s metering system.

Push-Pull versus Two-Touch zooms: Is there an advantage to one over the other?

Zoom lenses come in two design styles: push-pull and two-touch. Push-pull zooms utilize a single zoom/focus ring that you push and pull to zoom, and twist to focus. A two-touch design utilizes two separate rings, one for focus and one for zoom, with no pushing and pulling. Push-pull designs tend to be found on less expensive lenses.

While there are more than a few push-pull design zooms on the market that are optically fine, they can become problematic to use over time when the guides and threading within the zoom barrel start to wear down and get wobbly.

The first sign of this problem is when you mount your camera aimed up or down on a tripod, only to find the law of gravity tugging the lens barrel downward when you let go. Two-touch zooms do not suffer from this “zoom creep.” Another good argument for two-touch zooms is that you can adjust the focus of the lens without changing the zoom position, and vice versa, whereas with a push-pull lens you invariably change both the focal length and focus every time you touch the zoom/focus ring.

And as mentioned earlier, the push-pull action also creates more of a vacuum effect, which sucks dust and other forms of airborne particles into the lens barrel as you zoom in and out, unlike two-touch zooms, which are less prone to dust issues.

What’s Internal Focus and how important is it when considering a lens?

Internal focus (IF) is an important design feature that’s usually (but not always) found in pricier zoom optics. You can usually tell if a lens isn’t an IF design if it protrudes two to three times its normal length as you zoom and focus it. If the lens remains the same size as you zoom in and out, most likely it’s an internal-focus lens.

If you plan on using a polarizing filter, you should stick to an IF lens since most non-IF lens barrels rotate as you zoom and/or change focus, which can prove to be a real nuisance with polarizers. Another advantage of IF zooms is that when mounted on a tripod they don’t creep like an old push-pull zoom when you aim them up or down. And because the lens barrel on IF lenses doesn’t zoom in and out (like a push-pull zoom lens), you are less likely to suck dust into the lens barrel every time you adjust the lens settings.

Image-stabilized lenses cost more than non-image-stabilized lenses. Are they worth it?

Image stabilization (IS), a.k.a. Vibration Reduction (VR), Shake Reduction (SR) and Optical Image Stabilization (O.I.S.) is another factor to consider when purchasing your next lens. Image stabilization works by counteracting any sharpness-robbing effects of camera shake and is quite useful especially if your lens is a slower, variable-aperture model and you plan on shooting under less-than-favorable lighting conditions.

Some manufacturers offer the same lens with or without IS, depending on your needs or budget. Keep in mind that an IS-enabled lens can make the difference between a tack-sharp image and a marginally sharp image, especially if you tend to shoot under low-light conditions.

Pentax DSLRs have internal image stabilization systems (Pentax Shake Reduction {SR} System) built into them, which enables image stabilization with all Pentax lenses, including older manual-focus lenses. Similarly, DSLRs from Sony feature their own SteadyShot INSIDE image stabilization systems built into every Alpha series camera to better ensure sharp imagery regardless of which Sony (or Minolta AF lens) you happen to be using.

What’s the difference between a macro lens and a lens that features close focusing?

The precise definition of a macro lens is a lens that focuses down to life size, or 1:1. But it goes further than that.

All macro lenses focus close, but not all close-focusing lenses are macros, and the difference lies in the design of the lens. Most lenses have a curved field of focus, which often complements the curve of the front lens element. What this means is that when shooting close-ups at the lens’s widest aperture, the center of the frame is sharp while the edges become progressively softer as you look toward the corners of the frame.

Macros are designed with flatter fields of focus, which means when focusing straight-on at a flat surface, the center of the frame and the corners are equally in focus. As a rule, macro lenses are also sharper overall at closer distances than non-macro lenses. While you can always stop the lens down with standard lenses, they never quite measure up to the resolving power and contrast levels of a good macro lens.

Macro lenses are available in various focal lengths, and almost all of them can focus down to life size. Depending on your needs and what you plan on photographing, you should consider your options when choosing the best focal length for your needs; it is often easier to photograph a subject at a respectable distance than something up close.

What about lens construction and weatherproofing?

If you pick up any number of lenses, you'll quickly note that some feel solid and hefty and some feel not-so-solid and hefty. As mentioned earlier, there are more than a few inexpensive lenses that optically speaking, are quite good but as a rule you get what you pay for. The more expensive lenses tend to have smoother, more positive action as you zoom or focus them, not to mention little or no wobble in the barrel and lens mount. If you plan on using your gear on a regular basis, and in less-than-desirable conditions, you’re hereby advised to bite the bullet and buy the better lens.

Weatherproof and dustproof construction are other advantages of pricier glass, and again, if you plan on going on safaris, to the beach, out on the lake, snowboarding, skiing and other activities or locales beyond your patio, the integrity of the lens you buy should be a consideration.

Weatherproof does not mean waterproof. Though some manufacturers make impressive claims about water resistance, be advised that if you go diving or venture out in a hurricane, you run the risk of voiding your warranty. Weatherproof lenses can weather tropical humidity and the occasional splash or rain shower, but not full submergence into water.

Will multi-layer lens coatings, aspheric surfaces and exotic glass elements make a difference in the quality of my photographs?

Just as construction and weather/dust resistance should be a consideration when purchasing a new lens, the quality of the glass is equally important. Extra-low dispersion glass (ED), Fluorite (FL) and advanced lens coatings are also important considerations.

When you focus your lens, the image you are viewing consists of all of the colors of the visible spectrum. Unfortunately, all of these colors―red, orange, yellow, green and blue―do not fall into focus at the same point.

The advantage of ED, FL, other specialty glass and advanced lens coatings is that together they help bring all of the colors into focus in a narrower plane, and when all of the colors fall into place together, your pictures contain better color saturation and look sharper overall.

Aspheric lens surfaces are employed in lens design since they help maintain even sharpness toward the edges of the frame, where many other lenses’ resolving power tends to fail. The net result of these combined optical features is better color saturation and contrast, which in turn results in better, sharper-looking images.

What kind of accessories should I consider to go along with my lens?

Two accessories you should consider are a lens shade and a UV filter. Many lenses come with a lens shade, some don’t. Lens shades (or hoods) help block stray light from striking the front surface of the lens, which can otherwise cause lens flare that reduces contrast and color saturation.

UV-reduction filters serve dual purposes, the first of which is to reduce the image-degrading effects of ultraviolet light, which is prevalent when shooting outdoors in bright sunlight. UV filters also serve as a protector of the front lens element, which is always subject to dust, smudges and worse. And as any pro can tell you, it’s cheaper to replace a damaged or scratched filter than it is to replace the front element of the lens.

Lens Attribute Comparison Chart

Lens Attributes

Advantage(s)

Disadvantage(s)

     

Fixed Focal Length Lenses

Faster (Wider) maximum apertures & closer focusing

You have to swap lenses to change focal lengths.

     

Zoom Lenses

Flexibility. There's little or no need to change lenses on the fly.

Zooms are usually heavier and slower (aperture-wise).

     

Push-Pull Zooms

Less expensive to produce/purchase

Prone to barrel wobble and barrel drift with heavier use. Every time you zoom, you need to check focus.

     

“Two-Touch” Zooms

More precise, no need to check focus each time you zoom. Less prone to barrel wobble and focus/zoom drift when shooting with lens facing upward or downward

     

Variable-Aperture Zooms

(Usually) smaller, lighter and less expensive than fixed-aperture lenses

Slower maximum apertures than fixed-aperture lenses and get progressively slower as you zoom in tight

     

Fixed Aperture Zooms

Wider/faster maximum aperture throughout focal range

(Usually) heavier and pricier than variable-aperture zooms

Original Manufacturer's Lenses (OEM)

Less likely to have CPU communication or focusing/sharpness issues than third-party optics

     

Third-Party Lenses

Less expensive than OEM optics

More likely to have CPU communication or focusing/sharpness issues than OEM optics

     

Internal Focus (IF)

More precise than non-IF, optic (usually) remains same size throughout zoom and focus range

     

Image Stabilization (IS)

Enables sharp handheld shooting at light levels 3-4 stops slower than comparable non-IS optics

     

Macro Lenses

Enable life-size (1:1) imagery that's sharp, edge to edge

Maximum apertures tend to be slower than comparable focal length, non-macro lenses.

     

Weatherproofing

Protects your investment against dust and moisture

     

Aspheric Elements

Help maintain higher levels of edge-to-edge resolving power

     

Extra-Low Dispersion (ED) and Fluorite (FL) Glass

Advanced glass manufacturing technologies that reduce chromatic aberrations, improve color and image tonality

     

Multicoating

Improves color rendition, saturation and reduces flare

     

The Takeaway

  • Fixed focal length lenses are smaller, have faster (wider) maximum apertures, (usually) focus closer, and are usually (but not always) sharper than zoom lenses.
  • Zoom lenses are more convenient. Rather than changing lenses, you merely have to turn a ring (or push and pull).
  • Two or three zoom lenses take up less space in your bag and are easier and lighter to carry around (and carry onboard an airplane) than six to eight fixed focal length prime lenses.
  • Lenses made by your camera manufacturer are sharper, better made and pricier than third-party lens manufacturers… except when they aren’t.
  • Variable-aperture lenses tend to be lighter, smaller and less expensive than comparable fixed-aperture lenses.
  • If you plan to use teleconverters and Polarizing filters, you should consider sticking to wider-aperture (faster) lenses.
  • Two-touch zooms are preferable to push-pull zooms.
  • Image stabilization is a good idea because there’s nothing worse than a photograph that would be terrific if it weren’t so blurry.
  • If you plan on shooting lots of close-up pictures, consider a true macro as opposed to a lens that features “close focusing.”
  • Weatherproof does not mean waterproof.
  • Multilayer coatings, ED glass, fluorite glass and aspheric surfaces are good things.
  • As a rule, you get what you pay for.

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very well put together article.  look forward to more.  Thanks IL

Thank you for doing this.  When trying to get into photography, there are so many little details that old pro's take for general knowledge when trying to help a beginning level enthusiast like me.  These comparatives that explain pros and cons of choosing different option in lenses really helped me a lot.  This was extremely informative to me.

Wow! This article really answers the beginner photographers basic questions.

Really helpful article. Very well written! Thanks for your time and knowledge!  :)

From the article:  "It should be noted that the effective “depth of field” of each aperture remains the same, which means that while the effective aperture goes from f/3.5 to f/5.6, the depth of field remains the same as an f/3.5 lens. You lose light, but the laws of optics remain unchanged."

Are you certain of this?  if you take a photo with a 70-300 f/3.5-5.6, and are at the extreme telephoto end of the zoom, wouldn't that photo have a dof consistent with a 300mm f/5.6 lens and not a 300mm f/3,5 lens?

To be fair, there are also disadvantages to using UV filters.  

Most digtal cameras are not sensitive to UV light, so there is no need to filter it out.

Although a filter can keep a fingerprint off your lens, a fingerprint on the filter is just as bad. Most lenes have better coatings than your average filter, so you are better off cleaning the lens than a filter.

Digital sensors are shiny, puting a filter on your camera greatly increases the chance that light will bounce back and forthe between the filter and the sensor which reduces image contrast.

Most importantly, contrary to popular belief, a good filter can cost more than replacing the front element of your lens.

On the plus side, UV filters are a high-profit item, and by buying them we make it possibel for the store to keep lower prices on other items.

I have *never* used a UV filter in the last 20 yrs.  I believe it is far more important to have a lens hood on the lens at all times.  this protects the front element better than a filter can and is far more beneficial for making sure you get the best images out of your lens

Yes I would agree with that. A hood offers more protection and doesnt lower image quality like another layer of glass - no matter how well cut - does.

"Most digtal cameras are not sensitive to UV light, so there is no need to filter it out."

Any rule of thumb for how a hobbyist might determine which digital cameras are sensetive to UV?

Great article, very informative.....thanks!!

The rule of thumb is that digital cameras are not sensitive to UV light. 

Hello,

Digital cameras are not sensitive to UV light because their sesnors have UV cut filters covering them. Digital camera sensors are sensitive to UV and IR and a common practice is to remove this filter for special affects imaging. We ofcourse do not recommend removing this filter.

So people actually open their cameras and remove the UV cut filter?  Wow, that effect better be really cool.....even if it is, I'd never do this......but then, I'm just a hobbyist.

Thanks for the explanations guys!

ISO is the International Standards Organization (or something like that), but the pertnient definition for photography is the light sensitivity.ISO 100 (and below) is for photography where you have a lot of ambient light- like outdoors during the day. Use the lowest ISO you can get away with, as when you increase ISO, you add noise (i.e. grain) to the image. The Panasonic FZ50 that you suggest is a great camera, but not good at high ISO shots, so if you want to capture fast action in anything but great light, or if you want to shoot in lower light situations (like indoors), it's not the best choice. (look at the Fuji's instead). DSLRs can shoot ISO400 very cleanly, and even very nice ISO800 shots. That gives you 8 times the light sensitivity of an ISO 100 setting. Or, 1/8th the shutter speed. Most compact cameras start to get noisy over ISO200 (except the Fuji F-series cameras).Choose a camera which you can manually select the ISO setting.

Thanks for the terrific information on Lens. This is a keeper! Look forward to future articles. Regards, Chuck

These guides are extremely well composed and very helpful. Thanks for including them.

I find articles like this extremely useful.  I would like to see them throughout the B&H Photo site.  For example, an article on choosing a light meter.  I particularly like the fact that these articles *explain* the equipment in detail, rather than merely try to sell.  Knowledge is power.  Thank you.

not sure if I would ever buy third party lenses.  I only buy nikon brand lens intended for USA market.  I also prefer fast lens over variable zoom lens.   But I do believe that two must have lenses are 50mm 1.4f and 28-300mm zoom.

Regarding "push-pull" zooms... the ones we called "one-touch" when they were first introduced back in the '70s.:

Are there even any of those produced for current cameras?  The only ones that I can remember seeing in the last 10+ years are units designed and produced for legacy manual-focus film cameras.  Like my old SrT-202.

I've never seen one that was compatible with any autofocus system.

It's not really a critical point except that it seems like a fair amount of effort was used in explaining a non-issue in an an otherwise very good article.

Hi Scott,

While you are correct in saying one-touch/push-pull zooms are a breed of another era, they are still around, Canon's EF 100-400mm f/4.5-5.6 USM L IS being one of them.

And thanks for the feedback.

Hi Allan,

Thank you for a very informative review.

I must say though that I'm a little bit lost with your comment regarding the Depth of Field. I also read your article under your link above (in your 1st response) but I continue to fail to see your point.

DOF depends on camera-subject distance, lens aperture and lens focal length. For a given distance, if you set your zoom (say) to a certain focal length, the DOF will depend on the aperture; the larger the aperture (i.e., smaller f/#) you'll get a smaller DOF.

So, if I set a 70-200mm f/2.8 to 70mm, say, as I close down the lens from f/2.8 to f/11, the DOF WILL increase (for an object at a given distance). This is due to the rays that form the image being closer, on average, to the lens optical axis (the so-called paraxial approximation) when the diaphragm is smaller.

So, I'm certainly missing something from your point or argument.

In any case, thanks for your article!

OK Antonio, let's give it another shot. Your 18-55mm variable-aperture zoom lens has a maximum aperture opening of f/3.5 at it's widest angle position (18mm) that "slows down" to f/5.6 at its longest focal length position (55mm). Now stay with me Antonio, here's the key to the puzzle; the physical size of maximum aperture of the lens - f/3.5 - remains the same regardless of what focal length you have the lens set at. What changes is the degree of efficiency with which the lens transmits the light through the lens as the various groups of lens elements shift back and forth inside as you zoom in and out. And as you zoom in with a variable-aperture lens, the amount of light that makes it through diminishes, turning your f/3.5 lens effectively into the equivalent of an f/5.6 lens. How does that work for you?

Alan,

Wow, that was the kicker. I'm not optics expert, but I've been shooting with a variable zoom lens since around 1978 and I've always assumed (like some others here) that the effective f-stop decreases because, as the elements move around, some of the actual aperature is reduced. now I see the folly in that thinking: if the actual aperture was decreased, the decrease would not be in the focal plane and .'. would be out of focus and creating a vignette  effect and not a uniform about of light across the frame. The way you explained it, I can see the light is getting stretched out like salt water taffey and thus the EV will decrease.

Light dawns over marble head. Thanks again!

Gene

Now you have me stumped Moby. After reading you feedback I went back to test my theory adding your hypothisis into the equation and now I have saltwater taffey gumming up my aperture blades!!! 

(Don't worry.... I didn't gum up my lens, and I'm glad I cleared up your 34 year-old lens question...)

(I typically use taffy to remove the dust from my aperture leafs, with care it won't stick, just be careful pressure pumping it through the seams).

As to the DOF question, I'm sure your right, but why doesn't zooming in decrease the ACTUAL DOF? I always thought that a sufficently wide angle lens at f/8 focused everything clearly enough. I seem to remember at Nikon house (remember Nikon House?) that the 6mm super wide angle didn't even have the ability to focus.

And, while I have you on the line, what are you thought re: a good kit for my brand new D7000 which Santa carefully slid down the chimney with? It came with the 18-105 VR, I've heard that the 18-200 VRII is "better" as an overall carry along and worth the $500 upgrade it will cost me as I won't have to swap lenses.

I'm coming from the Olympus world, and was quite spoiled by my 12-60mm f/2.8-3.5 and my 50-200mm f/2.8-3.5 as well. So, I'm not opposed to changing lenses, but I'm sorry to lose such a great piece of glass. I also considered the 24-120 F/4, though pricy and limited.

I'd prefer a one lens solution, but I'm willing to go for a two lens solution if the sharpness increase is significant. From the reviews, I can't see whether that is the case or not, so, why not ask the expert?

Moby

p.s. Still waiting for the super lightweight super sharp 12mm to 300mm f/2.0 compact zoom.

Hi Allan, thanks for commenting back. I now understand your point of view better. I was considering, at a fixed focal length, changing the lens aperture; you're considering a fixed physical aperture, and a changing focal length.

However, I still feel that, even in the situation you describe, the DOF will in general change (if you don't want to be bored just go straight to my one-before-last paragraph to see my take; otherwise, hang on ;-) ). This is because, even though the physical aperture stays the same as you zoom in, say, the effective focal length (fl) will change. The change in f/# won't be linear with the fl change because, as you mention, the lens elements will adjust their relative spacing internally and the entrance pupil, where the diaphragm sits, will in general move inside the lens. In any case, I feel that, with the change in fl, even though with the same physical aperture, the DOF will in general change.

(I did a coarse visual test with my BH-purchased Canon 40D and my Sigma 18-200 f/3.5-6.3 lens in my living room. With the lens at 18mm & f/3.5, I focused on the windon pane, some 3m/10ft away, paying attention to how sharp the background looked. It was easily recognizeable, since, with the lens set this wide most of the things from the plane of focus towards infinity will be very close to focus. When I zoom to 200mm f/6.3, focusing on the same spot 3m away, the background is clearly way out of focus. I have to add that, if the 18mm setup is to provide an image of the object being focused of equal size as the 200mm setup, then the camera will have to be very close to the object and the DOF in both such situations will be the same.)

The overall situation might be akin to that of using a tele-extender, isn't it (I know you're familiar w/ all this but please bear with me for a moment). Say you have your fixed fl or zoom lens set at 400mm f/5.6. If you include a 1.4x tele-extender, it will multiply your initial fl by 1.4x, of course (your lens diaphragm is still the same inside the lens). The illumination on the detector goes with the inverse of the f/# squared (or, which is the same, with the inverse of the fl squared at a fixed aperture), so we loose a factor of (1.4)ˆ2 = 2, or 1 stop. We then get 400mm at f/8.

The DOF depends linearly on the aperture (aperture = 5.6, 8, etc) and inversely on the fl squared. With the tele-extender, it will be then a factor of 4 smaller (than w/o it) because of the fl change but it will be a factor of 2 larger because it goes linearly with the aperture. The overall change will hence be that the DOF will be a factor of 2 smaller (than w/o the tele-extender).

From the latter paragraph, in the situation you describe of zooming in and at the same time changing the aperture, the DOF will NOT change IF the fl changes by a factor equal to the square root of the aperture change. In your example of a change from 3.5 to 5.6, the aperture change is a factor of 5.6/3.5=1.6. The sqrt of that is about 1.26 or so. So, if you increase the fl by this factor, say from 18mm to 18*1.26~23mm, then the DOF will be the same, as you say. But if the fl change is larger than this factor, such as going from 18mm to 55mm say, then I feel the DOF will be smaller; if the change in fl is smaller than that factor, the DOF will be larger than initially at the new fl setup.

Sorry for being so lengthy. Thanks for bringing up such an interesting topic.

Antonio, The flaw in your test is that you are comparing the DOF at different focal lengths, and since DOF APPEARS differently at different focal lengths (though actually ISN'T, but this is another story) it's extremely difficult to make judgement calls on matters like these.

If you want to run a true test of my explanation you should compare the DOF of two images; one taken with a fixed-aperture 70-200mm lens (as an example) set to f/5.6 and a variable aperture 70-200mm zoom lens set to f/5.6. Even though the meter will indicate the light value is closer to f/8 or less, the images will appear identical to one another in terms of DOF.

Thank you!
A great article, only wish it were available as a Pdf. file which I could save.

Thanks again.
dsp

Dec. 27, 2011

 Hi,

      I just took a scan of the information you e-mailed, it is very informative - great info - I will

      give it a thorough reading later - Thanks

          jr

Thank you , Junior!

I just bought the Nikon 28-300mm f3.5-5.6 VR, and sold my 18-105 mm f3.5-5.6 VR kit lens and 70-300mm f4.5-5.6 G VR.  It was a even investment and I'm happy with the simplified shooting and sharpness increase.

Generally a great article, but I’m not so confident about your discussion of fixed and variable apertures in zoom lenses.  The statement that worries me is:

“The reason for this variation in the lenses’ maximum aperture is that zooms, by nature, are inefficient at transmitting light from the front element to the imaging sensor (or film plane) as you zoom from wide to telephoto. The physical size of the aperture (or f/stop) remains the same, but the amount of light that makes it through from one end to the other greatly diminishes.”

I have always put the change in f-stop down to the fact that as you double the focal length with a fixed diameter of lens, the f-stop also doubles (according to the formula f-stop = focal length over diameter).  I’ve always assumed that fixed aperture zoom lenses were made so that the aperture was set at the long end but that the design of the optics is such that image degradation at the shorter lengths with the same diameter of glass is too great to be used.

I can imagine that amount of light due to changed optics could vary a bit, but not “greatly” as you suggest.  (This could introduce a discussion of t-stops, but that is scarcely relevant for us.)

(I’m sure some people might find a discussion of the numbering of f-stops interesting as well; how they relate to the square root of 2.  Wikipedia has a good article on this at http://en.wikipedia.org/wiki/F-number .)

Lens have always had grouping on how light is makes it to the film plane. I have use fixed focal length lens over the years with very good results. With all the zoom lens fast or slow it is a tough call depending on what your subject mater is mainly, & if you do shooting in the 2.8 as a rule? Lens performance has been a ongoing subject for every new optic that come's out nikon, canon, after market. Unless you are going very large on the print side this maybe where edge sharpness would be noticed. If there is a focal length zoom or fixed you are looking to own, rent the lens test it to see if meets your needs before you make that commitment $$$. Good luck & happy shooting! Ciao G.C.

Great article!  Everything you need to know to select a great lens.

Well written and very informative, although I knew over 50% of the article I really enjoyed the article. Each time I take the time to read a B & H article I learn something that helps me to capture moments or objects I wan to share with folks.

Crisp

Question on the Nikon AF-S Nikkor 17-35mm f/2.8 ED -
This lens has never-ever been sharp. Tests done across the zoom range and at f/4, f/8, or f/16... even tried two other lenses (one from a rental house) and still cannot get sharp results.
I'm not being real picky here.. but having no luck or confidence as I use this lens the image is going to be sell-able.

My question is simple:
I need a replacement lens for this range... any recommendations?

Great lens artical! Thanks.

Great article and guide.
Can you pls explain what is a "Prime Lens" and how that designation is disclosed to the customer.

Great article as usual! I thoroughly enjoy the technical articles by the B & H Technical Experts. They are so well done and generally are some of the best written in the industry!

Can you give me an example of a Canon 'two-touch' lens? Thanks.

i'm looking at buying a travel lens. i wouldnt normally want to drop down in quality, but i'm doing a 'people' piece in Calcutta over the Xmas break, and want a 'snap it quick' lens that will suit any situation. i have been looking at some 18-135mm, 28-135mm, 18-200mm (Canon lenses and third party lenses) (for a Canon body btw - 7D), but can't decide because i'm worried about losing sharpness in my images. Anyone have any thoughts, advice, tried these lenses, are they sharp at all? Is it possible to get a sharp kit lens? Ive never used one.

Cheers!

Great article! Very informative.

A very helpful and informative summary. Clearly and understandably written. I am making a copy for future reference when purchasing my next lense. I also looked at my four existing lenses to review their features.
Suggestion for another topic: Compare the features of cameras with a "full-frame sensor" to cameras, such as Canon's 7D, that do not have a full-frame sensor.

Many thx
!
Very helpful

I really like the article on lenses. There were several key factors that were left out on stabilized lenses. One is the time for a image or picture stabilization to work, can you imagine how long it takes to spin that large ring in your favorite Canon lens. I have missed some shots waiting for the stabilizer to work. Another key factor, is battery life of those big stabilized lenses, most forget to mention that you will need to carry several camera batteries with you for your favorite lens. Thanks

I'll take that Nikon 200 f2.0 in the photo please !!! Oh that is one sweet bad-boy.

Thank you for the informative explanation among the various differences in lenses. This helped my understand on Macro's and lens coatings.