Rules of Thumb - Finding Your Lens' 'Sweet Spot'

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When defining the term image quality there are several qualifiers that go into the mix. Among them are tonality, contrast, brightness, and dynamic range, which is the degree of detail one can detect in the deepest shadows and brightest highlights. And then you have sharpness, which might be the trickiest to define.

'"Is it sharp?" might just be the number-one question people ask when shopping for lenses. After all, if you're going to be taking pictures with the camera you worked long and hard to purchase, you want to be assured the lens you're contemplating to buy will perform as advertised and deliver pictures you'll want to share with others.

What many shooters do not understand - and this includes many advanced shooters as well as newbies to the sport - is that lenses are not uniformly sharp at every aperture, nor at every focusing distance. And in the case of zoom lenses, not equally across the zoom range. A good example would be macro lenses, which as a rule, deliver higher resolving power at closer distances compared to non-macro lenses, which as a rule perform better than macros at distant focus settings.  

Now while we can't possibly address the optimum shooting parameters needed to squeeze the highest performance out of every lens on the marketplace, there are a few guidelines we can offer up that might at the very least influence the way you go about taking pictures.

The first thing we have to do is define the difference between maximum sharpness and maximum focus. If your goal is to squeeze the maximum levels of image sharpness out of your lens you can achieve this by simply stopping your lens aperture down 2.5 to 3-stops from the lens's maximum aperture. As an example, if the maximum aperture of your lens is f/2.8, you'll want to shoot with your lens aperture set between f/5.6 and f/8. For a lens that has a maximum aperture of f/3.5, the sweet spot of your lens resides somewhere between f/8 and f/11. Similarly, if your lens has a maximum aperture of f/1.4, the sweet spot of your lens is located somewhere between f/2.8 and f/4. And this simple rule of thumb works with most every lens you'll ever own.

Now some readers are probably baffled by the last paragraph because they're confusing sharpness, which is a measure of resolving power, with depth-of-field, which is a measure of focus. When you stop your lens down to smaller apertures, you do in fact bring more of the image into greater overall focus. This is akin to reading a piece of newsprint held a few inches from your eye. By squinting at it - i.e., stopping down your eye - it suddenly becomes more readable. Similarly, pictures captured at f/11, f/16, or smaller appear 'sharper' than images taken at wider (f/2.8, f/2, f/1.4, etc) apertures. But while you might have more in focus in a picture taken at a smaller aperture, overall, it's probably not as sharp compared to the in-focus portions of the same picture taken at the 'sweet spot' of your lens.

What all this means is that when you're out shooting you have to decide whether you are after maximum resolving power or greater overall focus. And do keep in mind the actual differences in sharpness between the two may not be all that great (or even noticable) when viewing these images side-by-side on your computer screen or in the form of mid-sized prints, but the differences are there none-the-less.

Note - When you stop a lens down to smaller apertures the increase of focus is not equal fore and aft of your subject, but rather 1/3 foreward and 2/3 backward. In other words, when you stop down your lens you increase depth-of-field between your subject and the background at twice the rate of the increase of focus between your subject and your lens.

It's also worth noting that just as the widest aperture of your lens doesn't represent the true resolving power of your lens, the smallest aperture - even though it allows more of the image to appear in focus - is most often equally deficient in the sharpness department.

That said... happy shooting.

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When using a wide-angle lens, a fast aperture/shallow DOF, and focused on a close subject, the distance to the edges are farther away than the center, and can fall outside of DOF acceptable focus. It's blurry and people blame the lens, but the focal range is actually an arc, not a straight plane. 

When I am shooting I'm  more concerned with DOF than "apparent" sharpness, but when in the dark room, I was taught that the sweet spot on the enlarging lens was in the middle range of the aperture openings. So my 50 2.8 Schnieder Componon S would deliver the sharpest enlargement at F 8 to F 11, in essence the middle range.

I always assumed, that the same could be said of regular shooting lenses. If you wanted the maximum quality of sharpness a lens could provide, the middle range of the lens would give it. Certainly this would apply for primes, but zooms may introduce another factor to it.

Anyway, great post Allan.

David

With the introduction of digetal photography with the smaller than normal sensors, it would appear that if the factor for your digegal camera is 1.5 then you already are using the meat of the lens.  The rest of the projected image alls outside the sensor.  This may be a wrong assumption by me and I would appreciate more comments on this.

The article matches what I do now but it would sure be nice if the focused distance data was used to display a DOF range. I have my gut feel and I carry a DOF calculator but when I check the first histogram I’d give up the 7 or 8 milliseconds if it would just load a DOF indicator too.

The closer you focus the more your depth of field split goes towards 50/50 instead of 1/3 - 2/3.

Lome ... you're correct, except that a Flat Field lens, typically your Macro lenses, are corrected to reproduce a flat field rather then a curved one.

With a Pentax camera it is quite easy to select the sharpest aperture for the lens in use. In Program you select MTF and the camera tries to keep the aperture as close as possible to the best one recorded in the chip in Pentax lenses. For other brand lenses it applies the same rule of thumb you told us.

Nice job Pentax! (and shame for others)

Lorne,

Frequently expressed, but is it necessarily so? Has anyone actually tested this against a curved subject and a flat subject at the same aperture to confirm it, or are we just repeating it because it seems to make sense? (Reminds me of the woman who always cut the end off the roast before cooking it. Why? Because her mother did it that way. And why did the mother cut off the end? Because her pan was too small!)

Beginners reading this article, please remember that basics of photography. While decreasing the size of your aperture to a certain degree (what specific degree is arguable) will improve the sharpness of your photos, it will also have other effects. One of these effects, as noted, is a broader depth of field, which means a greater distance in front of and behind your subject will be in focus. This effect is sometimes desired and sometimes not depending on what you are shooting. Another important effect to note is that stopping down the aperture means that, to achieve proper exposure, you must increase the power of your flash, increase exposure duration, or use faster film (or increase ISO digitally). Remember that using faster film or inreasing ISO on the camera will result in grainier images, and the grain may be more damaging than a very slight bit of softness.

Experiment a bit with your lenses. Read reviews, and, if shooting digitally, take more than a few sample shots and inspect them at the pixel level (100% magnification). This will help you get an idea of how impactful the sharpness hit is at different apertures. With some lenses, there isn't much of a difference at all, and sometimes it is more desireable to leave your lens fairly or wide open than to give up the ghost for proper exposure.

The above article is filled with mistruths - especally the following...

"[I]f the maximum aperture of your lens is f/2.8, you'll want to shoot with your lens aperture set between f/5.6 and f/8. For a lens that has a maximum aperture of f/3.5, the sweet spot of your lens resides somewhere between f/8 and f/11. Similarly, if your lens has a maximum aperture of f/1.4, the sweet spot of your lens is located somewhere between f/2.8 and f/4. And this simple rule of thumb works with most every lens you'll ever own."

Fast lenses like a 1.4 are the worst in sharpness due to optical compromises to gain speed, and they typically need to be stopped down to f8 to acheive the best images.

F2.8 lenses clean up around f5.6, and f3.5 lenses will typically be clean at f3.5 or 4 if they are good designs.

And a top quality f3.5 macro will be as sharp at infinity as it will be at 1:1

Thank you everyone, and I think Richard hit on something that I've been struggling with. As a newbie with a nice camera and lens (Canon 5D markII and 2.8L 24-70mm), I have not been able to obtain DOF while trying to learn the technical side of photography and how to use this camera at the same time.  (I've had a camera in my hand all my life, but always used auto). Now if I focus on the foreground, the background is fuzzy and vice versa.  If I focus in between, everything is not so great.  Then longer exposure with no IS and no tripod didn't help. I don't know if I played with ISO much. Is there something else I'm missing?  If I shoot at anything other than 2.8 they are not sharp and/or in focus. Will the angels sing for me if I get a lens with IS? I've gotten some stunning portraits with boquet, but no DOF when I want it.  So then I started defaulting to auto, which drives me crazy with trying to manage the camera to the focus points I want (again, sometimes multiple focus points, but not one that can be achieved as a quick snapshot, like two people playing ping pong - how to get both opponents in focus... but that's another story). Also, what is actually happening when I zoom from 24 to 70 and somewhere in between? - there also seems to be some sweet spot of everything working together perfectly but I haven't got the feel yet of which combo it is. If anyone has any magical advice (ha), a resource/web/book to point me to, it would be greatly appreciated, and I will continue to practice till I get it.

Your observations about lens performance and Depth of Field are correct, but you are using incorrect terms for your explanation.

RE: FOCUS -- the definition of FOCUS is as follows:

There is one (and only one) distance from the lens that is "in focus" -- this is referred to as the plane of focus and any objects not in this plane are "out of focus". Parts of the image that are "out of focus" may appear "sharp" due to the phenomena of Depth of Field. It is correct that the best focus (and best quality overall) for a lens is generally 2 to 3 stops down from wide open. Depth of Field is dependent on 3 parameters --  focal length, focusing distance, and aperture.

Note that view cameras can be configured so that the plane of focus is not parallel to the film plane -- causing different distances to be "in focus" in different parts of the frame.

Monica... the larger the sensor (5D has a relatively large sensor), the less depth of focus you have at any given aperture as compared to smaller sensor cameras.  With your 5D you will need to stop down significantly for more depth of focus and will, therefore, sometimes go beyond the "sweet spot" of your lens in seeking greater DOF. In fact, you may not be able to achieve the greatest amount of DOF that you seek with your 5D in some cases and may need a smaller sensor camera to do that.

Use a smaller sensor camera, like a quality compact camera (Canon G9 for example) for situations where you want great DOF.  Pocket/compact cameras by virtue of their design yield great depth of focus even at wide apertures.  And they are designed to be at their sharpest with apertures at or close to wide open.  You will notice that with many compact/pocket cameras, the aperture range may only be 3, 4 or 5 f-stops. They don't need to have f/22 to achieve significant DOF because their smaller sensor size give you great inherent DOF, even at max aperture.

What I find baffling isn't 'the last paragraph', but rather that this article contradicts itself and doesn't explain things accurately.

It says 'The first thing we have to do is define the difference between maximum sharpness and maximum focus', but it doesn't define this at all it just goes on to talk about sharpness...

first it explains that maximum sharpness is obtained by stepping down the aperature.  Then the next paragraph it says the exact opposite, it says ' But while you might have more in focus in a picture taken at a smaller aperture, overall, it's probably not as sharp compared to the in-focus portions of the same picture taken at the 'sweet spot' of your lens.'... so now it is saying that in fact the picture will be sharper with a larger aperture.

It never explains maximum focus at all, and I am more confused than ever about how to actually take sharp photos.

By stopping my lens' aperture down 2.5 to 3-stops from my lens's maximum (f4.5 max down to f7 ~ f7.5) aperture, would this still apply when using a 80mm ~ 400mm VR zoom lens?  Does it matter if focusing to the 400mm limit and will this technique continuing to help me achieve finding my  "lens' sweet spot"?

Monica: "Now if I focus on the foreground, the background is fuzzy and vice versa."

Back in the "good ol' days" it was pretty easy to find DOF. Usually there was a scale on the top of your lens that would show you the approximate DOF range for the aperture used. Also, there was a DOF preview lever or switch that would stop down the lens so you could see what was in focus and what wasn't. The lens-top scale appears to have been eliminated on digital cameras, but my Nikon D70 still has the stop-down feature and I'd be surprised if your Canon didn't.

 Nice helpful discussion.

Thanks to you all.

Catman...

Why would you buy a 1.4 lens if you had to stop it down to F8 for it to be good?  Just so you would be able to stop it at 1.4 if you needed the light in a situation you couldn't control?

Interesting discussion.  Perhaps I missed it but I did not see any mention of diffraction which also degrades image at smaller (smallest) apertures.  But like everything else, if you have studied all the variables in optics and photography, each picture is a compromise.

Wow... first time i've visited this page... the enthusiasm everyone brings to the community is marvellous.  Like Monica, I'm sort of new to DSLRs and trying to soak everything in.

Thanks for all the comments!  Very valuable article, indeed.

The 'Pro' and 'Top-Pro' Zuiko lenses for four thirds DSLRs are sharp, much, much closer to wide open.

This means that those lenses are useable wide open and for most purposes plenty sharp at that F-stop. 

The sharpness/DOF sweet spot with quality four thirds lenses occurs between 1 and 3 stops from wide open,  This can mean faster shutter speeds for the same DOF / ISO rating than legacy-frame lenses. can give achieve. 

That quality also increases flash range and shortens recycle times or allows the use of smaller and lighter flashes than larger sensored cameras can give with flash the same size at the same ISO / DOF.

They DO fall off when stopped doem below F/16 but DOF is generally huge at f/11 so this is rarely needed anyway.

Good article though . . .

Many thanks to the Pentax owner who let us know that owning a Pentax can make this so much easier.

I do own a Pentax with a DA 200mm and a 50mm F Macro.  I am very happy with these lens.....now I know where to look, and what to do for sharpness, I'm even happy.

Cheers

The statement "At 2.8 F-stop, shallow depth of field, at 22 F-stop, very wide depth of field.  That is the only setting on your camera that adjusts the depth of field." is not true.

The following also affect DOF

Zooming or changeing your lens. DOF effected by focal length - the longer, the less DOF.

Distance from focal point. If focused close up, less DOF - far more DOF.

example:

If you are using long lens (150 mm), focusing at 5 feet, no matter how far you stop down the lens, you will still have relatively shallow DOF.

When setting up your shots with an idea of what you want to be look sharp and what you want to look fuzzy, you must consider the lens to use, your distance from the subject, as well as aperture setting. DOF has more to it than most realize - you must controal all aspects. Tamron's web site has a DOF calculator -- its use is very instructive.

Also some people have the idea that DOF is related to lens quality. Not so -- all lenses with same focal length and aperture setting and focused at the same distance will yield the same DOF. It is simply a matter of geometry.

Monica,

I shoot with the same camera and lense combo you are using.  I have found using the center focus point manually, aperture priority mode, F-8 to F-13 yeilds the sharpest photos for landscape or portraits.  For action shots I dial up the ISO. Any excessive noise evident can be removed in the Digital Photo Professional software that came with the camera.  I would also suggest shooting in raw to take full advantage of the software for image processing.

Thank you for your post Allan.

ioannis Gkatzelis wrote:

Thank you for your helpful post Tim Thumb.