Variable Apertures and Depth of Field

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It’s interesting to note how many photographers, even advanced shooters, are mistakenly under the impression that the depth of field (DOF) of a variable-aperture zoom lens changes in relation to the effective aperture of the lens as you zoom toward the telephoto end of the optic’s zoom range. But as the character Sportin’ Life sang so long ago in Porgy and Bess, “It Ain’t Necessarily So.”

The reason f/3.5 slows down to f/4.5, f/5.6 or f/6.3 as you pile on the millimeters is because variable-aperture zooms are inherently inefficient at transmitting light. You’re fine at the wide-angle end of the zoom range, but the further you zoom in, the light levels continuously drop off as you zoom into your subject.

What doesn’t change is the DOF at any set aperture. If you set your lens to f/3.5, f/8, or f/11, the DOF remains the same as it would be on any lens set to the same aperture and at the same focal length regardless of how much light is lost along the way.

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Great article on lenses! Your comment on the variable f stops on zooms was interesting since I had always thought that the depth of field would increase as the effective aperture changed. The clue is the "effective aperture". As you said the change is due to light fall off I assume caused by the inverse square law. The size of the aperture stays the same at say 3.5, so the circles of confusion created by the cones of light passing through the same sized aperture remain the same. Is that the gist of it? Thanks for teaching me something. I remembered all this stuff from school but hadn't connected it until I read the article.

Martin Greeson

I just came across this article and though it has been around for a while, I believe it contains some misinformation.

You said:

"The reason f/3.5 slows down to f/4.5, f/5.6 or f/6.3 as you pile on the millimeters is because variable-aperture zooms are inherently inefficient at transmitting light. You’re fine at the wide-angle end of the zoom range, but the further you zoom in, the light levels continuously drop off as you zoom into your subject."

Uhhh, NO!  F-stop is a ratio; the focal length divided by the effective aperture.  When you "stop down" a lens, you change the apreture.  When you zoom, you change the focal length.  There is a decent article on the subject on Wikipedia.

If you have a lens that you set for, say f11, and it stays at f11 throughout the zoom range, I assume that is because the lens is designed to adjust the aperture as it zooms to keep the f number constant.  Obviously, it cannot do so at the ends of the range.

As for the effect on depth of field, I really don't know. 

Hi Patton,

This is one topic that seems to make heads spin in all corners, and it's a topic we've bandied about the office on many occasions even before it appeared on our website.

When you say "F-stop is a ratio; the focal length divided by the effective aperture.  When you "stop down" a lens, you change the apreture.  When you zoom, you change the focal length." you are correct on all fronts. We are in total agreement.

The problem is when you get to the part that says "If you have a lens that you set for, say f11, and it stays at f11 throughout the zoom range, I assume that is because the lens is designed to adjust the aperture as it zooms to keep the f number constant." 

And here's where you're getting tripped up.  While some macros are designed to compensate for exposure loss by automatically opening wider as the lens extends towards its minimum focusing point (but only when stopped down!), but this is not the case with variable aperture zooms.

If the lens is set to f/8 or f/11, it remains at f/8 or f/11, as does the DOF at the respected set aperture and focal length. But because the lens cannot transmit all of the light that enters its front element efficiently, the 'effective' f/stop is knocked down a notch or two, while the lens setting remains whereyou left it.

And thanks for taking the time to write us a note.

 

Interesting article!

I was OK with the concept you are presenting until I read the closing paragraph : "What doesn’t change is the DOF at any set aperture. If you set your lens to f/3.5, f/8, or f/11, the DOF remains the same as it would be on any lens set to the same aperture and at the same focal length"

If I open up my variable-aperture zoom to it's widest aperture at the wide-angle end (say, 70mm/f3.5) and then zoom it towards the telephoto end, ending at (say) 200mm/f5.6; what I thought I understood you to be saying is that the aperture from a DOF point of view would still be f3.5, whereas the aperture from a light point of view (the "effective" aperture, perhaps ?) would be degraded to an effective f5.6.
But that doesn't seem to be consistent with your closing statement - am I getting the same result - both DOF and light - as I would on any other lens set to 200mm/f5.6 ? Surely there is no difference between "setting" the lens to 200mm/f5.6 and having it get there by zoom action from wide open ? And how about if I stop it down from f5.6 to f8 ? What then ?

Also, I would presume that the electronic aperture display on the camera would always display the aperture value corresponding to the "light" value - correct ?

Thanks for a thought-provoking article!

Hello,

“If I open up my variable-aperture zoom to it's widest aperture at the wide-angle end (say, 70mm/f3.5) and then zoom it towards the telephoto end, ending at (say) 200mm/f5.6; what I thought I understood you to be saying is that the aperture from a DOF point of view would still be f3.5, whereas the aperture from a light point of view (the "effective" aperture, perhaps ?) would be degraded to an effective f5.6.”

This is correct. The physical size of the 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 amount of light reaching the focal plane.

“Surely there is no difference between "setting" the lens to 200mm/f5.6 and having it get there by zoom action from wide open ?”

There is, 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 exposure wise.  The opening is still f/3.5.

“And how about if I stop it down from f5.6 to f8? What then?”

DOF for the true opening size or f/stop will remain the same.

“Also, I would presume that the electronic aperture display on the camera would always display the aperture value corresponding to the "light" value - correct ? “

Yes, information displayed in a VF is exposure related.

Hi Chuck - thanks for confirming the point.

So, if we take the same lens that has been "zoomed" from 70mm/f3.5 to 200mm/f5.6, you are indeed saying that the physical aperture (determining DOF) is f3.5, whereas the effective ("light") aperture is now f5.6.

Comparing this to a constant-aperture zoom lens - say, an 80-200/f2.8 - that is looking at the same scene but has been stopped down ("set") to 200mm/f5.6, you're therefore saying the light values will be the same (corresponding to f5.6), but that the DOF will be quite different : f3.5 for the variable-aperture zoom, but f5.6 for the constant-aperture one.

The implication of this is that when you set up the photo using the variable-aperture zoom. and you note that the chosen setting (as per the VF readout) is 200mm/f5.6, you actually have little idea of the physical aperture (ie the DOF) without using the DOF preview. This is unlike the constant-aperture zoom (or prime lens) that is set to 200mm/f5.6, for which "what you see is what you will get" - f5.6 for both DOF and exposure.

I think perhaps the closing paragraph of the original article should more accurately read as follows :

"What doesn’t change is the DOF at any set *physical* aperture. If you are able to set your lens to a *physical aperture* of f/3.5, f/8, or f/11, the DOF remains the same as it would be on any lens set to the same *physical* aperture and at the same focal length, regardless of how much light is lost along the way."

Many DSLR lenses don't have aperture rings any more, and those that do usually require to be locked at minimum aperture to work with the camera. So, most users "set" the aperture by twiddling a dial and watching the VF display, ie they are setting the lens to an "effective" aperture (which determines exposure), not necessarily a physical one.
So I think the "take-away" point of the article is that when using a variable-aperture zoom lens, unless you have a way to set the PHYSICAL aperture, you actually have very little idea what the physical aperture (and hence DOF) is.

I'm certainly getting more of an appreciation for why constant-aperture zooms are so much more expensive !

Hello,

Thanks for the follow up and keeping us on our toes. I will admit I had to research this myself. Photography is and has always been an art of compromise and trade offs.  What I wouldn't pay to have my D5000 have the same image quality as my Rollieflex.  :0)

Best wishes for 2012.

I think you do have a way to set the physical aperture. If you are shooting a SLR with veriable aperture zoom and it's max aperture is 3.5-5.6 over the zoom range and you select f/4.5 on the camera you will be using 4.5 through the entire zoom range. So DOF doesn't change through the zooming except for the fact that DOF is usually greater on wide lenses than on tele lenses.

The statement "variable-aperture zooms are inherently inefficient at transmitting light" is misleading, if not outright wrong.  Perhaps you ment to say "variable-aperture zooms tend to have smaller maximum apertures, and therefore don't offer the wider f-stops available on premium lenses.  Wider apertures are desireable to photographers who want shallow depth of field, or are working in low light."

Modern lenses are all very efficient in "transmitting light", and there are no siginificant differences in transmission between primes, variable aperture zooms, and fixed aperture zooms.    The differences you are talking about have nothing to do with lens efficiency.

 For instance, let's look at three Canon lenses:  the 200mm f/2L prime, the 70-200mm f/2.8 constant aperture zoom, and the 70-300 f/4-5.6 variable aperture zoom.

Set all three lenses to f/5.6 at 200mm, and all three lenses will pass the same amount of light and yield identical Depth of Field.  There are no differences in "efficiency" between these three lenses.

 Most people are confused as to what "f-stops" really are.  If you are curious, continue reading: The terms "aperture" or "f-stop" refer to the effective diameter of the len's opening.   The diameter of this opening affects the amount of light passing through the lense, and the depth of field of the resulting image.  Typicaly the diameter is expressed as the ratio of the focal length to the diameter of the opening.   On a 200mm lens, an f-stop of f/5.6 is 200mm/5.6 or about 36mm.

 If you zoom a lens, while keeping the diameter of the aperture constant, your f-stop changes.  A 36mm diameter aperture is commonly refered to as f/2.8 at 100mm but that same 36mm aperture would be called f/5.6 at 200mm.

"Constant Aperture" zoom lens actually change the diameter of the aperture as the focal length changes.  

Reading these comments has been an educational experience for me.

So, then, B&H guys, does this mean that a fixed aperture zoom lens - say the Canon 70-200 f2.8, has a "fixed" aperture because it transmits as much light to the sensor at 200mm as it does at 70mm?

Is the problem here that manufacturers of variable zoom lenses are not in their descriptions of their lenses - f3.5 to f5.6 - being accurate, that is, these are not actually variable "aperture" lenses, although the effect in terms of the quantity of light reaching the sensor is the same, but not the DOF (and perhaps other attributes such as distortion) as well?

It strikes me that this is a really important fact to get one's mind around.

Since I don't monitor these kinds of discussions, I would appreciate you emailing me your answer.

Thanks.

What im confused about is when people say to improve the sharpness then drop to F8 for more sharpness, this is on the canon 70-300mm lens which has F4-F5.6 stops.  So setting to F8 shouldnt be possible surely but im able to, so what gives????

When we discuss a lens and refer to it by its techncial designation, in this instance the Canon "70-300mm f4-5.6" what this designation specifically  is stating, is the widest possible aperture at each focal.  On this lens, at 70mm, the widest aperture one may shoot at is f4.  If you were to have the lens set to F4 at 70mm, and zoom out, by default of the nature of a vairable aperture lens, the lens would decrease its aperture to f5.6 at 300mm.  Therefore the widest aperture you could shoot at at 300mm is f5.6.  So with what I have just stated the lens' designations are only discussing the widest apertures (when discussing lenses, photographers first want to know what the widest aperture of a lens may be.  It is understood that lenses can be stopped down further - how far down they can be set at is dependant on the manufacturer and how they designed the lens).  Most in this category can be stopped down to about f22.  If you regard the specifications page on any lens' link on our site, we post the full aperture ranges for them.  With your current 70-300mm f4-5.6 lens, set the camera to either M or AV and you can then toggle through the various apertures the lens has to offer.