f/stop Changes on a Zoom Lens

This may be a stupid question but I can't seem to find the answer on my own. If you have a zoom lens, is there any way to tell in the written specs, where the f/stop changes and to what as the zoom increases? For example, the Canon 100-400L goes from 4.5 to 5.6. Is there any way in the specs to see (without purchasing/touching the lens on a camera) where the lens changes from being 4.5 max to 5.6 max? It happens somewhere in the zoom. Some lenses change 2-3 times as the zoom changes.

Thanks!

The review site "Digital Picture" gives this data. For instance, for the 100-400mm:
http://www.the-digital-picture.com …L-IS-USM-Lens-Review.aspx
(Scroll down, search on aperture.)

Archibald wrote in post #17990744
The review site "Digital Picture" gives this data. For instance, for the 100-400mm:
http://www.the-digital-picture.com …L-IS-USM-Lens-Review.aspx
(Scroll down, search on aperture.)

In a variable aperture zoom, the focal ratio changes continuously as it zooms. It doesn't "snap" from f/4.5 to f/5 and then to f/5.6. That's just how it is reported by the lens to the body. The values of f-stop versus focal length you read on your camera or in reviews are actually points on a continuous curve.

Mike

That makes a lot of sense, Mike. I never really thought about the physics of it but I guess that's almost the way it would have to happen. Nonetheless, I do think that full-stop zoom point maximums would be great information when purchasing a lens. It's also the reason we probably don't get it.

WilsonFlyer wrote in post #17991498
That makes a lot of sense, Mike. I never really thought about the physics of it but I guess that's almost the way it would have to happen. Nonetheless, I do think that full-stop zoom point maximums would be great information when purchasing a lens. It's also the reason we probably don't get it.

It's actually MUCH easier than you are making it out to be. You first have to realize, though, that the f-stop numbers are not linear. By that, I mean that the difference between using f/8 and using f/4, for example, is not twice the exposure but four times the exposure.

The difference between f/4.5 and f/5.6 is only about two-thirds of one stop. At half the focal length range of the lens, you're about 1/3 stop from the same setting used at either end of the lens' focal length range.

The real fact is that, unless you're using the lens wide open, the camera will make up the difference for you by doing what you've told the metering system to do. In the "olden days", those folks using zoom lenses usually had to be aware of the exposure shift and do something to compensate because most metering in those days was typically done with a handheld meter. A similar problem with view cameras was even more difficult to calculate because the distance between the lens and film was sometimes changed quite a bit and that affected the exposure more than today's typical zoom lenses.

I solved the problem with zoom lenses for myself when I got my first Canon DSLR by choosing only those zoom lenses having a constant aperture across the focal length range.

mcoren wrote in post #17991479
In a variable aperture zoom, the focal ratio changes continuously as it zooms. It doesn't "snap" from f/4.5 to f/5 and then to f/5.6. That's just how it is reported by the lens to the body. The values of f-stop versus focal length you read on your camera or in reviews are actually points on a continuous curve.

Mike

The lens stays ahead of the curve. It's not like you take a shot with a 100-400 V1 at 5.6 which is the widest selectable stop at 250mm and then take another shot at 400mm using 5.6 and get a different exposure given a constant ISO and shutter speed.

FEChariot wrote in post #17991984
The lens stays ahead of the curve. It's not like you take a shot with a 100-400 V1 at 5.6 which is the widest selectable stop at 250mm and then take another shot at 400mm using 5.6 and get a different exposure given a constant ISO and shutter speed.

That actually makes a lot of sense if I understand correctly. You're saying that at 250mm, the "wide open" aperture (un-energized diaphragm) is probably somewhat faster than f/5.6 but not fast enough to be called f/5. When you take the shot, the diaphragm probably has to close slightly to f/5.6.

Mike

SkipD wrote in post #17991955
The real fact is that, unless you're using the lens wide open, the camera will make up the difference for you by doing what you've told the metering system to do. In the "olden days", those folks using zoom lenses usually had to be aware of the exposure shift and do something to compensate because most metering in those days was typically done with a handheld meter. A similar problem with view cameras was even more difficult to calculate because the distance between the lens and film was sometimes changed quite a bit and that affected the exposure more than today's typical zoom lenses.

Are you saying that the camera compensates by increasing or decreasing the exposure time slightly? Is that even desirable on a pro lens? While that would make the EV the same, it could be noticeable in terms of DOF. Since the camera displays aperture in 1/3 stop increments (at least, the enthusiast 70D that I am familiar with), doing it that way means the displayed aperture might be off by as much as 1/6 stop. I don't know how noticeable that would be for people who want precise DOF control. I'm pretty much an "f/8 and be there" kind of guy personally.

Mike

mcoren wrote in post #17992386
Are you saying that the camera compensates by increasing or decreasing the exposure time slightly? Is that even desirable on a pro lens? While that would make the EV the same, it could be noticeable in terms of DOF. Since the camera displays aperture in 1/3 stop increments (at least, the enthusiast 70D that I am familiar with), doing it that way means the displayed aperture might be off by as much as 1/6 stop. I don't know how noticeable that would be for people who want precise DOF control. I'm pretty much an "f/8 and be there" kind of guy personally.

Mike

Exposure time won't change DOF. Aperture will but if shooting wide open you have no choice but be limited to the aperture of the lens. Is it desirable is irrelavant. You make decision to buy a variable aperture lens (or rent or borrow) and accept its features. I desire a 100-400 f4 but bought a 100-400 f4.5-5.6 because it exists. I'd prefer 4.5-5.6 over constant 5.6 too.

Choderboy wrote in post #17992448
Exposure time won't change DOF. Aperture will but if shooting wide open you have no choice but be limited to the aperture of the lens. Is it desirable is irrelavant. You make decision to buy a variable aperture lens (or rent or borrow) and accept its features. I desire a 100-400 f4 but bought a 100-400 f4.5-5.6 because it exists. I'd prefer 4.5-5.6 over constant 5.6 too.

But you get the constant f/5.6, and the constant f8, f/11. f16, and all the third stops in between too. Thats the great advantage of the electronically controlled aperture, it can self adjust based on the set aperture value, once you select an aperture value that equals the smallest maximum value available.

Alan

Choderboy wrote in post #17992448
Exposure time won't change DOF.

Obviously. The original assertion was that the camera reports a wider aperture than it actually is and lengthens the exposure time to compensate. That *will* affect DOF versus what you expect.

mcoren wrote in post #17992386
Are you saying that the camera compensates by increasing or decreasing the exposure time slightly? Is that even desirable on a pro lens? While that would make the EV the same, it could be noticeable in terms of DOF. Since the camera displays aperture in 1/3 stop increments (at least, the enthusiast 70D that I am familiar with), doing it that way means the displayed aperture might be off by as much as 1/6 stop. I don't know how noticeable that would be for people who want precise DOF control. I'm pretty much an "f/8 and be there" kind of guy personally.

Mike

No but since we are now using electronically controlled apertures the control system is able to vary the size of the aperture, so that is you set it to any value that is equal to or smaller than the maximum aperture at the longest zoom setting the f/number remains correct. This is possible with an electronic aperture diaphragm, but on old lenses with a manually controlled aperture diaphragm the control ring has predefined click stops at fixed diaphragm diameters, that are marked with the f/number that applies at the shortest (usually) focal length. Even for lenses that have auto stop down systems, it is still the clickstop control ring that sets the diameter that the aperture will be.

Constant aperture zoom lenses work differently, and the lens construction is more complex, hence why they are usually more expensive. In a constant aperture lens the zooming system is designed so that the physical aperture is placed in front of the zoom mechanism. This means that as you zoom the lens in and out, you also zoom the image of the aperture. If as far as the film/sensor side of the lens system is concerned it sees the size of the aperture change it causes the f/number to change. Because the focal length also changes, the two cancel each other out, and the ratio between the focal length and the aperture diaphragm diameter remains constant, and so does the f/number. This works just as well for manually controlled apertures as it does for electronically controlled ones.

Alan