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03-02-2022, 01:50 AM   #1
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On variable aperture zoom lenses...

Let's assume I have a zoom lens with a variable maximum aperture. The numbers don't matter but let's say it's a 24-70 f/4-5.6.

On a Pentax DSLR if I am in aperture priority and choke the lens to f/5.6 or f/8, will the camera maintain this aperture throughout the focal range if I zoom around?

For comparison, Nikons don't manage to remain constant, but for example M4/3 cameras do manage that.

Will a modern digital DA zoom lens remain "constant aperture"?
... will an A lens?
... will an M lens?
... how about even older ones?

03-02-2022, 02:23 AM   #2
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Easiest way to find out is to do tests at various apertures and focal lengths, then check the EXIF data.
03-02-2022, 02:24 AM - 1 Like   #3
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The aperture will only stay constant at the narrowest maximum aperture. Say with the 20-40 the aperture is f2.8-f4, if you have your aperture at f2.8, as you zoom out it will change accordingly but if you set it at f4, the narrowest maximum aperture, it will stay at f4 whatever you do
03-02-2022, 02:46 AM   #4
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QuoteOriginally posted by m96 Quote
Let's assume I have a zoom lens with a variable maximum aperture. The numbers don't matter but let's say it's a 24-70 f/4-5.6.

On a Pentax DSLR if I am in aperture priority and choke the lens to f/5.6 or f/8, will the camera maintain this aperture throughout the focal range if I zoom around?
In your example the selected aperture of f5.6 or f8 will remain constant throughout the zoom range. So will any slower aperture setting.

But if you set the aperture to f4 when the lens is set to FL 24mm, as you zoom toward FL 70mm, the aperture will change to f5.6. Equally if the lens is at FL 70mm you will not be able to select f4 at all.

QuoteOriginally posted by m96 Quote
Will a modern digital DA zoom lens remain "constant aperture"?
Some zooms are constant maximum aperture, like the DFA 24-70 f2.8. That can remain at f2.8 throughout the zoom range. Some are not like the 28-105 f3.5/5.6. Generally a constant aperture zoom will be more expensive.

Older lenses follow the same format, some are constant widest aperture, some are not. You can always tell from the name of the lens .......f2.8 vs f3.5/5.6 for example.

---------- Post added 03-02-22 at 09:51 AM ----------

QuoteOriginally posted by m96 Quote
For comparison, Nikons don't manage to remain constant, but for example M4/3 cameras do manage that.
both Nikon and M4/3 lenses follow the same format as above, some are constant max aperture, some are not.

03-02-2022, 03:29 AM   #5
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QuoteOriginally posted by Cerebum Quote
but if you set it at f4, the narrowest maximum aperture, it will stay at f4 whatever you do
QuoteOriginally posted by pschlute Quote
In your example the selected aperture of f5.6 or f8 will remain constant throughout the zoom range. So will any slower aperture setting.
Thank you, this answers it for DA and DFA lenses. You may have guessed it, my motivation behind this question is that I am considering my first Pentax camera and I like fully manual exposures (the M mode in Pentax nomenclature).


How about if I like an older lens, might I expect same behavior with an F zoom lens, or an A zoom?

---------- Post added 03-02-22 at 03:39 AM ----------

QuoteOriginally posted by pschlute Quote
both Nikon and M4/3 lenses follow the same format as above
I might have explained my problem a bit unclearly.

I know what a maximum variable aperture in a zoom lens is, perfectly well.

But I raised Nikon as an example because their older zoom lenses (manual-focus Ai zooms and screwdrive D zooms) behave as follows: the lens might be a f/4 - f/5.6 variable but even if I choke it down to f/8 the aperture still varies within f/8-f/11 as I zoom in and out, so it doesn't stick with an aperture even if it is shared and supported by the entire focal range.

Last edited by m96; 03-02-2022 at 03:40 AM.
03-02-2022, 04:11 AM   #6
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QuoteOriginally posted by m96 Quote
But I raised Nikon as an example because their older zoom lenses (manual-focus Ai zooms and screwdrive D zooms) behave as follows: the lens might be a f/4 - f/5.6 variable but even if I choke it down to f/8 the aperture still varies within f/8-f/11 as I zoom in and out, so it doesn't stick with an aperture even if it is shared and supported by the entire focal range.

That's very strange. If a "zoom" (really a varifocal) lens has a variable maximum aperture of f/4-f/5.6, then at any aperture setting smaller than f/5.6 it should remain constant. If you set the aperture ring to f/8 then f/8 is what you should get.

Could I politely ask on what evidence you're basing your belief that your Nikon lenses are varying between f/8-f/11? If it's true, it would mean that some pretty poor quality design work was going at Nikon in those days.


To add a bit more detail:

A true zoom will always have a constant aperture because it only changes the magnification of the element groups in front of the aperture diaphragm. Varifocals have a variable maximum aperture because they move the element group behind the diaphragm as well, changing the actual rather than just the effective focal length. It is possible to make a varifocal with a constant maximum aperture, but manufacturers usually settle for a variable maximum because it allows for a smaller front element group and a simpler and cheaper design. But the movements of the rear element group and the central variator and compensator groups should always make it possible to keep the aperture constant at anything other than wide open.

Last edited by Dartmoor Dave; 03-02-2022 at 04:27 AM.
03-02-2022, 04:12 AM   #7
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QuoteOriginally posted by m96 Quote
How about if I like an older lens, might I expect same behavior with an F zoom lens, or an A zoom?
As long as the aperture ring is set to the "A" position of an F or FA lens, the camera has full control of the lens aperture and will behave like a new DA of DFA lens.
However, A lenses don't communicate with the camera, therefore the current focal lenght is not recognized by the camera, as I only have a constant aperture zoom lens (A lens) I am not totally certain what happens in cases you select an aperture between the variable aperture values of the lens, when zooming (with a variable aperture zoom lens)


edit: when the aperture ring is set to something else than the A position, the lens will operate like a M or K lens.
The functionality will then depend on the camera body (the new K-3III offers more functions for M or K lenses than the older camera bodys do): the lens can be used in M or B mode without restrictions (with green button metering), or in Av mode (but the lens will not stop down in Av mode, no matter where the apeture ring is set to -> the new K-3iii will stop down the lens in Av mode as far as I know)


Last edited by othar; 03-02-2022 at 04:23 AM.
03-02-2022, 04:14 AM   #8
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Short answer is Yes
03-02-2022, 04:15 AM   #9
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QuoteOriginally posted by m96 Quote
Thank you, this answers it for DA and DFA lenses. You may have guessed it, my motivation behind this question is that I am considering my first Pentax camera and I like fully manual exposures (the M mode in Pentax nomenclature).
Aaaah, well, pentaxians (us lot) love the ability to use older lenses. All Pentax bodies from the early 2010s have in body stabilization. This means any lens you put on your camera is stabilized. You can also use any K mount lens on the Pentax body. Further than that, the M42 mount is the same lens to sensor distance as K mount meaning any M42 screw mount lens works perfectly (bar the very odd one). You just need an easily obtainable adapter. This means you have masses of cheap potential lenses. Re. using them, if you put the camera in manual, pressing the green button gives you perfect exposure. If you fancy using bold lenses, Pentax is the only DSLR (as opposed to mirrorless) alternative. Regarding zooms, vintage zooms aren't great but there are plenty of bold Pentax zooms out there. When it comes to Pentax bodies, there is plenty of guidance here, regarding lenses, if you see a lens you fancy, google "[lens name] review Pentax" and you will see a link to lots of reviews by pentaxians. Any questions, just ask
03-02-2022, 04:49 AM   #10
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QuoteOriginally posted by m96 Quote
How about if I like an older lens, might I expect same behavior with an F zoom lens, or an A zoom?
Exactly the same. With FA F and A lenses you set the aperture on the camera scroll thumbwheel, just like a DFA lens. With a variable max aperture zoom the same restrictions apply as above

Older K mount lenses the "K" and M series, rely on the aperture ring to select the aperture to be used. The M 28-50 f3.5/4.5 has a 3.5 setting on the ring, but when set here it acts as f4.5 when using the 50mm FL setting.
03-02-2022, 04:49 AM   #11
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QuoteOriginally posted by Dartmoor Dave Quote
Could I politely ask on what evidence you're basing your belief that your Nikon lenses are varying between f/8-f/11? If it's true, it would mean that some pretty poor quality design work was going at Nikon in those days.
Well it is not a Nikkor but what I have mounted right now is a Sigma 70-300 f/4-5.6 lens on my Nikon and I dialed in f/8 at 70mm and zoomed in to 300mm and the aperture narrowed to f/11 on the camera's LCD. Shot each end of the focal range with full manual exposure, the resulting files appear to be also about one stop different in their exposures.


I am happy to repeat the same experiment for you with a genuine Nikkor, at a later date, if you want.

---------- Post added 03-02-22 at 04:53 AM ----------

QuoteOriginally posted by pschlute Quote
Older K mount lenses the "K" and M series, rely on the aperture ring to select the aperture to be used. The M 28-50 f3.5/4.5 has a 3.5 setting on the ring, but when set here it acts as f4.5 when using the 50mm FL setting.
Thank you for confirmation! Just to make it 100% clear for me, if you set this lens to f/5.6 will it remain f/5.6 throughout the entire range?
03-02-2022, 05:01 AM   #12
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QuoteOriginally posted by m96 Quote
Well it is not a Nikkor but what I have mounted right now is a Sigma 70-300 f/4-5.6 lens on my Nikon and I dialed in f/8 at 70mm and zoomed in to 300mm and the aperture narrowed to f/11 on the camera's LCD. Shot each end of the focal range with full manual exposure, the resulting files appear to be also about one stop different in their exposures.


I am happy to repeat the same experiment for you with a genuine Nikkor, at a later date, if you want.

---------- Post added 03-02-22 at 04:53 AM ----------


Thank you for confirmation! Just to make it 100% clear for me, if you set this lens to f/5.6 will it remain f/5.6 throughout the entire range?
Variable aperture zoom lenses are variable because of light transmission fall off as focal length increases. Constant aperture zooms have light fall off, however they have cams and mechanisms to adjust the physical aperture opening to maintain the same amount of light to the film or sensor, regardless of selected focal length. A variable aperture zoom, say a 28-105 f4~5.6 set to f5.6 at 28mm will, unless it is a modern digital lens, drop to f8 at 105mm even though the camera will read 5.6.
03-02-2022, 05:03 AM   #13
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QuoteOriginally posted by m96 Quote
Thank you for confirmation! Just to make it 100% clear for me, if you set this lens to f/5.6 will it remain f/5.6 throughout the entire range?
It will. I own plenty of Pentax lenses both modern and old. I have never experienced what you described with the Sigma.

---------- Post added 03-02-22 at 12:13 PM ----------

QuoteOriginally posted by m96 Quote
Shot each end of the focal range with full manual exposure, the resulting files appear to be also about one stop different in their exposures.
At 70mm versus 300mm you will have two very different scenes. How can you compare metering results accurately?
03-02-2022, 05:13 AM   #14
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I just checked this with my 20-40mm f/2.8-4 Limited zoom in Av mode (which is what I usually use).

If I have the lens at 20mm and f/2.8 and zoom out then the aperture changes in accordance with the maximum available at that focal length. When I zoom back to 20mm then the aperture goes back down in the same way.

However, if I manually set the aperture to f/4 at 20mm then it won't change as I zoom to 40mm and then back down to 20mm again.

Essentially, the camera understands that if I'm at maximum aperture then I want to stay there as much as the lens will allow but if I set something manually then I want to stay there instead.
03-02-2022, 05:22 AM - 1 Like   #15
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QuoteOriginally posted by m96 Quote
Well it is not a Nikkor but what I have mounted right now is a Sigma 70-300 f/4-5.6 lens on my Nikon and I dialed in f/8 at 70mm and zoomed in to 300mm and the aperture narrowed to f/11 on the camera's LCD. Shot each end of the focal range with full manual exposure, the resulting files appear to be also about one stop different in their exposures.

Now that really is very strange, although I suspect it's more to do with the camera's metering system rather than any actual change in the size of the entry pupil.(*)

Unfortunately this thread is already starting to get tangled up with internet myths such as light fall-off at different focal lengths being a factor. (It isn't. It's entirely down to changes in the apparent size of the entry pupil when seen from the front of the lens.) So I'm not sure if we're likely to really get anywhere with your original question. Sorry about that.

(*) Using your Sigma 70-300mm as an example, with the lens set to f/8 then at any focal length the apparent size of the entry pupil when seen from the front of the lens should be that focal length divided by 8. If that ratio is changing as you zoom then it's a truly badly designed lens.

Last edited by Dartmoor Dave; 03-02-2022 at 05:46 AM.
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