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09-13-2017, 11:48 AM   #1
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Aperture control problems with variable aperture lenses (with aperture rings)

Dear Pentax Users!

I have a problem regarding variable aperture zooms, particularly lens with an aperture ring and an A setting, KAF mount.
(More precisely: my Tamron AF 70-300mm f/4-5.6 and my Vivitar Series 1 19-35mm f/3.5-4.5)

Here's what I noticed:
When I have my tamron at the 70mm end and set the aperture on the body to f/8 and use DoF preview, I get the appropriate aperture. When I do so at the 300mm end, I do NOT get the appropriate setting physically. The aperture is more open, than it should be. How do I know? If I use the aperture ring instead and set it to f/8, it is constant throughout all the way to 300mm and does not vary a tiny bit.
Similar is the problem with my Vivitar wide-angle zoom. Except, that lens does not even set the aperture properly, which is a fault of its' kind, but that is well known. So you have to compensate for it by using EV compensation. (-1 EV)

On the tamron, using the body vs the aperture ring, the ring does always set the aperture two stops from wide open, regardless of the zoom setting. When using the body however, using setting f/8 at the 300mm end is only about one stop.

Is this behavior normal or is there a problem with the aperture control in the body?

09-13-2017, 12:32 PM   #2
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Maintaining F8 on the 70-300mm requires a larger opening at 300mm than at 70mm, so it sounds like it is working correctly with the aperture set on the body. The camera knows the current zoom setting (on AF lenses) and can compensate accordingly to maintain the same aperture throughout.

An easy way to test this would be to put the camera in Av mode, select F8 at 70mm, and note the shutter speed.
Then, put the camera in M mode, select F8 and the same shutter speed, and zoom to 300mm. The center of the frame should have the same exposure, not counting vignetting.

As far as I understand, the aperture ring only tells the camera how many stops to stop down the lens. So, with a variable-aperture lens the number displayed on the ring will only match the true aperture at the shortest focal length, and at longer zoom settings the aperture will otherwise be narrower (i.e. F8 on the ring could actually be F11 at 300mm). However, as you correctly observed, the diaphragm opening remains constant in this case.

TL;DR, autofocus zoom lenses should always be used in the "A" setting on the aperture ring

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09-13-2017, 12:37 PM   #3
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Question...Which exposure is correct with the Tamron? My guess would be that using the aperture ring at f/8 in M mode results in one stop underexposed (shutter speed) compared to using the "A" position (shutter speed held constant). At 300mm, the body has to open the lens up more to provide f/8 than at 70mm (f-number = focal length/effective aperture*, a ratio).

I don't think I did a good job explaining and perhaps following comments will do a better job. (Edit: Adam beat me to it and his explanation is clearer.)


* Effective aperture is the diameter of the entrance pupil; though in practical terms, the diameter of the iris opening is a reasonable approximation with most primes. Zooms are a little more complex, though it expected that the marked numbers on a variable maximum aperture lens will only be accurate at the widest setting.
09-13-2017, 01:11 PM   #4
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Thank you both! I understand now. Then I am glad that the camera is working correctly.


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