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04-07-2013, 12:34 PM   #1
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Weird aperture behaviour with K-r and Cosina 28mm 2.8 MF

Greetings,

I have a Pentax K-r with firmware 1.13 (I think that's the latest version) and a manual focus Cosina 28mm f/2.8 MC Macro lens.

The lens has an aperture ring, allowing the aperture to be set manually or automatically with "A".

I have "Using Aperture Ring" set to "Permitted" as recommended by others when using older manual focus lenses.

If I shoot with the aperture ring set to "A", with the camera in aperture priority mode:
  • I can choose any aperture up to f/4, but not wider than that, ie the e-dial won't let me choose 2.8.
  • The aperture that I have chosen in Av mode is actually used for the shot. I can verify this by looking at exif data on the jpeg, and also because when shooting the same area of plain wall in the same light (with iso kept constant), as I widen the aperture the shutter time increases.
  • So this is all correct except not being able to shoot in 2.8, which is occasionally useful in very low light or when more background isolation is needed.

If I select the aperture manually on the aperture ring, while the camera is in manual mode:
  • Everything works out correctly. I verified this by keeping the shutter speed and iso constant, and adjusting the aperture ring while shooting the same scene. The aperture is not in the jpeg exif data, but it is clear that the exposure and depth of field changes with each shot. There is also a clear difference between f/4 and f/2.8, as well as a difference between 4, 5.6, 8, 11 etc so by induction I know that 2.8 is actually "working".
  • However, I am not completely clear on the best way of metering in this mode, other than guessing and then taking a picture and reviewing it.

If I set the aperture manually on the aperture ring, while the camera is in aperture priority mode:
  • Looking at the exif data, the aperture is always f/4. I think it is always choosing f/4 regardless of what I choose on the ring. Evidence for this is with a fixed iso and shooting the same scene, the shutter speed stays the same (from jpeg exif) regardless of aperture, confirmed by reviewing the images and also confirmed by the fact that in low light when the aperture was at f/22 the shutter speed still sounds fast, instead of the half second or more shutter speed it would be if it was really at f/22.

If I set the aperture ring to "A" while in the camera is in shutter priority mode or manual mode:
  • The widest aperture available is f/4, and the aperture that is displayed actually ends up getting used, so this is correct apart from no 2.8

Essentially, I would like to be able to shoot in aperture priority (or manual with some way of metering) at f/2.8. It's not a huge issue as when the lighting is good I prefer to shoot between f/4 and f/8, but in very low light f/2.8 is helpful.

I think this might have something to do with having a pentax 16-45 f/4 AF zoom lens, and the camera not being able to "forget" what the maximum aperture is. However, I tried with my Sigma f/1.8 20mm AF and that works fine when setting the aperture, but when I put the Cosina on immediately afterwards it still has a max aperture of f/4.

Any advice would be gratefully received

04-07-2013, 12:44 PM   #2
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If you plan on using the aperture ring, you have to be in M mode. Follow this guide for the proper metering technique:

https://www.pentaxforums.com/forums/pentax-lens-articles/110657-how-use-meter...k-x-k-7-a.html

Adam
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04-07-2013, 01:23 PM   #3
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Join Date: Apr 2013
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QuoteOriginally posted by Adam Quote
If you plan on using the aperture ring, you have to be in M mode. Follow this guide for the proper metering technique:

https://www.pentaxforums.com/forums/pentax-lens-articles/110657-how-use-meter...k-x-k-7-a.html
Thanks for this. This is a good temporary solution for when I want to use f/2.8 - however still not as convenient as putting the ring to "A" and the camera in aperture priority mode, as it meters correctly when doing this but won't let me chose an aperture wider than f/4.
04-07-2013, 02:24 PM   #4
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You might want to check the contacts on the lens mount for dirt or damage. It sounds like it is reporting its widest aperture to the camera as f/4 instead of f/2.8. If I read the tables right on Bojidar Dimitrov's Pentax K-Mount Page, the second contact from the bottom is the only one that is different between an F/2.8 lens and a f/4 lens.


04-07-2013, 06:50 PM   #5
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THoog is correct only his post doesn't include a link.

Go to THIS SITE for an explanation as to how the various contacts tell your body what the aperture range is for the mounted lens. While it sounds like you should clean them all, if per chance the lens contacts are defunct, you can do a temporary repair with a bit of magic tape and aluminum foil.
04-09-2013, 01:49 PM   #6
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QuoteOriginally posted by JimJohnson Quote
THoog is correct only his post doesn't include a link.

Go to THIS SITE for an explanation as to how the various contacts tell your body what the aperture range is for the mounted lens. While it sounds like you should clean them all, if per chance the lens contacts are defunct, you can do a temporary repair with a bit of magic tape and aluminum foil.
Thanks to all of you for this info. I carefully wiped down all the contacts with a glasses cloth. It now works fine and allows me to select f/2.8. I will have a detailed look at the links tomorrow (posting from phone). Thanks!
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