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11-15-2013, 12:14 AM   #1
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Cosina 55mm f1.2 Stays Wide Open in Aperture Priority Mode

I prefer to shoot in aperture priority mode with lenses without aperture contacts so the camera can adjust shutter speed for exposure but my Cosina 55mm f1.2 stays wide open when I do this. When I switch it to manual mode and use the aperture ring, select a shutter speed the camera closes the aperture. The lens mount isn't anodized, it is bare metal which is the only thing I can think that would cause the problem. So what could be causing this?

11-15-2013, 12:21 AM   #2
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Av mode will only work with manual lenses that also have a manual diaphragm, such as takumars. So, you have to use M mode with that particular lens (I know, it's a silly issue that could be fixed via a firmware update).

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11-15-2013, 01:37 AM   #3
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I use Auto Revuenon 55mm f1.2 manual lense and have the same "problem" - it doesn't work in Av mode. But that is normal for all manual lenses without automatic aperture (A), like Pentax K 50 f1.2 or Pentax M 50 f1.7. You need to use M mode and green button every time before shooting, when lighting conditions have been changed. Or You should make special changes in the lense bayonet to shorten 1 contact on the camera's bayonet (drill 1 hole and make contact. It's not easy, precise job). Or You can mount the lense on camera, but screw it not until the end (controlling exact position by your eyes and stop screwing when diaphragm begins to close when wide open). In this case You can use the diaphragm ring for selecting aperture and don't need the green button, but focusing can be difficult at f5.6 or higher, because the amount of light comming through the lense is small.
11-15-2013, 06:23 AM   #4
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Well, you don't "need" to green button before every shot - green button merely does a stop down metering and adjusts the shutter speed according to the aperture you selected on the lens. It is useful, but not necessary.
Av mode will keep the lens wide open, but it meters continuously and allows auto-ISO.
M mode lets you use any aperture you select on the aperture ring (as long as you enabled aperture ring in the back of the camera menu), but it will only meter when you press the green or +/- button, and ISO is fixed.
Any other mode will default to Av.

There are some ways to "trick" the camera or lens to allow more automation, I think, but I am not quite sure. You can try searching the forums, but any modifications are at your own risk. I think you need to modify the camera mount a little, to short some contacts.

Btw look into Catch in focus aka focus trapping, is a neat Pentax function that gives you something almost like AF with manual lenses.

11-15-2013, 12:20 PM   #5
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Since the mount is exposed metal, presumably conductive, if the camera A pin was connected to it, the camera would think a lens with f1.2 to f22 aperture range was mounted. Which sounds ideal, just avoid trying to use f22 because the Cosina only does f16, maybe deal with exposure errors if the lever doesn't move the same distance per stop like the camera expects. You have a few choices about making that contact between the camera A pin and the lens mount.
11-15-2013, 12:34 PM   #6
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Yes, absolutely normal behavior for all bayonet non-A lenses. Which is why people complain about the crippled mount...
11-15-2013, 01:28 PM   #7
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QuoteOriginally posted by vonBaloney Quote
Yes, absolutely normal behavior for all bayonet non-A lenses. Which is why people complain about the crippled mount...
What he said.

Automatic aperture (stop down at exposure time) only works for A-contact lenses when using Av mode on your K-7. This makes sense if you consider how modes other than M work.


Steve
11-15-2013, 01:51 PM   #8
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I have no trouble with Green Button metering. My most commonly used lenses are three DA lenses and two M lenses. No hassle. It only takes 0.5 seconds to use Green Button metering with the M lenses. Push Green, push shutter. Given a certain number of years of using manual only cameras, I don't see the problem, and actually use the stop down scale more often than the green button. If it is a hurried shot, green button.

11-22-2013, 01:11 PM   #9
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Sorry for the delay but thanks for the responses everyone!
11-27-2013, 12:53 AM   #10
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Can't one just dial in Ev compensation after selecting a different aperture? For instance shooting the Cosina at 2.0 would require +1.33 compensation. Then since you can set the front e-dial to control Ev, the AV mode process is just two-step - set aperture ring, turn the front dial the same number of stops you would have before, and shoot?
11-27-2013, 02:18 AM   #11
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QuoteOriginally posted by jeffshaddix Quote
Can't one just dial in Ev compensation after selecting a different aperture? For instance shooting the Cosina at 2.0 would require +1.33 compensation. Then since you can set the front e-dial to control Ev, the AV mode process is just two-step - set aperture ring, turn the front dial the same number of stops you would have before, and shoot?
No, it will not stop the lens down ever unless in M mode.
11-27-2013, 09:02 AM   #12
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QuoteOriginally posted by vonBaloney Quote
No, it will not stop the lens down ever unless in M mode.
Well that's a shame. Seems ridiculous - the mode should have nothing to do with action on the aperture lever.

I wish Pentax would just release a firmware SDK like Canon, there are multiple things that could so easily be improved in their software.
11-27-2013, 11:22 AM   #13
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wait...you are STOPPING DOWN a Cosina 1.2?
11-27-2013, 11:28 AM   #14
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QuoteOriginally posted by mikeSF Quote
wait...you are STOPPING DOWN a Cosina 1.2?
As nice as 1.2 is, that is a wickedly sharp lens stopped down so it is good for more than just speed and insanely thin DoF. But if you do just want to stick with wide-open, then you can go ahead and use Av mode.
11-27-2013, 01:52 PM   #15
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QuoteOriginally posted by jeffshaddix Quote
Well that's a shame. Seems ridiculous - the mode should have nothing to do with action on the aperture lever.
It makes perfect sense if you consider that the mode is Av, meaning that the body would normally control the aperture setting. This is done by a calibrated movement of the actuator lever. The actuator lever on the Cosina 50/1.2 (as with other non-A contact lenses) is not calibrated against the Pentax-A spec. There is also the other issue of just what you would expect the camera to do.

Consider:
  • No A contacts
  • Av mode
  • Camera is not able to set aperture from body so aperture is unknown
In the above case, what shutter speed should the camera choose?

The fallback is to not trip the actuator and provide metering (shutter speed) for the lens full open with the cautionary statement in the manual that the exposure may not be proper.


Steve
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