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03-13-2014, 08:18 PM   #1
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Manual lens that won't change aperture on camera

I recently bought on fleabay a manual K-mount lens for my K-5 II, a Cosina 35mm f/2.8 similar to this one Cosina 35mm f2.8 Cosinon W MC Lens Reviews - Cosina Lenses - Pentax Lens Review Database.


The problem I'm having is that the camera won't change the aperture when shooting (or checking metering/DOF). I know the camera should be able to do it because I have another manual lens that works fine, but this one is stuck open at 2.8. Off the body, the aperture ring works perfectly. On the body, it never changes (I've tested it by putting the camera in M mode, taking a shot at f/2.8, then taking another at f/22, and they are identical). Any suggestions? Am I just missing something (I'm new to Pentax)?

03-13-2014, 08:28 PM   #2
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The mount is anodized black and non-conductive, it must be conductive to work correctly on newer Pentax DSLRs.

The body is assuming it is an adapted M42 lens, so it doesn't attempt to release the aperture lever. Remove the anodizing or add some conductive foil so the contacts of the body are shorted so it will see it as an M lens and work as expected.
03-13-2014, 08:30 PM   #3
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I don't use manual lenses, but this is the tip in the review...


Set the aperture on the lens then just before shooting press AE lock to meter/set the shutter speed. (Green button on some cameras).
03-13-2014, 08:31 PM   #4
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QuoteOriginally posted by elliott Quote
The mount is anodized black and non-conductive, it must be conductive to work correctly on newer Pentax DSLRs.

The body is assuming it is an adapted M42 lens, so it doesn't attempt to release the aperture lever. Remove the anodizing or add some conductive foil so the contacts of the body are shorted so it will see it as an M lens and work as expected.
Thanks! I had no idea. Is there a particular section of the mount to make conductive?

03-13-2014, 08:43 PM   #5
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QuoteOriginally posted by bar_foo Quote
Thanks! I had no idea. Is there a particular section of the mount to make conductive?
Where it meets the contacts of the camera. I'm not entirely sure but I think it is mainly the Data contact, the one right by the screw on the bottom of the lens mount.

If you are going to be removing the coating it would be best to remove the mount from the lens if possible. Otherwise you can tape everything up with masking tape except where you want to sand. The anodized coating can be hard to get off, it isn't just paint.

I took the lazy way out when I had to do this to one of my extension tubes, I removed the mount and chemically stripped it with oven cleaner. I don't know how possible that is on that particular lens.
03-13-2014, 08:55 PM   #6
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It is not an A lens--the aperture is set by mechanical linkage only--the lever sticking out the back of the lens. You may need to push it slightly inwards or outwards--as it may be misaligned/bent.

---------- Post added 03-13-14 at 08:56 PM ----------

And in M exposure mode.
03-13-2014, 08:57 PM   #7
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QuoteOriginally posted by elliott Quote
Where it meets the contacts of the camera. I'm not entirely sure but I think it is mainly the Data contact, the one right by the screw on the bottom of the lens mount.

If you are going to be removing the coating it would be best to remove the mount from the lens if possible. Otherwise you can tape everything up with masking tape except where you want to sand. The anodized coating can be hard to get off, it isn't just paint.

I took the lazy way out when I had to do this to one of my extension tubes, I removed the mount and chemically stripped it with oven cleaner. I don't know how possible that is on that particular lens.
Thanks! Turns out to be incredibly easy to remove that plate on this lens, so I'll try sanding it down.
03-13-2014, 08:58 PM   #8
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Unless there is a change since the K20d/k-x?

03-13-2014, 09:08 PM   #9
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QuoteOriginally posted by dms Quote
Unless there is a change since the K20d/k-x?
I believe this started on the K-x and K-7, it seems to have shown up with the introduction of video in Pentax DSLRs.

The older bodies still used the difference between conductive/non-conductive mount to identify M42 or M lenses. They set the value in the EXIF, but didn't seem to change behavior based on it.
03-13-2014, 09:15 PM   #10
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QuoteOriginally posted by dms Quote
t is not an A lens--the aperture is set by mechanical linkage only
The mechanical linkage that sets the aperture is the aperture ring. The lever on the back of the lens actuates the stop-down mechanism to the opening set by the ring. The body sets nothing.


Steve

---------- Post added 03-13-14 at 09:17 PM ----------

QuoteOriginally posted by dms Quote
Unless there is a change since the K20d/k-x?
There has been...quite a few changes in fact. Strangeness with non-conductive lens bases started with the K-30/K-50 (K-01 too?) and carried over to the K-3.


Steve

---------- Post added 03-13-14 at 09:34 PM ----------

QuoteOriginally posted by elliott Quote
I believe this started on the K-x and K-7, it seems to have shown up with the introduction of video in Pentax DSLRs.

The older bodies still used the difference between conductive/non-conductive mount to identify M42 or M lenses. They set the value in the EXIF, but didn't seem to change behavior based on it.
Yep...My older body (the K10D) is completely agnostic in regards to base conductivity except for Catch-in Focus and writing K,M as the lens type in the Exif if the base is conductive and A contacts are not present. As for video being the start of the strangeness with non-conductive bases, I don't remember any reports before the K-30.


Steve

Last edited by stevebrot; 03-13-2014 at 09:38 PM.
03-14-2014, 04:01 AM   #11
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QuoteOriginally posted by bar_foo Quote
Thanks! Turns out to be incredibly easy to remove that plate on this lens, so I'll try sanding it down
I have that very same lens. And here's how it looked and looks:

03-14-2014, 05:56 AM   #12
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QuoteOriginally posted by stevebrot Quote
There has been...quite a few changes in fact. Strangeness with non-conductive lens bases started with the K-30/K-50 (K-01 too?) and carried over to the K-3.


Steve

Yep...My older body (the K10D) is completely agnostic in regards to base conductivity except for Catch-in Focus and writing K,M as the lens type in the Exif if the base is conductive and A contacts are not present. As for video being the start of the strangeness with non-conductive bases, I don't remember any reports before the K-30.


Steve
K30/K50 are very close to (identical to?) the K-01 electronically. There is considerable discussion as to whether the K-01 operates well with non-conductive lenses but fairly strong agreement the 30/50 don't. My opinion is my K-01 doesn't work fully with non-conductive lenses.
03-14-2014, 10:04 AM   #13
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QuoteOriginally posted by Stone G. Quote
I have that very same lens. And here's how it looked and looks:
Thanks! That photo helps a lot. I actually have a very slightly different version, but close enough.
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