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07-11-2020, 08:32 AM   #1
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Guess i was wrong?

This may be common knowledge but it was a new discovery for me. Some years ago I managed to acquire a M42 mount Vivitar Series 1 135mm f:2.3. I had played with it a few times and had always assumed that an M42 mount on my K-5 would only work if I set the exposure myself in manual mode. I took it out into the garden this morning to shoot some flowers, got set up and started shooting and was getting some good shots. I was quite surprised when i realized I'd left the camera in Aperture priority, and while the camera didn't know what the aperture setting was, it was adjusting the shutter speed to come up with a properly exposed image. When you stop down the aperture on a manual lens with the camera in manual mode the image in the viewfinder gets progressively darker, and i noticed this wasn't happening in aperture priority.
This lens in now a little more fun to use.

Note: see my final post on this topic. it seems my conclusions were not correct.


Last edited by BobCPO2; 07-11-2020 at 09:21 PM.
07-11-2020, 08:47 AM   #2
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If the viewfinder isn't getting darker you are shooting wide open. K-mount cameras don't automatically stop down M42 lenses. If the lens has an A/M switch, and it is set to M, the aperture will stop down when you turn the aperture ring.
07-11-2020, 09:41 AM   #3
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QuoteOriginally posted by boriscleto Quote
If the viewfinder isn't getting darker you are shooting wide open. K-mount cameras don't automatically stop down M42 lenses. If the lens has an A/M switch, and it is set to M, the aperture will stop down when you turn the aperture ring.
Boriscleto, You're quite correct and I already understand all that you've said. Everything i had read about using the pentax K mount to M42 mount adapter led me to believe that I would need to set the aperture manually, but that was not my experience today. There is an A/M switch on the lens. With the lens switch at M, the aperture of course stops down when i turn the aperture ring. When the lens switch is set to A, I can turn the aperture ring (in my test shots i went to f11) and the viewfinder remains fully bright, then the aperture stops down when i press the shutter. That's what I didn't expect to happen and why i was surprised.

Following this reply, and after some further review on my part, I did conclude that Boriscleto was right and my assumptions about what had happened when shooting with that lens were not correct. See my final post on this topic.

Last edited by BobCPO2; 07-11-2020 at 09:18 PM.
07-11-2020, 10:01 AM - 1 Like   #4
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QuoteOriginally posted by BobCPO2 Quote
When the lens switch is set to A, I can turn the aperture ring (in my test shots i went to f11) and the viewfinder remains fully bright, then the aperture stops down when i press the shutter. That's what I didn't expect to happen and why i was surprised.
No, it does not. A K mount camera lacks the mechanism to push in the M42 aperture pin. You can set the ring to f11, but the exposure will be for the maximum aperture. Set the lens to M and the viewfinder will darken, and the exposure will be for f11.

07-11-2020, 10:05 AM   #5
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QuoteOriginally posted by BobCPO2 Quote
When the lens switch is set to A, I can turn the aperture ring (in my test shots i went to f11) and the viewfinder remains fully bright, then the aperture stops down when i press the shutter. That's what I didn't expect to happen and why i was surprised.
Yeah, the new cameras do this with all manual lens that are native K mount or that have a provision for the diaphragm mechanism on the adapter. It can actually be a bit confusing if you're used to working on an all-manual camera; I'm always wondering why my diaphragm has stopped working before I realize that the camera is purposefully holding it open.
07-11-2020, 03:54 PM   #6
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After investigating this matter once again, I must concede that you are correct. Because I've always used full manual in the past, I confused the fact that the camera was adjusting the shutter speed to mean that for some reason the aperture was being auto adjusted in the lens, and it wasn't. I went back to B&H where I bought the M42 adapter and read the documentation where I noted that it says that the adapter does not maintain auto lens control, and only aperture priority or manual modes should be used, and I had always thought only manual. Until today I've shot only with a full open aperture. I suppose if I want to shoot with a higher aperture number on this lens and still see what I'm doing, i will need to compose, focus, then adjust the aperture as the last step. Let the fun continue, and thanks for clarifying this issue.
07-11-2020, 08:14 PM   #7
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QuoteOriginally posted by BobCPO2 Quote
After investigating this matter once again, I must concede that you are correct. Because I've always used full manual in the past, I confused the fact that the camera was adjusting the shutter speed to mean that for some reason the aperture was being auto adjusted in the lens, and it wasn't. I went back to B&H where I bought the M42 adapter and read the documentation where I noted that it says that the adapter does not maintain auto lens control, and only aperture priority or manual modes should be used, and I had always thought only manual. Until today I've shot only with a full open aperture. I suppose if I want to shoot with a higher aperture number on this lens and still see what I'm doing, i will need to compose, focus, then adjust the aperture as the last step. Let the fun continue, and thanks for clarifying this issue.
You should probably change the earlier post adding an edit to avoid people getting that far and replying.
07-11-2020, 08:15 PM   #8
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The a/m switch lets you focus and stop down by switching it. It is easy to adapt to. Jusy one extra step.

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