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03-06-2021, 02:29 PM   #1
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Non Pentax Lens with Adapter on Pentax P3 35mm Camera


I recently bought an excellent vintage Vivitar Komine 55mm 2.8 macro in a Canon FD mount to use on my son's Rebel T7i when scanning 35mm film negatives. I recently had the shutter repaired on my Pentax P3 and have been shooting black and white and Portra 400 film. I would like to buy an adapter to use the Vivitar FD mount lens on the KA mount Pentax. I have seen a few adapters online, but they all say they are K adapters (not KA) and in researching the mounts I see that the KA mount allows the camera to know the aperture used (the viewfinder only shows the shutter speed). The Vivitar lens has an aperture ring, so my main question is if I use a Canon FD to K-mount adapter, will the camera know what aperture the lens is set to? I use an external light meter, so I am assuming that in manual mode I will still be able to set the shutter speed correctly even with the adapter. Just wanted to make sure so that I am not throwing my money away on something that will not let me set the proper exposure. Also, please let me know if any of you have had success with Canon FD lenses (Canon or other brands) on K mount film cameras. Thank you in advance for any help or advice.


03-06-2021, 02:58 PM   #2
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QuoteOriginally posted by Steven Stahl Quote
I would like to buy an adapter to use the Vivitar FD mount lens on the KA mount Pentax.
Welcome to the Pentax Forums!

Short answer #1 is that the lens flange distance for FD mount is shorter than that for K-mount, meaning that you would lose infinity focus unless an auxiliary glass element is part of the adapter (reduces optical performance). However, a glass-less version would extend the macro magnification.

Short answer #2 is that there are no adapters I am aware of that allow other than full manual aperture (always stopped down) with FD lenses on K-mount.

03-06-2021, 06:52 PM   #3
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Thank you. The adapter I purchased says that it has a glass element for infinity focus.
03-06-2021, 07:31 PM - 1 Like   #4
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There used to be a Chinese canon FD to PK adaptor, with an element for correcting the registration distance, it turns out to effectively be a 1.45 x teleconverter. I tried one just to see, I used it on a canon 50mm f1.8. It is really soft wide open, but acceptable stopped down

03-06-2021, 11:36 PM - 1 Like   #5
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Since it is a macro lens and if used primarily for macro work you may not even care about infinity focus. You can often find glass-less adapters. Fotodiox has some where the glass lens can be removed. You still won't have aperture linkages but glass-less adapters (essentially a short extension tube) are a possibility.
03-07-2021, 01:42 PM - 1 Like   #6
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I got a Vivitar 55/2.8 macro in FD mount for free and converted it to K mount myself. I ended up with infinity focus being about 4 feet. (The conversion is not practical - way too much work to make a lens that's hard to use.) An adapter without glass elements would be worse than that because the adapter would be thicker.
03-07-2021, 01:47 PM - 1 Like   #7
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I used a FD 50/1.4 on a K mount adapter with the corrective element removed, iirc infinity was just a few inches.
03-08-2021, 05:00 AM - 1 Like   #8
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One thing we seemed to have overlooked in the discussion is the OPs question about how to inform the camera the aperture the lens is set to.

This is not the function of the lens or adaptor, but the camera, P-KA mounts tell the camera the aperture range of the lens, the body controls the actual aperture. The FD to PK adaptors are a mechanical mount only, they do not offer aperture control of the lens.

For all adapted lenses on Pentax cameras you need to work in manual mode with the function “aperture ring enabled” in the menus.

I missed this in the initial response


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