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03-09-2020, 10:15 AM   #1
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Pentax 50mm 1.4 SMC K Mount - Removing the Aperture Pin for full frame?

Hi Everyone,

I've used pentaxforums for a few years to get lens info, but I've never posted. Here's my problem:

I own a BlackMagic Pocket Cinema Camera 4k with a Metabones .64 MFT to Canon EOS EF speedbooster. I've recently run into a problem when trying to use it with a Pentax 50mm F/1.4 SMC K-Mount lens.
I bought a cheap glassless K mount to Canon EF mount adapter to mount this lens on my Metabones speedbooster, but the aperture pin (what sticks out next to the bayonet mount for Pentax SLR cameras to control the aperture) scrapes against the inside of my Metabones mount. It doesn't press against the glass itself, but rather a layer of felt that surrounds the glass on the inside of the speedbooster. I later discovered online that you cannot mount this lens inside full frame mounts because of this aperture pin that sticks out.

Has anyone run into this problem and ended up removing this part that sticks out by essentially sawing it off? I don't own a camera that would need to use this part of the lens; I bought it specifically for my BlackMagic. I'm going into the vintage camera store that I bought it from to ask them for some solutions. I may ask them to remove this part for me or possibly pay them to exchange it for an M42 version of the lens.

I could just use the Pentax K to Nikon F glass adapter, then a Nikon F to Canon EF adapter, but the glass adapter softens the image especially when it's wide open, rendering it useless because I bought this lens for its sharpness at F/1.4.

Also, does anyone know if the M42 version of this lens has the same problem if I buy an M42 to Canon EF adapter? I really want to use this lens for filming but I'd hate to damage it unless it's my only option.

Thanks guys.

03-09-2020, 10:50 AM   #2
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Welcome to the forums

I've moved your post to the non-Pentax section, since although your question is regarding a specific lens, this is really about modifying lenses to work on non-Pentax bodies.

I'm sure you'll get some helpful responses, but brace yourself for a little push-back on hack-sawing a 50/1.4 beyond repair
03-09-2020, 10:53 AM - 1 Like   #3
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QuoteOriginally posted by BigMackCam Quote
Welcome to the forums

I've moved your post to the non-Pentax section, since although your question is regarding a specific lens, this is really about modifying lenses to work on non-Pentax bodies.

I'm sure you'll get some helpful responses, but brace yourself for a little push-back on hack-sawing a 50/1.4 beyond repair
Thanks Mike! I'd hate to damage this beauty so I'll understand if I'm criticized for considering that as an option. In the future I'll use this non-Pentax section for any other questions in the same topic.
03-09-2020, 11:10 AM - 1 Like   #4
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I only know enough to say some M42 Taks don't need the pin cut based on posts I've seen of a Canon 5D with an unmodified 50mm Tak according to the poster. It's your property. But removing the lens from the pool of available K-mount lenses for future generations would weigh heavy on my mind.

03-09-2020, 12:50 PM - 1 Like   #5
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QuoteOriginally posted by jhickson Quote
I bought a cheap glassless K mount to Canon EF mount adapter to mount this lens on my Metabones speedbooster, but the aperture pin (what sticks out next to the bayonet mount for Pentax SLR cameras to control the aperture) scrapes against the inside of my Metabones mount. It doesn't press against the glass itself, but rather a layer of felt that surrounds the glass on the inside of the speedbooster. I later discovered online that you cannot mount this lens inside full frame mounts because of this aperture pin that sticks out.
K-mount lenses don't have an aperture pin. There is an actuation lever, however ("I" in the image below).



Is that what is causing your problem? If so, your options are to saw the lever off (ruins resale value) or not pair the lens with the speedbooster. As noted above, you might have better luck adapting M42 lenses, but even then, the actuator pin with that mount may be a problem.

BTW...here is my push back regarding the amputation option...


Steve

Last edited by stevebrot; 03-09-2020 at 12:57 PM.
03-09-2020, 01:12 PM   #6
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I think Canon sold a few 50mm f1.4 lenses in EF mount. Maybe try one of those?
03-09-2020, 07:23 PM - 1 Like   #7
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QuoteOriginally posted by tuco Quote
I only know enough to say some M42 Taks don't need the pin cut based on posts I've seen of a Canon 5D with an unmodified 50mm Tak according to the poster. It's your property. But removing the lens from the pool of available K-mount lenses for future generations would weigh heavy on my mind.
Hi Tuco! No need to worry, I went to Glass Key Photo in San Francisco, CA and they were happy to exchange the lens for a similar M42 mount Pentax 50mm 1.4 for free. I'm sure that the K mount will go to a owner that can properly use it, and I won't have to modify my new M42 lens!
03-09-2020, 07:40 PM   #8
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Sometimes the lenses were built so the lever is removable in a less destructive way than cutting. If you can get the 5 mount screws off (a JIS screwdriver is very useful for this) the mechanism is right there. Pentax designs usually have a part which connects the aperture ring movement, the actuation lever and the blades together. It's worth exploring for a more elegant solution which may not exist.

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