How to Modify K-Mount Lenses to Fit a Full-frame Canon Camera

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Introduction: How to Modify K-Mount Lenses to Fit a Full-frame Canon Camera

This indestructable shows you how to modify a K-mount lens to clear the mirror on a full-frame Canon EOS camera so you can use a K-mount to EOS adaptor without smashing the mirror as it comes into conflict with the aperture lever of the lens.

I use lenses modified this way on a Canon EOS 5 film SLR, attached in this manner they're fully manual for both aperture and focus as the camera can't talk to them, and it's your own eyes that will be doing the focus confirmation - just as it always used to be.
My camera is perfectly happy with this and will figure out it's own shutter-speed to suit whatever aperture you manually set, or alternatively you can switch to full manual and figure the shutter-speed out for yourself for more control.

The same basic principles will apply to any other type of lens you can get a suitable adapter for, but couldn't normally use on a full-frame camera because of stuff sticking out of the back.

Please be gentle with me - this is my first Indestructable.  :-)
 

Step 1: What You'll Need:

What you'll need:

A K-mount lens

A tiny cross-head screwdriver

A lens-cap or similar container for the tiny screws

  A mouse-mat or similar to stop things sliding around

Some cutters that can cut both metal and hard plastic - I used a pair of pincers

A file

Good eyesight

Nimble fingers
 

Step 2: Dismantling the Lens

Dismantling the lens
Place the lens on the mousemat with the end that normally goes to the camera facing upwards
You'll see a few tiny crosshead screws, remove those and keep them safe - try not to drop them on the floor or you're going to curse trying to find them. Don't ask how I discovered this...
Once the screws are all removed, the plate with the bayonet fitting should be able to lift clear - be careful as some lenses may have spring-loaded parts hiding to poke holes in the eyes of the unwary... mine didn't so all was ok.
 

Step 3: Cutting the Lever

Step 3
Cutting the lever
I cut the lever by nicking the sides as low down as I could reach, then simply twisting it off. It's butchery that's taking place, but as it isn't going to be visible the finish doesn't matter. Be careful that you only twist rather than pulling otherwise you risk dismantling a bit more of the mechanism than you intended!
 

Step 4: Cutting the Guard

The metal plate that has the bayonet fitting on it also has a guard that sticks up to protect the lever while the lens isn't fitted. As we no longer have a lever, we no longer need a guard, and besides, it's in the way... just cut it off and file it flush. 
Make sure you get rid of any loose chips from the filing, you don't want then ending up inside the lens when you reassemble it
 

Step 5: Reassembling the Lens

This is simply the reverse of dismantling it.
On my lens, the hole-spacings for the tiny screws weren't all the same so the plate would only fit in one position.
This is good, it means I'm sure the markings will be at the top where I can see them!
 

Step 6: The Finished Result

The lens you just modified will now be safe to use with one of the many cheap adaptors you can buy on Ebay.

Enjoy bokeh heaven with an easy supply of fast prime lenses at prices that are a fraction of modern auto-focus lenses, and often perform substantially better than a zoom-lens. 

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    23 Comments

    @thelandlord Hey! Great article man. Something I've been wondering about. So below I have three lenses that I want to modify:

    Auto Chinon MC 1:2.8 28mm

    Auto Chinon Zoom MC Macro 35-80mm 1:3.5-4.5

    Auto Chinon Zoom MC Macro 75-200mm 1:4.0-4.8

    I recently bought a Canon 6D 24-105mm (which I'm loving) however I really love my Cinon lenses. I'm trying to get the PK mount but neither Ebay nor Amazon ship to my country. In any case I'll get one. Could these work? I'm really interested in how the picture result would be.

    Thank you!

    Just a question but once the k mount plate is removed could you not mod an EOS adaptor to actually fit as the mount instead?

    You 'could' do, but you'll throw out the distance from the back element to the focal plane, which is one of the things the adaptor is adapting.

    Don't be too surprised if it's possible to focus beyond infinity, or if the closest-focus distance is further away than you expected.
    It may be that neither of those things matter for your application.

    If you prefer your 'neater-but-nicer' method, it should be trivially simple to make a shim to fit behind the EOS mount to push it forward a bit... take your cue from the thickness of the K-mount you removed?

    Can I do this for my Canon 5D mark III ? I have a pentax 50mm 1:2

    Yes you can.

    Because the 5D is a full-frame camera with a full-size mirror *don't forget to trim the back of the Pentax lens flush* so nothing protrudes beyond the back of the adaptor to avoid clashing with the mirror - but as long as you remember that it will work perfectly. Have fun with your 'new' lens :-)

    Awesome thanks! What's the best tool to trim off those pieces of the Pentax lens?

    I was a bit 'agricultural' about it and cropped it as close as I could with a pair of pincers then filed it flush - but a junior hacksaw then file would do too.
    As long as it gets flat and flush and you don't mark the back lens in the process it's really not critical in the least.

    Excellent work but I removed the aperture control lever alltogether. U can reverse everynthing if you want.151 oftarym

    Does anyone know if you can still use the modified lens back on the pentax body?