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.  :-)

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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 Discussions


    4 years ago

    @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!


    4 years ago

    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?

    1 reply

    Reply 4 years ago

    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?

    thelandlordNational 2713

    Reply 4 years ago

    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 :-)

    National 2713thelandlord

    Reply 4 years ago

    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.


    6 years ago on Step 6

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


    Reply 7 years ago on Introduction

    Yes you can.

    As long as you haven't super-glued the EOS adaptor as some people do, it's removeable and the K-mount still fits a Pentax body and works just fine.

    Obviously as you've removed the lever you'll have to do DOF preview manually the same as on the digital camera, but you'd do that without thinking once your fingers get used to the idea... I have no problems switching between my Canon digital and back to my Pentax K-1000 35mm camera.


    Reply 8 years ago on Introduction

    Not only relevant, but genuinely useful.

    A 50mm f1.7 is part of my everyday kit alongside the more conventional image-stabilised auto-focus modern lenses.

    The Pentax glass gives me direct control over the depth of field, and it's a lovely 'contrasty' lens with some real punch to the images, great fun to use. :-)

    I'm keeping my eye out for a 28 or 35 version to add to the collection, which by the time the crop factor of my EOS20 is taken into account will be just nice for street photography.


    Reply 9 years ago on Introduction

    K Mount Pentax/Takumar lenses are pre-digital so they are full frame. 

    The only thing that would concern me is the adapter adding some spacing can possibly make you lose infinity focus.


    Reply 9 years ago on Introduction

    That was one reason for me looking to the K-mount lenses rather than, say, older Canon lenses like CFD - those would use a much thicker adaptor making loss of infinity almost inevitable. 
    On this particular combination of adaptor & lens I got away with it.
    Another point worth mentioning is that using a full-frame lense on a camera that uses a smaller sensor is there's a multiplier to take into consideration when looking at the length of the lens - if I remember right the full-frame lense will become about 1.4x longer or something like it , an advantage when you're using telephoto but potentially a nuisance if you're trying to go as wide as possible.


    Reply 9 years ago on Introduction

    There is no difference between full-frame-capable lenses and digital-only lenses other than image circle coverage.  A 50mm lens is a 50mm lens, period.

    However, a 50mm lens (whether a "digital-only" one or a FF-capable one) will give a FOV equivalent to a 75-80mm lens on an APS-C camera.

    The reduced image circle requirements do, however, make it easier to produce wide angle lenses with short focal lengths - 28mm is about the widest you'll see without getting into ridiculous price ranges for FF lenses, while it's common to see 17-18mm digital-only lenses.

    The nice thing about this instructable is that for quite a while, Pentax was ahead of most other manufacturers in optical coating quality.  Classic Pentax glass is generally regarded as VERY high quality.  Fast K/M series prime lenses are dirt cheap and provide excellent image quality.  A-series lenses are far harder to find since they have an aperture linkage that provides full auto-aperture even with modern Pentax DSLRs, however this feature is of no benefit to Canon adapter users.


    Reply 9 years ago on Introduction

    I'm glad one of us knows what they're talking about - I'm still on my voyage of discovery and fumbling my way on the theory... *blushes*


    Reply 9 years ago on Introduction

    That should work just fine as the full-frame Canons have less space behind the lens than the APS-C sized ones... that's why your 450d can use the EF lenses from mine, but I can't use the EF-S lenses from yours.
    The critical thing is the clearance to allow the mirror to move, and you have a lot more of that in Canons that use EF-S lenses.
    When you go shopping for the K-mount to EOS adaptor you can get two sorts, the cheap plain ones that only cost about £5 in Ebay, or the dearer chipped ones (about £12) that will let your autofocus confirm work. I prefer the cheaper ones because I grew up in the days before auto-focus, but your mileage may vary.
    Considering that a cheap F1.8/50mm prime lens costs about £100 new and you can find a better-quality K-mount with the same figures for about £10 if you're lucky there's a lot of money to be saved! Once you start looking at telephoto primes it gets even better, an 85mm portrait lens might cost you over £300 new from Canon but £25 as a good second-hand K-mount... collect a bagful to play with from a yard-sale and you'll keep yourself entertained for hours.  :-)