Instructables

How to modify K-Mount lenses to fit a full-frame Canon camera

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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

Picture of 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.
 
 
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thePurush1 year ago
Excellent work but I removed the aperture control lever alltogether. U can reverse everynthing if you want.151 oftarym
mloewen2 years ago
Does anyone know if you can still use the modified lens back on the pentax body?
thelandlord (author)  mloewen2 years ago
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.
Thanks so much for the quick response! Doing this asap...
Its_izzy3 years ago
That's why I love old Pentax lenses.... Still relevant after all this years... : )
thelandlord (author)  Its_izzy3 years ago
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.
ravvip04 years ago
 Will this mod allow the lens to be used on the APS-C sized canons? ie 450d etc?
ve2vfd ravvip04 years ago
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.
thelandlord (author)  ve2vfd4 years ago
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.
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.
thelandlord (author)  Entropy5124 years ago
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*
thelandlord (author)  ravvip04 years ago
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.  :-)
very nice mod, hope the additional spacing of the adapter plate doesn't cause any vignette.
now you need to find a way to change your focus screen to a split prism ground glass type to really let you focus easily  :)
(I know I know, you can focus easily enough and it'd be serious taking apart to do this, but it'd be cool right?)
Additional spacing usually doesn't cause vignetting, in fact it should result in a slightly larger image circle.  It can, however, cause problems with the ability to focus to infinity, by having the same effect as a macro extension tube.

However, each lens mount system is designed for a different film-plane-to-flange distance.  If I recall correctly, Canon's EF mount has a slightly shorter distance than the Pentax K mount - the end result is that K-mount lenses can be mounted on EF-mount cameras and still reach infinity focus even with a bit of spacing, but EF-mount lenses can never focus to infinity on a K-mount camera even if the adapter does not add any additional spacing, unless the adapter has optics added to it.
thelandlord (author)  lasersage4 years ago
The adaptor plate I used added a shade under 1mm additional spacing and there's no visible vignette.
I could easily be tempted to get hold of a spare camera to try the split-prism mod you've suggested - as you say, unnecessary, but it would be cool.  :-)