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05-05-2010, 12:54 AM   #1
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K mount measurements

Hey!

I'm doing a little experiment where I have mercilessly slaughtered an old, cruddy 35mm point 'n' shoot camera and salvaged its lens to adapt to K-mount (as a fixed focus, "toy camera" type lens) and I need to know (without trial-and-error) how far into the body (measured from the flange) the lens can protrude before colliding with the mirror.

Also, any other measurements would be appreciated -- I recall that the K mount used to be an open standard in the 70's, before Pentax turned it into the KA mount, so is there any official online documentation of the specs?

05-05-2010, 03:30 AM   #2
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have you tried bojidar dimitrov's k-mount site?
05-05-2010, 03:53 AM   #3
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QuoteOriginally posted by kytra Quote
have you tried bojidar dimitrov's k-mount site?
Yeah. There is some technical info on how it works, but no measurements that I can find. Thanks, though!
05-05-2010, 04:02 AM   #4
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i guess you need some blueprints of the k-mount - source could be Pentax or pentax repairers

05-05-2010, 07:04 AM   #5
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aside from figuring out if the mirror will hit the lens (a real possibility) the most important thing to kow is the system regestry

this is the distance from the lens mount to the film/sensor plane

for pentax the regestry distance is 45.46mm.

It is actually one of the longer regestry distances, which is why so few other brands are easily adaptable to pentax bodies.
05-05-2010, 07:18 AM   #6
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QuoteOriginally posted by Lowell Goudge Quote
aside from figuring out if the mirror will hit the lens (a real possibility) the most important thing to kow is the system regestry

this is the distance from the lens mount to the film/sensor plane

for pentax the regestry distance is 45.46mm.

It is actually one of the longer regestry distances, which is why so few other brands are easily adaptable to pentax bodies.
Yes, thanks. I'm aware of this, and naturally all 35 mm point and shoots will have a register distance far shorter than this, which means I will probably be making a macro/close-focus lens -- which is fine. This is just a fun experiment, but I want to know how far back I can get the focus point without breaking the mirror or anything stupid like that.
05-05-2010, 10:05 AM   #7
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Clearance

QuoteOriginally posted by Erik Quote
...I need to know (without trial-and-error) how far into the body (measured from the flange) the lens can protrude before colliding with the mirror.
I measure .36 inches free clearance for the PK mount on both an M-body and a K200D.

Quick an' dirty -- rest the edge of a piece of paper on the square frame inside the mount and activate the shutter. The edge of the paper will not offer significant interference as the mirror swings.

H2
05-05-2010, 04:15 PM   #8
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Could you measure it with the mirror locked up in the sensor clean position ?

05-05-2010, 11:28 PM   #9
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QuoteOriginally posted by pacerr Quote
I measure .36 inches free clearance for the PK mount on both an M-body and a K200D.

Quick an' dirty -- rest the edge of a piece of paper on the square frame inside the mount and activate the shutter. The edge of the paper will not offer significant interference as the mirror swings.

H2
Thanks! So that makes about 9 mm. Currently I'm 6 mm into the body and the lens focuses to about 1:3 magnification ratio, I don't really think 3 mm further would set the focus point much farther back, but I'm not sure how to do the math...?
05-06-2010, 02:29 PM   #10
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Erik, my first thought is to use a body cap - not the white translucent one, of course, but the black ones. You could drill a mounting hole for the lens and put it on that way.
05-06-2010, 02:50 PM   #11
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QuoteOriginally posted by Erik Quote
Thanks! So that makes about 9 mm. Currently I'm 6 mm into the body and the lens focuses to about 1:3 magnification ratio, I don't really think 3 mm further would set the focus point much farther back, but I'm not sure how to do the math...?
I can't help you much with the math. The calculations would be the same as those for an extension tube, wouldn't they? Going 3mm more would certainly help with getting closer to infinity focus. The flanged adapters move a lens out less than 1mm and significantly alter focus on the far end.
05-06-2010, 04:26 PM   #12
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If you are going into/behind the mount surface, be very, very careful. The mirror is quite fragile, and you might be a tad disappointed if you cracked it.

The math is fairly simple. A 50 mm lens at 50mm from the focus plane (sensor) = infinity focus. If you know the focal length of the P&S lens, that's how close to the sensor you must be to focus to infinity.
05-08-2010, 05:25 AM   #13
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The math is:

Magnification_change=(image_distance_change)/(focal_length)

I believe this is valid for complex as well as simple lenses.

Dave
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