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02-09-2009, 10:15 AM   #1
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How much focus would I loose mounting a m39 on my pentax?

How much focus would I loose mounting a m39 on my pentax?

Would it be useless to anything other than portraits / Macro / interior photography?

Thinking about the helios-40, cant find a m42 version out there.

02-09-2009, 11:54 AM   #2
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Depends on what adaptor(s) you use. My m39 to m42 adaptor has a flange (it sticks out from an m42 body) but I use it on a bellows lens so it doesn't really matter. I've read about m39 adaptors that will mount flush inside one of the genuine Pentax m42 adaptors but I don't know where you might be able to get one.
02-09-2009, 12:52 PM   #3
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QuoteOriginally posted by melander Quote
How much focus would I loose mounting a m39 on my pentax?

Would it be useless to anything other than portraits / Macro / interior photography?

Thinking about the helios-40, cant find a m42 version out there.
EDIT: discussion below is for m39/Leica rangefinder screw mount -- as Quicksand notes below, not applicable to Zenit SLR m39 screw mount.

I've never tried it, but i'd expect it to be useless for anything but macro/close focus. The flange focal distance for m39 (Leica Screw Mount) is 28.8mm, for m42 is 45.5mm, so putting a m39 lens on an m42 body is like using a 16.7mm extension tube.

Thin lens forumula says

1/object distance + 1/focal distance = 1/focal length

Suppose you mount a 50mm m39 lens focused at infinity. You've increased the focal distance from 50mm to 66.7mm; so you get

1/max object distance = 1/focal length - 1/focal distance
= 1/50-1/66.7

which works out to

max object distance = 200mm (somebody please check my math).

With wider lenses you're even more hosed.

Last edited by troyz; 02-09-2009 at 05:46 PM.
02-09-2009, 12:56 PM   #4
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troyz, you're right. I tested my M39 50mm lens from Zorky rangefinder on my K100D and it is useless.

02-09-2009, 05:30 PM   #5
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But there is the Zenit version of M39 (for SLRs, not rangefinders) which has a register distance comparable to (if not exactly equal to) Pentax.

It's confusing, and there's a lot of contradictory information out there.

Take a peek at this thread:

Manual Focus Lenses :: View topic - Camera mounts and register distance
02-09-2009, 05:57 PM   #6
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QuoteOriginally posted by Quicksand Quote
But there is the Zenit version of M39 (for SLRs, not rangefinders) which has a register distance comparable to (if not exactly equal to) Pentax.
Thank you for pointing this out. I've edited my previous post.
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