[mikechopragant]

Hi

I'm new to the world of DSLRs and have taken the plunge with a K-r, which I am very happy with. I've been playing around with an old CZJ Tessar 50mm F2.8 on it, as well some other older MF lenses. I really like the Tessar but one thing about it perplexes me. I use it on Av, which works fine, but underexposes by about two stops. It's not a problem as I just dial in the appropriate compensation but I was wondering what the explanation is. None of the other lenses I use require this. Is it a known characteristic of the Tessar and why?

[bobpur]

Hi
I have the same lens, excellent glass.
When the lens is painted so the camera contacts are not shorted out, It for some reason
will do that. Does not depend on the lens as it is the firmware that controls it.
I used to use 1.7 ev but now have stripped the contact area on all of my M42 lenses.
Just don't try to sand off the paint as you will get some grit into the lens.
Just remove the base ( 3 screws ) before. At the same time you can block the Auto pin to allow manual aperture settings.
Use very fine water paper, 600 or 1000 grit followed by scotchbrite to polish the surface so
as not to scrape the camera contacts. Not too hard and will work fine.

[mikechopragant]

Hi Bobpur

Thanks for this. Might give this a go when I feel a bit more confident about modifying lenses.

Mike

[RioRico]

Don't sand that lens! There is a simpler, non-destructive way to short the PKA-mount contact pins with an M42 lens. Assuming the lens has a wide enough base, just cut a strip of metallic tape and stick it onto the lens base in the appropriate position. A duct-tape-size roll of metal heating-duct tape only costs $US4-5 and provides enough tape for hundreds of lenses.

Dealing with the auto-aperture pin on an auto-only M42 lens is another problem. I have read that an M42-PK adapter ring with an inside flange to depress that pin is available... for something over US$100. Ouch. With my first such lenses, I superglued the pin down -- but that seems to be irreversible, and I wouldn't do that to any lens I might hope to sell in the future, not to use on an M42-auto body. I have modified a few lenses but I'm not comfortable with disassembling a lens to jigger the auto pin. So my other solution is: don't buy such a lens. Stick with M42 lenses that either have aperture presets, or M/A switches.

About the CZJ Tessar 50/2.8: I have two very different copies. Neither has a serious exposure problem with Av mode on my K20D. One copy is a big black M42 with an M/A switch and 5 iris blades. The other is a small aluminium Exakta-mount with preset aperture and 12 iris blades -- BOKEH MONSTER!! I mount the latter using a simple modified adapter I describe in another thread here. I don't know why some CZJ Tessars cause exposure problems on some cameras but not others. I can't just blame karma...