Screw mount Pentax adapted to Contax body. Focus?

[Larry Cloetta]

I wasn't sure how to word the thread title to get the right people to look at it, without wasting everybody else's time.
I recently acquired a Pentax SMC 50/1.4 lens and, separately, a Fotodiox m42 to Contax/yashica adapter to use the lens on a Contax ST. ( I know, "why don't you just get a Spotmatic body if you want to use that lens?" Because the ST goes up to 1/4000 shutter speed, which opens up a lot of possibilities for shooting wide open, that the Spotmatic can't handle.)

Anyway, I don't think it focuses accurately set up like this (no, not due to shallow depth of field), trying to decide if it is my paranoia or not.
Here, finally, is my question: to achieve accurate focus with an SLR the lens has to be exactly the same distance from the film plane with the mirror up, as the distance (never mind, not going there); the mount has to be precise with regard to the distance from the lens to the film plane, with the image reflected into your eye accurate as well, i.e. If it looks in focus to you, it has to be in focus on the film plane as well.
The Fotodiox adapter clicks into the Contax body pretty securely, but there is a lot of variability in how far, exactly, the lens screws into the adapter. There is sort of tight, tighter, pretty tight, tighter still, and "I probably should not screw this on any tighter," though obviously I could. Plus, none of these results in the aperture ring being anywhere near it should be. I've had Mamiyas with m42 mounts before and there was a much more definite, precise stop with those.

So, is my "seems to be real" focus error due to the sloppy Fotodiox adapter, probably, and incorrectable? Or, would it not matter? We're talking a quarter turn or more of slop.
Is there a better adapter out there which I just haven't found?
Or should I just get a Pentax screw mount body, live with the 1/1000 shutter speed and a lot of ND filters and be done with it?

I did buy a Spotmatic II off eBay a couple of days ago, but it arrived "not as advertised" to be kind, and unusable. Not really up to the build standards of the Contax, anyway, even if it had been perfect.

 

[Ranchu]

I have a contax 137 MA I use m42 with adapters on with no problems. M42 and the contax mount are the same distance from the film plane so the lens should rest directly on the mount, and stop. Is the M42 aperture pin getting pushed too far in by the inner ring on the adapter being too short, and that is causing the adapter to not screw on the lens all the way? You can put the adapter directly on the lens to check, it should go all the way down to the lens mount surface.

If your screen and mirror are in the right place in relation to the film you should get accurate focus no matter what, because you will just turn the focus to compensate for inaccuracy in the mount/adapter to film distance. The infinity stop and distance scales will not be accurate though, if the lens mount/adapter isn't in the right place.

I have heard that some versions of that lens stick out the back of the lens quite a bit, I have never used it though.

My adapters look like this

http://www.ebay.com/itm/M42-Lens-to...078?pt=LH_DefaultDomain_0&hash=item23131f2326

 

[Kate-the-Great]

If there is an issue with the lens not screwing in far enough and as a result being just a bit too far from the film plane, that could cause consistent poor focus at infinity- close enough to look in-focus in the finder, but not exact enough to hold up under close inspection of the images.


That would not affect the accuracy of focus at other distances though**- how is focus accuracy & sharpness at close & medium distances? Unless your finder is really messed up and out of alignment somehow, what you see through it will be what you get on the film. You have no focusing issues with native C/Y lenses?



**(except with lenses featuring CRC or some examples of internal focusing, but that's of course not applicable to your unit-focusing Takumar)

 

[mfogiel]

You have only discovered a fact known long ago to many: in SLR world you do NOT necessarily get what you see in the VF. Typically, the critical point is the angle at which the rays hit the mirror: The longer the lens, or the more "telecentric" if you prefer, the fewer issues you are likely to encounter.
Beyond this ( optical issues ), there can be other banal problems, like a misalligned mirror or focusing screen.
The best solution is to try various bodies with your lens and shoot focus tests in a controlled way, to discover which couple can be "married" to each other. A shortcut would be to get a Nikon F3 and try various types of screens, as you are likely to find one that works well. The problem with M42 to Nikon adapters is that you don't get the infinity focus.

 

[Larry Cloetta]

I have a contax 137 MA I use m42 with adapters on with no problems. M42 and the contax mount are the same distance from the film plane so the lens should rest directly on the mount, and stop. Is the M42 aperture pin getting pushed too far in by the inner ring on the adapter being too short, and that is causing the adapter to not screw on the lens all the way? You can put the adapter directly on the lens to check, it should go all the way down to the lens mount surface.6[/URL]

Thank you. It seems to be related to the adapter somehow; not exactly in the way you mentioned, but, in checking that, I discovered that the adapter, by itself, bayonets down to the body with no problem, and the adapter, by itself, screws onto the lens to a solid full stop with no problem, but the lens-adapter-body combination can't be made to work; the bayonet will then not snap into place, but binds.
So, screwing the adapter onto the lens is deforming the aluminum adapter enough to keep the bayonet from engaging correctly, as unlikely as that sounds, because nothing else seems to be fouling internally.
I should have tried them, separately, before this. Will try a different adapter. With as many m42 lenses and C/Y bodies as there are, I wonder why neither Novoflex nor Voigtländer makes an adapter.

Thanks to the other respondents as well, I always learn something.

 

[CK Dexter Haven]

I recently went through a bit of a 'photographic ordeal' with a Pentax LX and Pentax 50/1.4. Everything looked fantastic through the viewfinder, but the resulting film revealed significant back focus. I didn't figure out the technical reason, but suspect, as Marek suggested, a mirror misalignment. If you do another test, instead of shooting a flat target for resolution, try a front/back focus chart, shot at a 45-degree angle.

re: a more appropriate camera for that lens —
You were prepared to shoot it in stop-down mode on the Contax, so i might suggest a Pentax ME Super instead of a Spotmatic. The Super has a huge, glorious viewfinder, and goes to 1/2000. The body is even smaller than a Spotmatic, and can be picked up for nuts. I got a black one with a 50/1.4-M for $75. I never would have even looked at one previously, as i thought the shutter speed button set up was ridiculous, but now that i have it as a consolation prize for dealing with two bad LXs, i'm really happy with it. It's tiny, solid, and has a beautiful wind mechanism and soft shutter/mirror operation. And, again, that viewfinder is spectacular. Plus, you can get a genuine Pentax screw mount to K adapter, which should resolve all paranoia.

 

[raid]

In addition to the possibility of a camera mirror shift, there is also the possibility at times that there is an internal lens movement in a lens element. It once happened to my Canon 50/1.2 LTM. It took DAG to figure out and fix this problem.

 

[Larry Cloetta]

but suspect, as Marek suggested, a mirror misalignment.

re: a more appropriate camera for that lens —
You were prepared to shoot it in stop-down mode on the Contax, so i might suggest a Pentax ME Super instead of a Spotmatic.

Thanks for the tips. I think the body is fine as never had a problem with the Contax lenses, but, then, I've never checked it for focus alignment as you suggest. Difference could be I wasn't shooting everything at f1.4 with the Contax lenses, so I'll test the body to be sure.

I just ordered a different adapter, but if none of this pans out, I appreciate your suggestion of the ME, and will give that a try. I love a bright viewfinder, which is one of the reasons I like the Contaxes, but there is something to be said for having a Pentax lens on a Pentax body.
Thanks, again.

 

[CK Dexter Haven]

Thanks for the tips. I think the body is fine as never had a problem with the Contax lenses, but, then, I've never checked it for focus alignment as you suggest. Difference could be I wasn't shooting everything at f1.4 with the Contax lenses, so I'll test the body to be sure.

I just ordered a different adapter, but if none of this pans out, I appreciate your suggestion of the ME, and will give that a try. I love a bright viewfinder, which is one of the reasons I like the Contaxes, but there is something to be said for having a Pentax lens on a Pentax body.
Thanks, again.

Yeah, i'm going to have to start using the back/front test chart on everything going forward. I once had a Fuji GA645i that gave me soft images. I thought it was fine on my first 'walkabout test' roll, but i later realized it wasn't shooting anything at wider apertures.

The Contaxes have great viewfinders, as well. I'm probably going to re-acquire an RX or ST soon. I've had an RX and Aria, and one of the older ones (159?). I wish i could compare the Contax to the ME Super. The Super (and LX) viewfinder stands head and shoulders above everything i currently own (F3/F3-HP, FE2, F100, EOS3), which is kinda shocking. It's big, and clear.

 

[CK Dexter Haven]

In addition to the possibility of a camera mirror shift, there is also the possibility at times that there is an internal lens movement in a lens element. It once happened to my Canon 50/1.2 LTM. It took DAG to figure out and fix this problem.

That's a good point. Not the case in my situation. I had to buy a second Pentax lens to eliminate the first lens as a variable in the problem. But, i once had a lemon Leica 50mm Summilux-ASPH that Leica took two returns and four months to diagnose.