Warning! OT digital mystery!

[Joakim Ahnfelt]

This may not be the right forum for this mystery but I'm an LF photographer and the LF forum is where I hang out on the internet. Besides I'm in awe of the knowledge level here. So here goes.
On the internet I found an adapter to be able to use Zeiss lenses from my Contax RTS system on a Canon digital SLR. I purchased it and a second hand Canon 400D. My first autofocus as well as my first digital SLR.
It did not focus on infinity nor where I focused. Since it was very cheap I figured that it was crap and I bought another, much more expensive one, with electronics in it, making the focus indicator dots (or whatever the're called) in the viewfinder work. It does not focus on infinity either. This I can understand, just wrong thickness, like an extension tube.
I did the ruler test focusing manually and the focus plane is 30 mm closer to the camera than the point where I focus with a 50 mm lens wide open. (The same result with both the cheap and the expensive adapter.) If I use the focus indicator dots the focus is spot on.
Figuring somethings wrong with the mirror I took it to the Canon Service Center and had them test it. No problem with the either the mirror or the autofocusing stuff inside. The guy I talked to said that the mirror is not a part of the autofocus system but could not explain why the mirror was so ”off”.

And here's the mystery. We then put the camera on a tripod and shot a brightly lit candy wrapper about a meter away. With my Zeiss 85 mm 1.4 and their Canon 85 mm 1.2 lens stopped down to 1.4. Both lenses where focused manually. Result: The bloody Canon lens is spot on every time while my zeiss miss the spot where I focused!? How can this be???

If some one can explain this to me before I'll pull out what little hair I have left I'll be your humble servant for ever.

Cheers
Joakim

[Gary Beasley]

Have you checked to be sure the adaptor is not interfering with the mirror? If it does not seat completely the focus will be off.

[Daniel_Buck]

hm... I'm not quite sure. You are saying that what you see in the viewfinder appears to be in focus with both the canon and the contax, but the final image from the contax isn't focused where it looked like through the finder? Are you focusing by eye? or by the scale? The scale might not be accurate with the adapter.

I use Contax Zeiss lenses on my 1Ds2 with no troubles at all. I use 'dumb' adapters, just a metal adapter with no electronics.

[Joakim Ahnfelt]

Gary, interesting idea, no idea how to test it.

Daniel, I focus by eye. Do your adapters focus on infinity?

Perhaps I should add that I have the same problem with all three lenses. 28, 50 and 85 mm.

[Daniel_Buck]

my adapters go right close to infinity on most of my lenses. (the lenses 'infinity stop' can be adjusted if you really want to open them up, from what I understand) My 50mm 1.7 doesn't go quite to infinity, but it works when stopped down. my 28/2.8, 45/2.8 pancake, and 60/2.8 macro go to infinity, as well as my Nikkor 18/3.5. For my zeiss lenses, I have an adapter for each lens, swapping them around the 50 still doesn't quite reach infinity when wide open, so it's most likely the lens itself. If you have one adapter for all of your lenses, and none of them hit infinity, it's probably the adapter.

[Joakim Ahnfelt]

I've had a serious peek inside but I can't see anything coming even close to touching the mirror, sorry Gary.

[AJSJones]

http://www.fredmiranda.com/forum/board/55

There is a lot of experience with alternative lenses and various adapters there. Browse/search or post your question as above (identifying the source of the adapter will help)

Good luck!

[Joakim Ahnfelt]

Thanks, I'll have a look around and see what I can find.