How check infinity focus on adapted lenses?

[Hening Bettermann]

Hi!

I am trying some Contax primes on my new 5D2 via the CameraQuest adapter and wonder,
how to check if they really can focus at infinity? Since the focus ring can not be turned *beyond* infinity (which nobody has promised), how do you know if the focussing is at its optimum, or the lens is at its limits? At least with the Distagon 35, I feel not convinced that it really focusses at barely 50 meters distance. (Everything further than that is in a mist right now).

It also turnes out that there is a gross focus shift when stopping down. So that the lens can be focussed to infinity at f/2.8 does not guarantee that it will at f/22.

I used live view with the 10x enlargement of course, and viewed the monitor with a pair of +4 spectacles, on top of my +3 myopia.

The lenses in question are the Distagon 35, Sonnar 85 and Sonnar 135. Only with the latter I feel reasonably sure that i really can be focussed to infinity. Swapping adapters has not changed the picture.

Would anybody know of a procedure to clarify this question?

[rockrose]

Would anybody know of a procedure to clarify this question?

I don't have the answer, but I enjoy using old lenses on DLSR bodies, like on my new 5D mkII, maybe I can help thinking.
I presume it are bayonet lenses, so my method for m42 lenses won't work: sanding the adapter a little thinner. The only way to know if your lens can have better infinity-focus, would involve getting it a little closer to the sensor than the adapter allows. It will be difficult, but if I wanted to know, I would try this: set your camera on a tripod and point the lens at a good distant subject (like buildings) on a clear day. Memorize the 10x enlargement at infinity. Remove the adapter and hold the lens close to the camera mount (preferably with some homemade tube to avoid light leaking between lens and camera). While looking at the 10x screen move the lens close to the camera back and forth and look if you can see more detail than with the adapter. With some effort I think that can be done. Then you know if the lens is capable of better infinity focus. That won't help you getting it unless you can find a thinner adapter, or mounting your sensor a couple of millimeters forward ;-).

[marcmccalmont]

Just a thought? How about focusing at night on stars and see if the circle can become a point? Just a thought?
Marc

[Hening Bettermann]

Thank you for your suggestions! I'll try both when the mist has cleared...

[Hening Bettermann]

Now I had a little look at the Evening  Star. The variety with the adapter removed an the lens handhold turned out to be difficult. - According to my previous trials, the most critical lens was the shortest, and the most critical f-stop was the smallest. So using the Distagon 35 and f/22, I focussed until the star did not become smaller. From there, I could turn the focussing ring another say 10 degrees or so. - I tried to take shots, but can see nothing on them, probably due to exposure. Anyway, I feel now reasonably safe that the adapted lenses can be focussed to infinity.

Thanks again for your suggestions!