setting infinity using a second camera

[cluso]

Hi,
I've been trying to calibrate the infinity setting on my Olympus 35 RC and SP cameras. I've been using the method where a second camera (SLR), carefully set to infinity, is used to measure through the lens of the first camera. The method is for example explained on the web page of Michael Feuerbacher:
http://feuerbacher.net/photo/frame.html?repair/InfinityFocus/InfinityFocus.html~Main

The idea is to make a scratch or draw a line with an ink pen on a part of film and then use these lines as a reference. However, I find that there is quite a bit of a difference in the setting you get when using the line or the scratch. The line is on top of the film, the scratch lies withing the emulsion of the film, so one ends up with different settings. When I change the focus, I even find that different parts of the scratch come in focus as I tune the focal point through the film plane. So the method is quite sensitive.

I am wondering if the best method wouldn't be to use a photograph (best with straight lines) from an exposed film as a reference and then calibrate to the straight lines in the motiv. This should be the most exact reference.

Just wondering if this is correct or not...

Thanks,
cluso

 

[VictorM.]

I use a small piece of glass with a piece of frosted clear tape. The tape has a thin line cut into it using a very sharp Exacto knife. The tape faces the lens mount. I have also used an old Nikon F focusing screen.

 

[Al Kaplan]

Not many lenses, other than process lenses used in the printing industry, have anything aproaching a flat field. SLR lenses get focused in the center because the cameras are set up that way. The rest of the image is a wee little bit out of focus, getting worse as you get to the corners. Rangefinder lenses are usually designed with the rangefinder cam set to give a focus out a bit from the center so neither the corners nor the center is in perfect focus, but the focus is more even across the field. Now go take pictures and stop worrying about it!

 

[payasam]

As I have no film SLRs any more, I look up to the moon. Corrected the infinity setting on my M2 recently, which led (as I had hoped) to a distinct improvement in near focus.

 

[ferider]

I use a ground glass screen with pen marks on it.

It is important that the reference lens on the SLR has flat field and is known to be well calibrated. I typically use an OM Zuiko 50mm macro lens.

Roland.