Focus Testing

[jwinst]

I seem to remember seeing a mention of a technique some time ago involving a focusing screen from an slr. The way i seem to make sense of it is to place the screen as near to exactly on the film plane and focus accordingly using the screen as a check. Is that all there is to it? How do you get it accurately on the film plane, or do you just fiddle about with it till you get as close to a focused image as you can? I would be using the standard screen from a nikon f3hp if that matters.

And if it is off, can you tell if its the rf or the lens with this technique?

Thanks...

 

[Kim Coxon]

I think you may be a little confused. The use of a TV is usually to check the accuracy of the shutter speeds not the focus. Are you trying to check the focus of accuracy of the camera or a lens?

Kim

 

[awilder]

Try this: http://rick_oleson.tripod.com/index-123.html

 

[Xmas]

The lens can be out, (the lens focal length or the register or the cam)
the body can be out, register
the rangefinder can be out

all these unknowns makes life difficult.

You really need a known good wide aperature lens if you suspect the body register, leave lens at infinity on a star or street light several miles away.
A SLR sceeen is plastic and may be too small depending on screen, it needs to register on the inner rails like film, 3m magic tape (matt tape) may be good enough, but needs a high magnification loope to say you have a sharp focus point.

If you get the star sharp, then you can confirm the rangefinder image is coincident as well, or adjust it to be alinged horiiziotally and vertically.
Then you put on your own suspect lens and check it is sharp and the rangefinder still coincident.

Then you try with a focus test targets at 1m and 10m, to confirm that the image and rangefinder are both still good. If no it is the known good lens again.

This may take a lot of time if you have a problem.

If you have lens or register problems you need real expertise to fix, some ragefinders are relatively easy to adjust, some pigs.

If you have a Noctilux Leitz recommend that the body is sent back for a register, if you have a f3.5 or only use a 35mm for landscapes I'd worry less.

Noel

 

[jwinst]

Hmm, might be a bit over my head... How deep a hole in my pocket would a calibration/ cla for an M6 non-ttl and a CV 40/1.4 make?

Thanks though, and Kim, i think you read wrong when I said "screen." I was referring to the focusing screen, not a monitor or tv screen, although I have heard of that test before.

Thanks again.

 

[Kim Coxon]

Sorry, in a rush. I did indeed read it as a TV screen. I managed to get a batch of old Pentax screens which are perfect for such work. They are not the soft plastic of the more modern ones and are a bit larger so they fir on the film rails perfectly and cover the whole area.

Kim

Hmm, might be a bit over my head... How deep a hole in my pocket would a calibration/ cla for an M6 non-ttl and a CV 40/1.4 make?

Thanks though, and Kim, i think you read wrong when I said "screen." I was referring to the focusing screen, not a monitor or tv screen, although I have heard of that test before.

Thanks again.