Checking the focal plane

What ho one and all,

I have been watching too many videos on Vintage Resurrections site (https://www.youtube.com/@VintageResurrection) Amazing!

But I cannot understand one aspect.. Towards the end, he tapes a glass screen to the film plane, lit from behind and has his digital camera, lens to lens, to check the sharpness on the film plane. How does this work?

A good example is this Nikomat FTN restoration (https://www.google.com/search?clien...ate=ive&vld=cid:a76f3399,vid:c6X6vrUtSKY,st:0) at around the 1:21:50 mark.

What exactly is happening?

Thanks and toodle pip

Try this:

https://www.addicted2light.com/2013...sing-a-camera-as-a-collimator/?v=7885444af42e

Many thanks and interesting. I have a Nikon D300 but don't understand how, with the lens at infinity, it would focus on the film plane? The focus of the camera / lens to test is set to infinity, why would my Nikon focus so close to the film plan to show on the rear display?

The viewing lens will correctly focus subjects at infinity. If the pattern on the film rails is at the infinity focal plane of the taking lens, it will be projected by the taking lens as if it were at infinity (you'll need some backlighting). Thus it will appear sharp through the viewing camera. Conversely, if the pattern is anywhere else vs. the taking lens, it will appear blurry on the viewing system.

Also search for "autocollimators".

Rekusu said:

But I cannot understand one aspect.. Towards the end, he tapes a glass screen to the film plane, lit from behind and has his digital camera, lens to lens, to check the sharpness on the film plane. How does this work?

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Rekusu said:

What exactly is happening?

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Hi, the link from monopix tells roughly how to perform this test. And of course OAPOli is right.

But in order to understand the mechanism you might need a more detailed explanation, which I'll try to do. (I didn't watch the video too carefully, but will explain what I THINK he was doing.)

First, let me say that light doesn't really "care" which direction it's going through a lens - rays of light will follow the same path whether they are entering the front of the lens (going to the film plane) or starting from the film plane and exiting the front of the lens.

So... if you have a camera/lens properly set up for "infinity focus" on the film plane, this means that (nearly) parallel light rays from a distant "object point" (roughly something like a star, for example) will all be focused to a hypothetical "point" on the film plane. Right?

And... since light behaves the same regardless of which direction it's going, it should be possible to have such a "point source" of light to originate AT THE FILM PLANE and thus be projected out the front of the lens in a series of parallel rays. Right? So if one can look into the front of the lens, either by eye or by some sort of instrument, said illuminated "point of light" at the film plane will appear to be coming from an infinite distance. (This is ASSUMING that the camera/lens assembly is properly set up for "infinity focus.")

Now, given that this camera should have the illuminated target appear to be "at infinity" we should be able to use another camera, one that is properly set to "infinity focus," to look into the lens of the first camera and verify that the illuminated target is actually focused properly. Fwiw success at this requires that the second camera is known to have proper focus.

Many thanks. In words that five year old can understand, if the repaired camera is correctly set with shims on the lens panel so that infinity is at the film pone, then the digital camera, also at infinity, should show a sharp image on the rear screen?

And if the image is not sharp, then shims need to be added or removed?

I kinda get it but will give it a try to convince myself that it works. Of course, easier said than done since some of the bodies I bought on fleaBay to play at being a camera repairman, did not have lenses. Guess I will have to haul out my Nikon F.

Rekusu said:

Many thanks. In words that five year old can understand, if the repaired camera is correctly set with shims on the lens panel so that infinity is at the film pone, then the digital camera, also at infinity, should show a sharp image on the rear screen?

And if the image is not sharp, then shims need to be added or removed?

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Not really. One needs to check the flange focal distance with a depth micrometer (or a dial indicator stand) and adjust it with shims to get to a set tolerance. Then the lens is put on and is checked with an autocollimator or relay camera to make sure it reaches infinity at its stop. You can also check the focus screen through the collimator.

Rekusu said:

... if the repaired camera is correctly set with shims on the lens panel so that infinity is at the film pone, then the digital camera, also at infinity, should show a sharp image on the rear screen?

Click to expand...

Yes.

Rekusu said:

And if the image is not sharp, then shims need to be added or removed?

Click to expand...

Yes. And again, this is on the assumption that the digital camera also has an accurate infinity focus.

Rekusu said:

And if the image is not sharp, then shims need to be added or removed?

Click to expand...

I have a bit of time, so let me try to elaborate a bit more; it's not quite that simple.

In the couple dozen different slr type cameras that I've dug into a bit, it seems to be a standard thing to have several shims behind both the camera's lens mount flange AND on the lens' mounting flange. So if it is necessary to change shims, there is a question as to WHICH shims to change... those in the camera body or those in the lens?

I concur with OAPOli's methodology. That is, using mechanical measuring tools to measure distance, on the camera body, from the image plane to the lens mount flange. There are published specs on what this distance should be. If this distance is correct, within tolerances, then any shim errors are most likely in the lens.