Focus adjustment: Loupe/matte screen question

[elollo]

Hi

When adjusting a scale focus camera, I´m using a loupe and a home made matte screen with matte side facing the lens.
Does the thickness of the pastic screen come in to play (aprox 1-1.5mm) when the loupe is made to have focus on its "bottom surface"?
I´m guessing this is out of the tolerances that both I and the camera can achieve.

Olle

 

[loccdor]

A standard photographic loupe can handle this small variation in focus point.

The technical answer would be that it's not only up to the design of the loupe (its diopter strength and its length) but also the nearsighted or farsightedness of your eye, the effect of both of those combined.

 

[Ian C]

The standard tool for this job is a thin rectangle of flat sheet glass that has one surface lightly textured so that the image projected by the lens will form on the textured surface. The glass screen is sized so that its image-forming surface is in contact with the film registration rails. This duplicates the relationship between the front surface of the film and rails.

Such a screen is relatively simple to make. Thin window glass is sufficient. The thickness of the glass screen is irrelevant.

 

[elollo]

Thanks for the quick reply, fellas!

 

[reddesert]

In principle, the image is formed on the matte screen and your eye is focusing on that image, with the loupe as an aid. As long as you can bring your eye to focus on the matte screen, the exact focal point of the loupe is not material.

Many typical loupes have a focus point well beyond the base of the loupe (for example, if you have a 5x loupe with a focal length of 2", but the loupe is only 1-1.5" tall). This is desirable, because the focus range of your eye can then bring the object at the base of the loupe into focus.

Sometimes, people report focus problems that seem to happen if the screen is not-matte-enough and they actually focus through it onto the aerial image.

 

[elollo]

Thanks again everyone. I made sceen from glass and it worked well!