checking a readyload holder for registration

[Bruce Watson]

I've got a new Kodak Readyload holder - lastest revision "H." The end that you insert into the camera first, facing the lens, is visibly bowed. Kodak tells me that the holder is within tolerance. I am doubtful.

Clearly the way to find out is to expose some film. What I want to do is prove or disprove that when all four corners are sharp on the GG, that all four corners are sharp on film exposed using the holder. I'm going to do the same with a known good conventional two sided film holder for comparison.

Question is, how do I get the smallest possible depth of focus? Not depth of field (front of lens) but depth of focus (film plane).

I was going to pull down close on a nice flat brick wall with a 150mm Sironar-S. But something is tickling my brain... When do you get the narrowest depth of focus -- when focused at infinity, or when focused closer?

I am planning to shoot it wide open to minimize depth of field also.

Any thoughts, suggestions, comments???

[James E Galvin]

If you focus close, the cone of light between lens and film is stretched as you extend the bellows, making a smaller angle at the film, increasing the depth of focus. You want the fastest lens you have, and a distant subject, distant enough that you don't extend the bellows much. Say at least 10 times the lens focal length.

[Bruce Watson]

If you focus close, the cone of light between lens and film is stretched as you extend the bellows, making a smaller angle at the film, increasing the depth of focus. You want the fastest lens you have, and a distant subject, distant enough that you don't extend the bellows much. Say at least 10 times the lens focal length.

Yes, I thought that was the case.

Now, will I get less depth of focus with a shorter or a longer lens?

[Eric Jones]

Use the shortest lens you have to cover the 4x5 frame and focus at or near infinity. Depth of Focus is most "shallow" and critical with wide angle lenses.

[James E Galvin]

I don't agree with Eric. The angle of the cone of light at the film (if the subject is distant) is only a function of the f-stop. See the DOF discussion at http://www.largeformatphotography.info/
I think the depth of focus is not related to focal length, only f-stop.

[Michael Mutmansky]

Bruce,

I don't think that shooting will give you a definative answer unless the holder is really bad.

Since there are ANSI specifications for 4x5 holders (including tolerances), it is a simple matter to verify whether the holder is within the ANSI specifications. If not, send it back.

Testing with a lens is fine, but you introduce the variables of camera setup, lens condition and it's performance variables, and most importantly, human error into the test, all of which can obfuscate the results a bit in terms of establishing the source of a problem.


---Michael

[James E Galvin]

On second thought, the focal length doesn't matter at the center of the field, but at the edge the cone of light is tilted, making the depth of focus smaller. And a wide angle lens tilts the cone more. So Eric is right.
The best way is with a depth micrometer, or a microscope with a large precision stage. But require some setup.

[Brian Ellis]

The shorter the lens the narrower the depth of focus. Use the shortest lens you have.