Ground glass vs. roll film discrepancy

[Mark J]

I had at least one of those, many times !

 

[r_a_feldman]

Older folding cameras suffer from greater problems in this way. I ran foul of this when using an Ensign Autorange 6 x 9 cm many years ago. The rollers and film gate are not as sophisiticated as modern MF cameras, and shots at f/5.6 and 8 would be a bit hit & miss. The worst thing to do was wind on the camera after a shot was taken, then close it up, come back later and open and deploy the camera. Something about opening the thing up probably caused suction and pulled the film further away from the platten. You had to have discipline to only wind on just before you were going to fire the shutter, then the film would be tight.

The Olympus Chrome Six III (circa 1952) had a spring-loaded tensioning mechanism in an effort to keep the film flat. What I have read is that Olympus was concerned that film bulging would defeat the quality of their lens at f2.8 (wide open), so they did research and developed the tensioning mechanism.

BTW, does anyone have any first-hand experience with the Chrome Six III?

 

[DREW WILEY]

Some older roll film mechanisms and backs tended to have a bit of slop in them, along with reverse curl issues. Short story : you might need to stop your lenses down somewhat to handle these potential issues.

 

[OAPOli]

Here are some test pictures. 100% crops. RF was focused on the piece of tape which lined up with the 50 mark on the rule. 3.5ft distance.

90mm lens at f/4 then f/8. Focus seems to be slightly behind target.

58mm lens at f/5.6 then f/11. I think the focus is in front of target.

Focus was on the knot. 90mm f/5.6.

I'm not sure what to conclude regarding the focus shift due to presumed bulge. Maybe that "f/8 and be there"?

 

[snusmumriken]

In my opinion this discussion is flawed. It neglects the depth of field in the subject space at the actual taking aperture in use, which is greater for wide angle lenses at a given f-number. The depth of focus at the film plane, to first order, depends on the f-number in use, and not on the focal length. Because the f-number defines the taper of the cone of light toward/away from the point of best focus at the ideal film plane. If you have a 0.2mm focus offset, but an acceptable circle of confusion of 0.03mm, then the f-number that allows depth of focus to cover the error is 0.2/0.03 = f/6.7. This is true whether you are using a 200mm lens (where the focus offset at the subject is small, but the depth-of-field is small), or a 20mm lens (where the focus offset is past infinity, but the depth-of-field is large in the subject space).

Pictorially, the effects of a film plane offset might still be worse for wide angle lenses because one is trying to get the entire picture sharp from near to far, as opposed to tele lenses where one accepts or wants some blur of the subject away from the focus point. But this text gives a misleading impression.

I agree with Dan that the proof is empirical, at the small-offset level one has to test the system on film before knowing whether to get agitated or modify the camera over a nominal 0.2mm offset.

I hope you understand that I quoted the Merklinger passage because it seemed so relevant to the original question. I certainly don't have the wit to agree or disagree with your counter-argument!

 

[reddesert]

I hope you understand that I quoted the Merklinger passage because it seemed so relevant to the original question. I certainly don't have the wit to agree or disagree with your counter-argument!

I understand that. I reviewed that post a couple of times because Merklinger has a reputation as an authority on the subject. But what he says there is totally wrongheaded. I haven't gone back to read his whole discussion to understand if this was a mental slip on his part or a deeper misunderstanding.

 

[snusmumriken]

I understand that. I reviewed that post a couple of times because Merklinger has a reputation as an authority on the subject. But what he says there is totally wrongheaded. I haven't gone back to read his whole discussion to understand if this was a mental slip on his part or a deeper misunderstanding.

I would certainly feel more at ease if you could check his whole discussion rather than rely on my selective quote!