Bulging of 8x10 film causes BIG focus errors???

[Sol Campbell]

Hi,

Last night, while I was loading my holders with some 8x10 film, I noticed that a t the center, the film was bulging quite a bit. I estimate the bulge to be 1-2 m illimeters.

So today I did some calculations to see how it would affect the focus, and I was astounded by the results.

For example I calculated that a 2mm bulge would cause the focus to shift from 10 feet to 9.31 feet on a 300mm lens. On a 110mm lens the focus would shift from 1 0 feet all the way to 6.57 feet!

A 1mm bulge would cause the focus to shift from 10 feet to 7.93 feet on a 110mm lens.

Then I calculated what the lens need to be stopped down so that the depth of fie ld could "cancel" this shift and the answer was f45 or smaller! Even at 1mm!

Does this sound right? Or are my calculations ways off? Is my estimate of the bu lge too pessimistic?

What is the average bulge for 8x10 film? And what about 4x5?

If this indeed is the case, then film flatness is VITAL. And anything but a perf ectly flat film will TOTALLY waste the capabilities of a fine modern lens.

Comments please...

[Sean Billy Bob Boy yates]

So Sol, have you made any exposures yet? If so, how were they? Sharp enough or not?

[M.]

Sol, your numbers sound about right (unfortunately). They probably are, however, pretty much the outside limits of the problem. It obviously depents a lot on the design of the holder and the thickness of the film base. 4x5 is much, much better at holding the film flat, for obvious reasons. In the old days, when shooting with film pack which was basically coated on roll film stock, we always had to allow an extra stop to compensate for the lack of film plane flatness.

[Doug Paramore]

Now you know why the folks at the observatories use glass plates to take pictures of the stars and why other scientific photographers still use glass plates. In actual use, small aperatures make up for a lot of the errors. Film bulge is not an inconsequencial problem, especially with 8x10 and larger negs and to some entent with 4x5. It accounts for a lot of those shots where you know you focused carefully and the neg looks soft. Also why two negs made without touching the camera controls will not havethe same sharpness. The main thing is that most of the time we LF'ers can live with the result. Incidentally, it isn't just LF. Medium Format SLR cameras can really bulge and buckle film, especially the backs with reverse roll, which seems to be most of them made today. I agree with you on the lens issue.

Regards,

[Ellis Vener]

When shooting jewelry at greater than a 2:1ratio (i.e a 2x magnification) several years ago, the only problems we had with 8x10 was when the camera was pointing downwards. In all cases however we were at ?/45 or ?/64.

A couple of years ago, "View Camera" did a survey comparing various 4x5 format holders --Ready Load, Quick Load, Fidelity, Lisco, Sinar adhesive, and glass plates among others-- and found the results to be very similar in all cases in terms of variations, although the author didn't care for the Quick/Ready Loads at all.

Most interesting was his conclusion that some individual holders were better at holding film flat than others that were otherwise identical. One of the morals of the story was to carefully check your individual holders for flatness and discard those that didn't meet the acceptable standards.

My understanding of astronomical photographers still use coated glass plates is the stability of the underlying media during long exposures as the temperature or humidity levels change.

[Sol Campbell]

Sean,

I don't shoot 8x10 much. From the shots I have taken, yes, on a few of them I have noticed unusual focus planes. Not terrible but definitely annoying and worse, unpredictable.

So far the response seems to suggest that my calculations are correct. Which is very scary. I am surprised it is not addressed more. What is the point of Schneider and Rodenstock designing these incredible lenses, only for them to be wasted?

This will definitely make look into the Sinar Adhesive Film Holders. I wonder why they don't make those for 4x5?

Thanks.

[Bill Glickman]

Sol... I am not following your math. Here is what I come up with to calc. the depth of focus at the film plane. f stop * cc * 2

So, at f45 * .042 cc (allows for 5x enlargement and still maintain 5 lpmm to print) = 1.9mm film play in either direction and still maintain the desired cc. To accomodate 1mm of film buckle, you only need to stop down to f22 to negate it. What formula are you using to calculating the depth of focus at the film plane?

Your point is well taken though, film buckle is a risky part of 8x10 film. I often struggle with this, and try to keep my film holders upright when loaded. I think when they lay flat for awhile, the bottom film can buckle towards the dark slide but the top one lays flat against the film holder middle.

Considering I always shoot 8x10 at f22 or higher, I have not had focus issues relating to film buckle. If you are having problems, I would first check gg / film alignment before thinking the buckle would be worse than what you mentioned. 4x5 is much more reliable in this area, my guess is it buckles way less than half vs. 8x10. I will agree with the poster above, roll film backs can be very vunlerable to this issue, specially if shot at wide apertures... i.e. mainly used with MF lenses, not LF lenses.

[Micah Marty]

Re: Ellis's post: Does anybody remember which issue of ViewCamera tested film flatness? In my recollection, the Kodak Readyload design actually did quite well in ViewCamera's test (because unlike with standard holders, there's a pressure plate evenly pushing the film forward) but there's also a good chance I'm mistaken. Anyone want to dig up the test and tell us abou

[Micah Marty]

tit?

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[james mickelson]

8x10 is normally shot at apetures of f32 and smaller. If you are going to buy adhesive backs anyway, why the question? I never have a problem with film bulging as much as focusing errors and FP-GG alignment. James