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

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Hi,

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

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 10 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 field 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 bulge 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 perfectly flat film will TOTALLY waste the capabilities of a fine modern lens.

Comments please...

-- Sol Campbell (solcam31@hotmail.com), December 31, 2000

Answers

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

-- Sean yates (yatescats@yahoo.com), December 31, 2000.

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.

-- Bill Mitchell (bmitch@home.com), December 31, 2000.

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,

-- Doug Paramore (dougmary@alaweb.com), December 31, 2000.


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.

-- Ellis Vener (evphoto@insync.net), December 31, 2000.


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.

-- Sol Campbell (solcam31@hotmail.com), December 31, 2000.



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.

-- Bill Glickman (bglick@pclv.com), January 01, 2001.


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 (micahmarty@aol.com), January 01, 2001.

tit?

/\/\/\/\

-- Micah (micahmarty@aol.com), January 01, 2001.


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

-- James (james_mickelson@hotmail.com), January 01, 2001.

The article on film holders was written by Cervin Robinson and appears here: Battle of the Bulge - Sheet Film Holders, Mar/Apr 1996. pp 62-67.*

What kind of work are you doing Sol? Unless you are shooting for paying customers I am inclined to think you are overly concerned.

Even then you might be. The last studio I worked in used all conventional holders Fidelity, etc. except when they shot Polaroid of course. They were exposing over 30 sheets of 8 X 10 chrome per set- up (they shot furniture sets) and 2 - 4 sheets of B&W & Color Neg. Other than cleaning the holders religously they never took any special precautions. One photographer always raped the holder smartly in his hand before inserting it into the back, but he was the exception. The stuido is still in business and brings in a substanial $um/year. I don't think they'd be competative if they were handing in poor results.

Think of Ansel Adams and Brett & Edward Weston, not to mention Paul Strand, etc. etc. etc. They all shot 8 X 10 with wooden holders and yet their prints are the standards by which we judge our own work, right? What kind of holders do Ron Wisner and Michael A. Smith and Tilman Crane and Linda Connor and Christopher Burket and Clyde Butcher and other current masters use?

-- Sean yates (yatescats@yahoo.com), January 01, 2001.



I'm sorry, he RAPPED the film holders, before inserting them into the back.

-- Sean yates (yatescats@yahoo.com), January 01, 2001.

Sol, The ANSI spec for an 8x10 film holder states depth should be .260" +/- .016". This is a fairly generous tolerance, but as others have mentioned, ground glass/film plane coincidence is of vital importance. If the ground glass is in the perfect plane, the film can lie anywhere within that .032" range and still render good results assuming you are using an aperture that is appropriate. You might test this with lens wide open using a stepped target with super fine detail such as the one I wrote about in my Nov./Dev. 1996 article in ViewCamera Magazine.

-- Robert A. Zeichner (info@razeichner.com), January 01, 2001.

I'm not smart enough to take issue with your math, Sol, but I wonder about the basic premise.

"While I was loading my holders with some 8x10 film, I noticed that at the center, the film was bulging . . . 1-2 millimeters." Although I'm not sure how you noticed this in the dark, I did check to see how much 2 millimeters is, and it's a lot. Those of you who don't have unsleeved 8x10 negs lying around can take an 8.5x11 piece of paper, lay it on a table, and slide a stack of two dimes just under the edge in the middle of one end of the piece of paper, letting the corners of the paper rest on the table. That's about 2mm of "bulge," and I can't picture 8x10 film bowing that much in any reasonable holder.

I guess I'm with Sean on this one; there's theory and there's practice. I've exposed thousands of sheets of 8x10 (in standard Fidelity holders) and never once noticed a focus differential between the corners and the middle due to film bulge (I do manage to make lots of other focusing mistakes!). But I also stop down to at least f22 and do mostly contact printing rather than enlargements.

Please let us know if "your mileage varies" in this respect in field us

-- Micah (micahmarty@aol.com), January 01, 2001.


Rapping the bottom edge of the holder should be standard practice. It seats the film in the holder so it doesn't slowly slip down as you expose the sheet of film. Pull it out after the first exposure and rap it again to reseat. I've seen images that are ever so slightly out of focus due to this problem especially on my 8x10. Sexton said he does it all the time. and Ray McSaveny does it too. Just good mechanics. James

-- lumberjack (james_mickelson@hotmail.com), January 01, 2001.

According to Stroebel (View Camera Technique) an acceptable circle of confusion is in the range of 0.01in for an 8x10 neg (contact printed; for a 4x5 enlarged to 8x10 print is ca 0.005in). Equation for the depth of focus is (as previous respondent pointed out) is 2 x Fnumber x acceptable circle of confusion or (using F22) DOF=2 x 22 x 0.01 inches = 0.44inches. If you wish to use the tougher COC standard of 0.005in, DOF is then 0.22 inches. Only below F11 (or at very large magnifications of the large format negative-such as murals)does this present a problem but few 8x10s are shot at F11 or wider.

The only fly in this ointment is if you are using 100% of your depth of field.........

Conclusion is stick with your regular holders

Alan

-- Alan Barton (cherylannschieber@worldnet.att.net), January 02, 2001.



Alan, that figure of 0.01" circle of confusion is outside the limit of what most users would find acceptably sharp. Half that would be a more reasonable figure.
Also, the formula you quote is for total depth of focus, from the point behind the film plane where acceptable focus is lost to the equivalent point in front of the film plane. In practice we're only interested in how much the film can bulge outward, since it can't sag behind the film holder.
Your figures should be divided by 4 at least, to be workable.

-- Pete Andrews (p.l.andrews@bham.ac.uk), January 03, 2001.

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