Depth of Field, Depth of Focus, and Film Flatness

[Eric Biggerstaff]

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[Oren Grad]

Having said that.... will I do anything different than I have been doing since 1970?

Kirk, for my purposes the practical value of this sort of knowledge is primarily for troubleshooting - understanding the different failure modes of our tools and the circumstances under which they're likely to come into play is a big help in that respect.

For example, the likeliest source of weird focus problems when I'm working with my 4x5 wood-field is flex in the standards. On the other hand, with my 6x9 technical cameras, where the rear standard is absolutely rigid and I'll tend to be working at larger apertures, roll-holder film flatness and ground glass register are relatively more important.

I don't want to leave the impression that I need to be doing depth-of-focus calculations or fine tweaking of my equipment all the time just to get a usable negative - far from it. Most of the time I'm working at apertures and with subjects that are tolerant of quite a bit of slop. But sometimes things do go wrong, and when that happens it's nice to have a sort of mental checklist of different technical issues that might be involved, ranked in order of their likelihood as a cause.

[bglick]

Hi Steve

> I never made any statement about"anal photographers"

Your could be right, in that thread, you referred to them, as only the "most obsessed" photographers, then others, shortened your reference to the other term.... Sure glad you mentioned this, I feel much better.

> I did know about depth of focus, I said so in the suspended thread. I just questioned its day to day relevence. If you disagree fine.

When that thread comes back up, I think by reading it, it will become obvious to everyone, even by your own posts.



> Applying numbers to Cof C does not give a magical answer because the required size for a Cof C depnds on many variables.

Steve, from my post above..... Of course, the CoC can be effected more, or less, by lens MTF performance, aperture diffraction and depth of focus. The point being, Depth of Field alone, is rarely EVER the only determing factor of the CoC diam. This is NO small detail. It was me posting this to remind you of such, not sure why you feel compelled to then, re quote my post?????



> There are numbers considered acceptable but in a visual world they are not absolute.

This is my exact point, there is no absolute, in photography, most issues border on "acceptable" or "not acceptable" of the finished product. As several posters have pointed out here, there is many reasons why results can be....

acceptable, even with depth of focus shortcomings....

unacceptable, with NO depth of focus shortcomings.....

It's all based on what your trying to accomplish and the gear you're using. With the hodge podge of film holders and cameras in LF, it all can add up to potential problems. Hence why I take the opposite position as Steve does...... LF is the ONLY format whereas depth of focus should be of concern. And as Ted mentioned, it's all part and parcel of the final on-film resolution. It doesn't matter where the weak link in the chain is, it will surface on the film, so Steve, why dismiss depth of focus so quickly because you and your associates and 3 other books did not reference it? That reasoning still just doesn't make sense to me, that's one of the many things that baffle me, specially being an author on technical books in this field? I keep trying to understand you Steve.

> Bill's post seems particularly directed at me and seems like an attempt to start a fight. He could have made his points, and they are his opinion and he is welcome to them, without going on and on about me.

I was responding your YOUR post Steve, of course I was directing it at you. Just so its clear, I have zero desire to pick a fight with anyone in the real world or in a forum, specially you. I don't have the energy to withstand your persistence to "never be wrong", and you can be assured I too will fall victim as others have who challenge your statements. They get burnt out, trying to convince you 2+2 = 4, so they eventually move on. But, I hope these brave posters have helped other fellow photographers along the way, as I have sure benefited from many of them through the years on this forum.

In all fairness Steve, I spent more time trying to give credit to other posters on the suspended thread who relentlessly tried to help you in a nice professional manner before you went on the defensive. I personaly hope we don't loose these posters, due to the frustration level they experienced in the suspended post, as I did not notice their participation on this post. If anything, I like to think of my role in this and the suspended post, as a "forum medic", risking my life, running through war zones, saving the casualties in threads you particpate in. But you will be happy to know, my "term of duty" is up soon, so you can relax :-) BTW, I never attacked your books or magazine one bit, I only comment what you write on this forum, and isn't that the purpose of the forum? In the suspended thread, I did it as peaceful as possible, I will continue with this approach Steve. And it's OK you failed to comment on the "meat" of my post, I fully expected this, as you have trained me well in the suspended post. But I must admit, I was shocked when you did not defend yourself when Oren pointed out that your OWN magazine did an article on this subject in 1996 and concluded this was an issue LF users should be aware of. I respect your silence.....

> Despite what he says the required size of the CofC will vary depending on the circumstances (how much the image is enlarged, how closely it is looked at, etc, etc,).

Steve, please, you need to re-read my post, it was ME bringing this to your attention, not sure why you are tyring to turn-the-tables here...and I will not waste bandwidth constantly re pasting my own posts.

> Bill also questions the poll I took amoung working photographers. Since he was not sitting next to me whern I made the calls I am curious why he brought this up.

I only commented on your reasoning of why depth of focus should be dismissed, and then your conclusion, "relax, let's go photograph." It was the same poll, and the same books you quoted in the suspended thread. The key is here, you never commented on the meat of the subject matter and the discrpancies many posters have above, with your first post here. You find the issues irrelevant. Instead, you operate under the premise, if you can't beat the message, beat-up the messenger! Make sense? Same "mode of operation" in the suspended thread. Anyway Steve, I do wish your books and magazine success, as the LF community needs such. I think it's obvious your products would benefit by treating everyone with respect and fairness on this forum, as this forum probably respresents the largest customer base for your producuts, right? It's hard to beleive I have to "state the obvious", but, such is the case.....

Eric...... >Being the Non- Technical person I am, I was following this thread (sort of) until I got to Bill's post which left me completly confused.

I am sorry for such..... and I certainly don't suggest everyone to get burried deep into these issues as I mentioned in the suspended post, photography has everything from artist to scientist (and everything in between) participating. Let this post serve as a "heads up" of things that can happen.

If you're not happy with your current results, do some very simple tests, such as photographying a flat wall (but not too close 40ft+) Take a few shots, one focussed one right on the wall, one slightly beind the wall, and one slightly in front of the wall. Use the widest f stop lens you have, shoot at 4.0 or f5.6, this will exaggerate any problems. Be sure to put a label on the wall, (1,2,3, etc.) so you dont' confuse the chromes. Use the same technique you normally use to focus, use the film holders in question, or test them all, and once again, mark the scene accordingly. View the film on a light box and be sure the chromes which were focussed correctly, are the sharpest.

If you pass this test with flying colors and normally shoot at f16+, your technique and photo gear is completely void of all these issues. If you notice different results, well, based on how concerned you are, you can go to the next level. You can also try to shoot an open area, with signs posted at different distances from the focus point.... just use some text that comes out of your office printer.....this will save on film cost, as you always focus at the ONE sign (target) in the middle. Be sure to put min. 5 targets total at varying distances, as if the alignment is off, you can tell in which direction and how far. Again, this is a rough-cut test. But it demonstrates how complex issues can be reduced to simple tests to either confirm no problem exist, or maybe discover an alignment issue.



Oren, THANK YOU very much for your roll film holder contribution! I had problems with this in the past and never wanted to spend the time to investigate it, as I felt it was easier to simply buy Toyo roll film holders as I use all Toyo LF gear. This immediately corrected the alignment issues. It was cheaper then doing extensive testing and mechanical corrections. This is one of the benefits of trying to stay with one maker for all your gear. Of course, not too probable for most.

The .25mm you quote above, demonstrates just how radical the introduction of one piece of gear can toss the alignment issue into anarchy. My post mentioned the .3mm margin of error the depth of focus has at f16 for "all" issues, whereas in this case, just using the "wrong" roll film holder can account for all the depth of focus allowance, leaving nothing available for other mis alignment in the view camera system. In my opinion, this is one of the many reasons it's risky business switching from MF cameras to view cameras with RFB's of similar format size. Yes, you gain movements, but sometimes at the expense of many other negative issues. Of course, with prudence and testing, much of this can be avoided or corrected.

Oren, also your previous posts completely sums up how I feel about this entire issue. It's a trouble shooting tool, and a useful set of knowledge to 1) avoid buying gear that might create the alignment problem or 2) knowing what to look for to see if the problem existed. Again, thank you for the contribution.

[Kirk Keyes]

To see if any of my photo books had anything to say on about depth of focus, the first one I reached for was my copy of "View Camera Technique", by Stroebel, 6th Ed. On pages 138-9, it lists the 4 significant relationships that I think Steve recounted at the start of this thread. In closing the section, I think Stroebel makes the best arguement for caring about depth of focus:

"If a subject exactly fills the depth of field space, then there is only one position the film can occupy, and there is no tolerance remaining for the photographer or the camera."

I would like to tie this into what the other Kirk wrote: " Having said that.... will I do anything different than I have been doing since 1970?"

Maybe not for you... But for me, what Stroebel stated had a big affect on my choice of camera.

When I started, I took the route of using a lightweight 4x5 camera, a Nagaoka. It was nice and light at a bit over 3 pounds, and pretty compact. But I kept noticing that it flexed all over the place - putting the dark cloth one, placing the loupe on the gg to focus, placing the film holder in...

Now I knew that the flexing, when compared to the depth of field, really had no affect on anything. But then compared to the depth of focus, it was a huge amount of movement. And I lost several shots because the camera was just not rigid enough to place a loupe on the ground glass and not have it move. Having used a Linhof Tech IV for a while earlier, I decided to ditch the Nagaoka and get a Technika.

Now I don't mean to badmouth wood field cameras, as I'm sure many of them are much more rigid than the good old lightweight Nagaoka was, but I have the comfort of knowing that I can shoot without having to take actions to insure that the film will end up acceptably close to the plane of focus.

Kirk, I think that at this point, you probably do not have to take depth of focus into consideration on a daily basis as your camera kit is most certainly pretty well tested by now. But for others, that are getting both new or possibly old or poorly adjusted or poorly functioning equipement, remembering and understanding the principle of depth of focus and Stroebel's summation of it may someday be useful.

Kirk - www.keyesphoto.com

PS - Mark, glad you enjoyed the photos on the web site, and I'm planning on getting more B&W work up there some day soon...

[Kirk Gittings]

Kirk understood.
Kirk

[Kerry L. Thalmann]

Wow, between this thread and the suspended one there has been a LOT of heat generated and even some light shed on the topic of depth of focus. There also seems to be some sort of technician vs. artist debate brewing. This would seem to imply that one has to make a choice between the technical and artistic aspects of photography and that the two are mutually exclusive. I strongly disagree (not with anyone in particular, just with this premise in general) and believe it is possible, even desirable (but not required) to be both an artist and a technician. Ansel Adams used the technical understanding of his materials and processes to produce beautiful photographs. Edward Weston used a much less technical approach to also produce beautiful photographs. The important thing is they both chose the approach that worked for them and even though they chose different methods, there is no documented evidence that they ever called each other names or publicly denigrated the work of the other. In fact, they were close friends who respected and admired each other's work. We should all learn from their example and learn to not just tolerate each other's opinions, but to welcome them and embrace them (OK everyone, let's all join hands around the campfire and sing Kum Ba Yah).

On the subject of depth of focus... Although I am a technical person by training, I always look for a simple solution to any problem. In my simple-minded approach, this entire issue can be summed up in one equally simple-minded sentence:

It's only a problem when it's a problem.

I consider depth of focus issues to fall into the same category as things like light leaks. You can go years, even a lifetime if you are lucky, without ever seeing any evidence of a problem. If that's the case, as others have recommended, forget about it and keep making photographs. Until you actually notice a problem, whether it's a light leak or a depth of focus issue, there is nothing to fix. So, why worry about it. If you do experience a light leak (or a depth of focus problem), then it's time to take action, determine the cause of the problem and implement a solution. As with light leaks, there are some very simple tests that can be used to check for depth of focus problems. With light leaks, you can check your bellows for pin holes, check that your film holders are light tight, etc. With depth of focus issues check the alignment and registration of your ground glass, check for warped or out-of spec, film holders, etc. The point is to diagnose the problem, identify the source and fix it. Then get back to making photographs and living a happy and productive life. No point in arguing about it. Unless you share a camera and film holders with someone else on this forum, whether or not you experience a depth of focus problem is specific to you and your equipment.

Of course, there are specific applications and circumstances where depth of focus problems are more likely to be noticed, but again sweeping generations should be avoided. There are just too many variables to assume somebody else will or won't have a depth of focus problem.

So, it is certainly possible to spend your entire life making beautiful photographs without ever encountering even a hint of a depth of focus problem. On the other hand, it is equally possible for a great artist's vision to be thwarted by a camera with an improperly registered ground glass or out-of-spec film holders.

Sorry if my comments seem to re-state the obvious and repeat what others have said with more vigor and passion. I'm just finding the aggressive intolerance and lack of respect for other's differing (but no less valid) opinions a bit tiresome. There's room here for a whole wide range of opinions and this is one of those issues where both you and the guy you are so desperately trying to prove wrong could both actually be "right".

Kerry

[William Mortensen]

While Bill's above-recommended method of shooting several chromes of a flat wall (I'd shoot paper negatives for this myself) might be more accurate, I check my focal plane registration as follows: take the back off the camera, lay a rigid straight edge across the inside of the back and measurefrom the straight edge to the gg center and four corners. Then put in a filmholder with a crappy old negative in it (no dark slide) and make the same measurements. This seems as accurate as one could make the measurements, (I used a digital calliper). Anybody see any problems with this method?

I use Fidelity and Lisco holders, and checked one of each in each of the three 8x10's I use. No problems found, and I've never had any depth of focus issues with my negatives. As Oren noted, this is probably mostly a trouble-shooting issue, and I suspect it will never arise for most photographers. Then again, I hope to finish building a ULF and a film-holder or two this summer, and it will be *THE* critical measurement.

[bglick]

Kerry.....

> There's room here for a whole wide range of opinions and this is one of those issues where both you and the guy you are so desperately trying to prove wrong could both actually be "right".

Those who know me, since 98 on this forum, I have never been one whose goal it is to
"prove people wrong." This was a rare case of a suspended thread being "continued" for the sake of re affirming a position many posters rightfully challenged in the suspended post.

When I think back to many historical posts on this forum looking for information, I recall often someone challenging a position, or possibly even making a very vague refernce more clear and give it some legs. This is what happened in the suspended thread, an inaccurate and vague postion, and now the same in this thread. I applaud those who stood up in the last suspended thread. But, yes, I marvel how I got suckered into this myself, but, when I look back at the posters threads, I now realize I am not the only one who "took the bait." In the end, I hoped some people benefited from the information, if not, I will clearly avoid such in the future, as other posters have done. I apologize to anyone who found my post the least bit unfair in any way.

As for light leaks vs. alignment issues...... in my opinion, a weak comparison, as light leaks represent a system break down, and are "usually" obvious on film. Alignment issues are impossible to see, can come and go based on using different film backs, can appear only when shooting flat subjects, but not as objectionable shooting near/far subjects, etc. Quite a tricky variable. It's not always noticeable if your not pushing enlargement factor, etc. Then when you notice soft chromes, was it your focus, was it camera shake, was it subject movement, etc. etc. etc. Much easier in my opinion to know your gear well. As I mentioned many times in this post and the supsended post, this is not everyones cup of tea, many prefer to just wing it, and when a problem surfaces they consult a camera repair technician. Of course as with most issues, if one never experienced the problem, they have a hard time relating how others can experience it.

[Henry Ambrose]

Aaron,

I answered QT's "good enough" post because it does have something to do with Depth of Focus in that Depth of Focus is a range of acceptability just like all photo work is, whether commercial or private.

What Evan Clarke wrote is most all the answer - the film plane and the ground glass must agree, all that's left is to focus the camera and expose the film without disturbing your setting. "Perfectly" would be great but within tolerance gets the job done. There is only one perfect film/image plane junction, anything else is to a tolerance of acceptability (or not). I suspect that Kodak has some lab equipment that can tell when the focus plane is precisely in a particular layer of the film emulsion - now that is real precision - but we will never approach that level of precision and don't need to do so. Most standard film holders will hold the film in the right place when all is right with the world and the holders and camera back are to specification. You'll get "good enough" results, which might be "quite good" or "sorta good" maybe even "excellent" -- if the print you make is what you wanted, you did it "good enough".

Depth of Focus is a description of a natural or optical phenomenon that you really can't do anything about other than to be sure your film gets exposed within the range of acceptable Depth of Focus for your printing requirements. In less words, if your gear is all correct in its dimensions and functions and you have focused properly, Depth of Focus is still there and you are operating within it. So when you are shimming your plate holders for film use (or whatever thing it is you are measuring or adjusting) follow Evan's advice and you should be ready to make pictures.

[Kerry L. Thalmann]

Bill,

Those who know me, since 98 on this forum, I have never been one whose goal it is to "prove people wrong." This was a rare case of a suspended thread being "continued" for the sake of re affirming a position many posters rightfully challenged in the suspended post.

My comment was not directed at you, or anyone in particular. It was just my general impression as to why the previous thread (and now this one) went on and on well after everyone had made their points about the subject. Again, this is not directed at you (or any particular individual), but at some point all the useful information has been shared and people are just repeating their message with different words, or worse still, getting off-topic and personal.

As for light leaks vs. alignment issues...... in my opinion, a weak comparison, as light leaks represent a system break down, and are "usually" obvious on film. Alignment issues are impossible to see, can come and go based on using different film backs, can appear only when shooting flat subjects, but not as objectionable shooting near/far subjects, etc. Quite a tricky variable. It's not always noticeable if your not pushing enlargement factor, etc.

That was a deliberate oversimplification on my part. My whole point is if you can't see a problem on your final output, then no problem exists. It's the old "if it ain't broke, don't fix it" cliche. Yes, alignment errors can be harder to detect and diagnose than light leaks. That was not my point (my fault for not expressing myself better). If there is a problem (or if someone just wants to make sure their gear is up to snuff), it can be checked with a few simple tests and/or measurements. On the other hand, if someone is getting acceptably sharp results at their maximum enlargement size, do they really need to spend time running these tests and worrying about their ground glass alignment and registration? Of course, "acceptably sharp" is a subjective term - but that's part of the point, everybody has different standards (as well as different equipment).

Much easier in my opinion to know your gear well. As I mentioned many times in this post and the supsended post, this is not everyones cup of tea, many prefer to just wing it, and when a problem surfaces they consult a camera repair technician. Of course as with most issues, if one never experienced the problem, they have a hard time relating how others can experience it.

I'm all for people knowing their equipment. I admit I'm a bit anal retentive when it comes to my camera gear. I try to identify and eliminate anything in the chain that would impair my ability to make the kind of photographs I desire. Then I can't blame anything or anyone, but myself for my poor results. That then allows me concentrate on improving my abilities and not worry about the equipment being a limiting factor (at this point, I am quite confident it is not my gear that limits the quality of my photography). I recently sold some 40"x50" prints (made from 4x5 chromes). The client was absolutely thrilled with the sharpness and detail in the final prints, and I admit I was pretty happy with the way they turned out, too. I have no plans to make anything bigger than a 10x enlargement from my 4x5 chromes (and I don't even go that big very often). So, I'm confident my equipment is up to the task and not a limiting factor.

For the record, I have had holders that were out of spec. I threw them in the trash and went back to shooting. I also once had a camera with a mis-registered ground glass. I installed some shims to correctly position the ground glass, re-tested and went back to shooting.

Ultimately, I consider myself an agnostic in this whole technical vs. artistic debate. I'm a technical person by training. So, technical concepts that may seem simple to me may confuse others with a less technical background. That doesn't make me a better photographer than them, but I do believe the technical knowledge I have aquired has helped improve certain aspects of my photography. However, as I type this, hanging on the wall above my monitor is one of the best photos I've ever made - way back when I was new to large format, was using a "sub-standard" camera and lens and lacked the technical understanding I have acquired in the 16 years since (and still looks great at 20"x24"). So, I can see both sides of this issue. Yes, it helps to know your gear (and your materials) like the back of your hand (and I do), but it is not always necessary. To each his own.

Kerry