LARGE formats: 12x20, 16x20?

[Mark_437]

(Tuan - Thanks for sending the link, I appreciate it.)

I usually avoid these sorts of discussions because: a) I approach photography primarily from an empirical standpoint, and b) they can lead to an affliction which Jeff Cooper has termed P.I.I. (Preoccupation with Inconsequential Increments). However, in this instance I think there may be some practical reasons to discuss what seems like a purely theoretical circumstance.

Yes, the standard formula which N. Dhananjay was probably referring to (r=0.00061 f/number) does indeed seem to indicate that f-stop is all that matters. There are some caveats, however. The first is that the formula commonly thought of as "the diffraction formula" does not apply to the entire diffraction pattern, only the most significant part (the radius of the Airy disk in the center of the pattern). There are other components involved here - such as the rings around the Airy disk - which are usually ignored due to the fact that they are 12 times (3 1/2 stops) less bright. (* see pg. 68 of "Image Clarity - High Resolution Photography" by John B. Williams.) Nevertheless they are there and contribute to image degradation.

In a more practical vein, it may be more accurate to look at the correlation between diffraction and f-number as something analogous to a film curve. The majority of the graph is a straight line, with a constant correlation between image degradation and f-number. However, there is a definite "toe" to the curve, and - I posit - a "shoulder". For the toe you can refer to the thought experiment I described above (miniature camera such that even at f/2 the absolute size of the aperture is smaller than the wavelength of visible light and thus suffers from extreme image degradation due to diffraction.) For a more practical example of this, see the illustrations (Figures 1-3 [A] and [B]) on page 4 of Ansel's "The Camera". It shows a photo made with a 1/64" aperture (pinhole), then the same image made with a 1/100" aperture. This is only a 1-stop (approx.) reduction in aperture size, yet there is a HUGE loss of resolution with the smaller aperture, much more than you would typically see with "normal" aperture sizes. So, obviously at some point the absolute physical size of the aperture has an effect on diffraction, beyond simply the relative dimensions of aperture and focal length.

At the other end of the scale, I believe it is also important to look at the system as a whole (i.e. absolute physical size of aperture and image) and not just the ratio of opening to focal length. As the format increases (i.e. physical aperture and image size increase) diffraction becomes much less of an issue than many people would think by just looking at the standard diffraction-induced formula. Let's hear from someone far more knowledgeable than I on this subject: Leslie Stroebel. In Sec. 3 (Image Formation) of "View Camera Technique" (5th Ed.) he talks about avoiding apertures which can result in "...an objectionable loss of resolution". More interestingly, he talks about comparable f-numbers between formats which would result in similar diffraction-induced loss of resolution. He starts with something we are all familiar with; f/16 on 35mm. To get to even this point with 4x5, you have to stop down to f/64. On 8x10 it takes f/128. And to bring this whole thing full circle to what started it all (diffraction worries - and my cavalier lack thereof - on 12x20), Stroebel states that with 16x20 (close enough for me) you would have to stop down to f/256 to have a problem. 'Nuff said.

Cheers!

[N Dhananjay]

I wish I knew of some data to confirm or disconfirm these hypotheses.

A couple of points. It is probably defensible to say that the Airy disc is the most significant component of the diffraction pattern i.e., it is an order of magnitude brighter than the brightest ring that sorrounds it and as such is the only part of the pattern that is photographically important in its contribution to the spread function.

The argument about diffraction being a function of only f stop explains why a larger format does not gain lower diffraction as a function of a larger physical aperture. That is, larger formats have larger physical apertures but the beam of light has to travel a longer distance and the longer distance gives the angular spread more room to spread).

It is also worth noting that the degree of degradation due to diffraction varies with the proportion of diffracted light around the edge of the image beam to the undiffracted light in the center. Which explains why a larger physical aperture cuts down on diffraction losses (the undiffracted central AREA increases much more rapidly with increasing radius than the diffracted PERIMETER does). In other words, at small apertures, the diffracted light at the edge or perimeter of the beam is so large a part of the total light that it dominates in the exposure and becomes a visible part of image points. However, again, longer focal lengths will lend a longer linear distance over which the angular spread pattern will detreriorate further. There does not seem to be any way to get around these limits. I've done some macro work at 2-4 times life size and the degradation due to diffraction certainly looked a function of the effective aperture rather than the marked aperture (again illustrating that the physical size of the aperture is not the issue, it is a combination of the physical size coupled with the linear distance it has to spread over, which is the f stop for normal photography).

The idea of a 'toe' and a 'shoulder' is intuitively appealing (if only for the reason that such a curve turns up repeatedly in nature). However, I have not seen any data that would actually speak to this issue (that doesn't say that there is no such data, just that I haven't seen it). Stroebel's arguments pertain to prints made to the same size and they do explain why you can get away with stopping down further with larger formats. But its also worth keeping in mind that typically a move to larger formats is typically a commitment to a contact printing process (to eliminate a second optical system with enlarging and its inherent degradation). So I'm not sure how relevant Stroebel's arguments are in this light i.e., you are not going to enlarge the negative but will be contact printing them.

If you are contact printing, the diffraction pattern is probably a function of only the f stop. That means the diffraction at f/90 on 8x10 is the same as the diffraction at f/90 on 20x24 (and by extension, will be worse at f/256 on 20x24). I would venture that it probably does not matter because we view a 20x24 contact print from further away than we view a 8x10 contact print. That helps. However, I would argue that there is no magic breaking of diffraction chains.

I also have to agree that these are probably baroque worries (a.k.a. as P.I.I.) because, we are not talking of going to monster formats for enlargeability or for resolution figures alone. We typically contact print these formats (although it is not unheard of for enlargements to be made - the latest View Camera has an article on enlargements from 12x20). However, if the move to these larger formats is a function of a commitment to contact printing, the only way to get larger pictures is by starting with a larger negative. If you want to make Pt/Pd prints to large sizes like 20x24, you're probably going to have to work with ULF and work hard at dealing with the problems inherent to that format. Also, there are other gains which are very real such as increased microgradation - there is very little discussion about how these different things that make up "picture quality" interact with one another - who knows, maybe a drop in resolution from 25 lp/mm to 15 lp/mm is perfectly OK if microgradation increases. However, it is hard to make a case for larger formats based on resolution figures alone. I agree that it may not matter given that the picture size has increased and will be viewed from further back etc. Also given that f/256 will still deliver about 7 lp/mm and f/64 will deliver about 25 lp/mm, all of this does make for stuff not worth worrying.

Cheers, DJ.

[james mickelson]

Of course if you are going after large negs for contact printing then it would be cheaper to just get an enlarged neg from a service bureau. Much easier too. The cost of the ULF equipment and materials, and schlepping the thing around is really something so why not just an enlarged neg from 6x7 or 4x5? I've seen work from enlarged negs and it is superb. James

[Mark_437]

D.J. - As usual, a well-reasoned response. The "toe" of the curve, at least, is fact and not theory (for the reasons expounded on above). And as for the shoulder, who knows, but consider that the effects of diffraction are reduced as the wavelength of the light is decreased (i.e. w/ UV, etc.). Then consider that, as you scale the entire photographic system up, the only factor NOT scaled up is the size of the light wavelength as it passes the edge of the aperture. But you're right - the discussion is mostly academic, except for the admonishment not to worry unduly about diffraction with the really big formats (where DOF is a MUCH more real problem). And I agree completely with your comments about micro-gradations: I've long thought that this aspect of LF was overlooked as an important quality.

James - You're right, you can get some superb images by enlarging negatives from 6x7 or 4x5 to 16x20, and then printing them. But it's not the same thing as a first generation contact print from a large negative. Without making value judgements, consider that - if nothing else - one way requires the image to be enlarged by a lens twice while with the other process the image only passes through a lens once. And also consider D.J.'s comments above regarding micro- gradation (which is limited by the original neg size and can't be improved on by enlarging). Again, both ways have their advantages (ex: you have another chance to manipulate/adjust the image when you make an enlarged negative from a small one) and I would not claim that one is "better" than the other, but they are not the same.

Cheers!

[james mickelson]

I meant a digitally enlarged negative. The old fashioned way makes a very nice and tack sharp enlarged negative. It has been done for years. I've seen some incredibly sharp (16x20 from 4x5 and 6x7)second gen negs and the prints from them for contact printing in Pt/Pl. But the negs from a digital platform are incredibly sharp and the interior gradations are superb. David Fokos and others are scanning negs down to 35mm and boosting them to 11x14 and larger with no degradation at all. As an old school backward kind of guy it took me awhile to accept the digital realm but after seeing so much work done with digitally enlarged negs I have come around. I'm setting up my system to scan my negs and enlarge them for contacting. Once scanned and manipulated to my liking I can have a bureau make an enlarged neg and just print it and have the same quality throughout the series. James

[Daniel Loshbaugh]

This isn't an answer...more of a "hey guess what, I'm crazy too" question kind of thing. I came to this page because I want to do very large pinhole camera stuff. I have a 1980 VW bus with a sliding side cargo door and I want to build a box that fits inside the cargo area with the pinhole pointing out the door so the distance from the lens to the paper will be far....like in feet. And the paper will be large too...like in feet. I've had a lot of fun with other pinhole cameras and wanted to knowif this idea of turning my VW bus into a camera sounds like something that may work. The other thing was...where could I get photo paper that's say...three foot by four foot? And would any careto guess what size my pinhole would be if the hole were say, three feet from the paper? Also how do I keep my wife from trying to kill me? Ha ha ha. Thanks! ( think I can already hear some or all of you laughing...I don't care, I have got to try this)

[Daniel Loshbaugh]

I had a typo in my e-mail address (not to mention all the ones in my post) on the last post so this is just to fix that.

[kthompson]

Daniel, I think the car/van pinhole thing has been done before..so it's really not that crazy. Especially compared to some of the other pinhole ideas out there, like hollowing out a tomato and using ortho film, or using your mouth...

Try getting some mural paper if you want something 3'x4'. 30"x40" is about the largest cut sheet size in paper. Sorry, can't help you with the hole size or distance, but I'm sure somebody out there knows the equations. I think your bigger problem will be in how to handle and process the paper out in your van.

[Mark_437]

Dan - GREAT POST! I couldn't agree more. Everyone has to find his/her own way, and telling someone dedicated to fine art b&w contact prints to digitally enlarge smaller negs is like telling dedicated jazz drummers to quit hauling all that gear around and just use a drum machine. Nothing *wrong* with it, but that's simply not why they're in the game. And again, without making value judgements, the results are not the same. (And even if they were, a large part of any experience is the process itself.)

Cheers!

[mirek lawrowski]

I have thought of one way of doing very large format on a small budget. of course the lens will be the real cost. 1. buy a small square van , with lens carrier in the side of van. 2. an interior moving film holder , focus from inside van , by moving film holder towards or away from holder (you sit inside van to focus and take the photo.

I have been thinking of doing this for a photographic tour of the uk ,you would of course be limited to taking photos only where you can take the van. which in the uk is not sutch a limitation.

The only problem is ?? where can you get a standard lens for( 16 x 20 )?? I would be intrested in hearing from anyone who would be intrested in this idea.