Millimeter scale for focus - maximum delta?

[Roger Rouch]

On the Large Format homepage there is a link to focus techniques where a method is described using a millimeter scale on the bed with a pointer on the focus rai l. This is also described in an old Photo Techniques issue, I belive. Basicall y, once movements have been applied to minimize the difference between near and far focus, the difference is split for the optimal focus. I have glued a millim eter scale to my Tachihara 4X5 and am trying to use learn this technique. I am using a 16X20 reproduction size as a standard for sharp focus. At this point I have considered a delta between near and far focus of 2mm to indicate an f stop of 22 and if the delta is 3mm an f of 32 would be neccessary. Beyond that, I've considered that the delta is too large to produce a decent image and bypass the opportunity. I actually have avoided a delta of 3mm in some cases. Of course is the delts is less than 2mm, things get a little easier. I'd sure be interest ed in the experience of others using this technique and if I'm on the right trac k. I haven't been at this long enough to verify some of this and would like to avoid errors and get better at it.

[Robert A. Zeichner]

I've been using this focusing technique for about three years with what I would characterize as excellent results. Normally in 4x5, I try to avoid anything smaller than f32. I have, under certain cicumstances used an aperture of f45 with a delta of as much as 5mm. It's not my preference, but if the opportunity means getting an important image and living with slightly lower resolution, I expose the film and decide after I've developed the negative. Sometimes I'll bias the position of the lens standard toward the far if the characteristics of the foreground are such that slightly soft focus wouldn't be all that noticeable. Apparent sharpness is a function of a number of factors, one of them being what I call detail scale. If the foreground objects have relatively large elements of detail in them, even if the focus is less than optimal, at the normal viewing distance those objects might seem acceptably sharp in the finished print. Again, this is not ideal, but sometimes the importance of capturing the scene outweighs living with these imperfections.

[Jeff White]

I agree with your basic approach and use this same method for focussing. I use less than 1 mm as f/16, 1.5 mm as f/22, 3 mm as f/32, 5.5 mm as f/45 and 11 mm as f/64. I agree with the goal of keeping your f/stop in the f/16 to f/32 range, BUT I would not hesitate to stop down to f/64 if that is the only way to solve problems. I think we get caught up on trying to make "perfect" images so much that we forget that excellent images are also ok. Many of the best known images have been made at f/64 and smaller. I believe that Ed Weston estimated his F/stop to be f/128 for Pepper #30, I for one am glad that he made that image even though it didn't fall in the "ideal" range for focus spread. Have fun.

[Robert A. Zeichner]

One point that I feel should be made here is that the smallest useable aperture (before diffraction becomes noticeable) changes with the format of film you are using. On 4x5 that aperture might be f32, in 5x7, f45 and in 8x10, f64. You can't necessarily draw a conclusion about what seems good enough at a particular aperture unless you know what size the original negative was. In the case of Edward Weston's work, I believe he worked almost exclusively with an 8x10. Perhaps one of you historians out there could confirm this, but I was once told he never even owned an enlarger. Is this true? His f128 would be like f45 on a 4x5 and with nothing bigger than a contact print, f128 would seem pretty sharp!

[Doug Paramore]

Reply to Robert Z. Ansel Adams, who was pretty close to Edward Weston, wrote that as far as he knew, Weston never used an enlarger after he returned from his time in Mexico. I believe that statement is in Adams' "Letters and Images etc." In the same book, a letter is printed from Weston to Adams that he had the aperature of his lens modified to stop down "two stops from the f64 it came with". However, Weston was writing to say that he was having problems with the focus of his convertable Turner Reich at that aperature. I have also read that Weston copied smaller contact prints onto 8x10 to make larger prints.

[QT Luong]

A focus spread of 3mm is nothing. I read a lot that one shouldn't stop down beyond f32 in 4x5. However even at f64, the diffraction limit is 23 lp/mm for in focus subjects and still 16 lp/mm for subjects with a focus spread of 11mm. There is still enough resolution to make a 11x14 which will appear critically sharp at minimum viewing distance.

I will be releasing soon an article on which f-stops to choose. If you'd like to preview it and comment on its readibility or technical contents, it's for now at: http://www.ai.sri.com/~luong/photography/lf/fstop.html

[Roger Rouch]

Thanks for the help on this one. Jeff, since you printed one of my photos last year I've gotten better at this, but am still working on it. Thanks for the help.

Q.-Tuan, I compared your article to the two articles refereced in Phototechniques. I am not a great mathematician, but your article as well as Mr. Peterson's article express the understandable theories relevent to my question and to general focus theory. I do tend to comprehend visual renditions somewhat better, and found some of the charts in Mr. Hansma's article complementary to the theory and math. I especially found the chart using the axis of f-stop and resolution (lp/mm) and plotting focus spread to illustrate optimum f-stop to be helpful (fig 5. in his article) Like you, I tend to walk into a photo for a closer look regarless of the enlargement size. I would estimate this might be constant at, say 18"-24" somewhere. It would be interesting to add horizontal shading or other indications to this chart to further indicate maximum print size or enlargement relative to negative size to correspond to the theoretical resolution. This would specifically answer my question and might give me a little more latitude in focus spread than I have perceived, as I'm using a standard of a 16X20 enlargement (or other sizes for that matter). Maybe this is there somewhere, but in these articles, but I've not got it yet.

[Brian Ellis]

I use this same method, which I originally learned from the two articles in "Photo Techniques" that you mention. I'm probably confused but my recollection was that one of the articles set out optimum f stops for varying degrees of difference between the near and the far. Again as I recall, without having re-read the articles, the theory was that for any given distance between the near and far (as determined from the milimeter scale you've taped onto your camera) there was an optimum aperture size, and that using a larger or a smaller aperture would degrade the image to some extent. If my memory is correct (always a question these days), then what difference does the size of the enlargement make? If you're using the optimum aperture size, it should be the optimum regardless of the size of the print. But, again, I'm probably missing something here. ure smaller or larger

[QT Luong]

Brian: You're correct in saying that the f-stop is optimal, in the sense that it will yield the sharpess possible image at the depth of field limits. However, the larger your focus spread, the less sharp it will be. Sometimes you want to know whether it results in enough resolution for the enlargements you want to make.

[Robert A. Zeichner]

Brian, theoretically, if you view a print of any size from a given negative at a viewing distance that renders it's apparent size to your eyes/brain at the same magnification, the apparent detail or defects will look no different. With larger prints, there is a tendency to get closer to the image. This is where you will begin to see things you might miss in a smaller print. As print size diminishes, you reach a point where you just can't get any closer without optical aids. I've also noticed that defects also become masked by the texture of the paper at those smaller sizes. Another strange thing I've noticed is that other photographers always seem to "get out their loupes" when viewing other's prints.