Focussing (again!!!!!)

[Michael Phifer]

I have been using the method and values that Robert Zeichner describes, since I read about it a few years ago in Photo Techniques. I thought the focus spread, a perture values (0.7mm=f16; 1.3mm=f22; 2.7mm=f32; 5.4mm=f45) were for the optimu m sharpest possible. However, because of reading Patrick's comments, I was not s ure. This made me hunt for the original article describing the View Camera Focus ing Method. (From my findings below it appears that Patrick is off by about a fa ctor of two in his defocus circle of confusion. You should get about a 10% sharp er photo using the above table as is, than if you divided by 0.6 as Patrick sugg ests)

In the March/April 1996 issue Paul Hansma's (excellent) article does describe an "Optimum f-stop Method" - that is, the f-stop (N) that makes the resolution (R= 2/C) as high as possible for a given focus spread (dv). Mr. Hansma shows that th ere is an "optimum" f-stop based on the balancing of the diffraction circle of c onfusion (Cdiffraction =N/750 ) , with the defocus circle of confusion ( Cdefocu s = dv/(2N) ). The resultant combined (diffraction & defocus) circle of confusi on (C) is the square root of the sum of the squares of the diffraction and defoc us circle of confusion { C=SQRT((N/750)^2+(dv/2N)^2) }. If one cranks some numbe rs, it can be seen that there is an optimum f-stop which occurs, when the diffra ction and defocus circle of confusions are equal. Mr. Hansma (fortunately) did the calculus for us and the Optimum f-stop, (N)= SQ RT(375*dv).

Please don't let the math scare you away. All you need to know is the method tha t Robert Zeicher described so very well above. It has helped my photographs imme nsely. (Say, "good by to Hyperfocal", and "Thank You" Mr. Hansma !!!!)

Mike Phifer - Just an Amateur

[Glenn Kroeger]

I agree with Robert Zeichner's answer in principle, and use that technique, although I have found myself modifying it due in part to dissatisfaction with the results and in part to a different perspective on depth of field offered by Harold Merklinger in his books and articles.

Harolds writings can be found at:

http://fox.nstn.ca/~hmmerk/HMbook14.html

and represent a tour-de-force in mathematical analysis of geometric lens behavior. My problem, and Harolds original problem, was the impression that infinity was not sharp enough even when DOF numbers indicated it should have a small enough circle of confusion. Harold's math points out that even though mathematical depth of field extends further behind the plane of best focus, perceptual depth of field often favors nearer objects. He suggest always focusing at infinity (for no movements). My take on this is a bit different. I think the perception is due to smaller detail in the distance. In a typical landscape shot, there is not much small detail in a nearby blade of grass, it's close so it's pretty big, but the eye wants to see all of the tree branches on the distant slopes. What I have found works well, is to find the near and far focus points, but set the focus 2/3 of the way toward the FAR point, then using a smaller f-stop based on 4/3 the distance the standard moved to catch the near. This results in the DOF extending somewhat beyond infinity which some would find a waste of good DOF. I find it makes distant images conform to a higher standard of focus that my eye seems to demand.

[fw]

Glen - interesting response. Could you please elaborate on how you compute your f-stop - i.e. how you convert 4/3 of the distance into an f-stop equivalent?

[Glenn Kroeger]

fw:

It's pretty simple, and I don't use a calculator! Take the numbers from Robert. Lets say the travel from far to near is 0.7mm. With Roberts table that suggests f/16 and move the focus 0.35mm back toward the far. I would move the focus 0.5mm back toward the far, about 2/3 of the way, then set the aperture as if the total travel had been twice that (0.5 * 2 = 1.0mm). So I would end up using an aperture about halfway between f/16 and f/22. So it's like using a tighter circle of confusion but biasing the focus toward the far.

Glenn

[fw]

Glenn ; Thanks, I understand what you're doing now. In about 50% of my photographs, I can get the movements set in such a way that almost everything of importance is in focus anyway, even at full aperture, and then I would normally operate at f/16 or f/22, depending on wind/shutter speed required etc. What you have described probably applies in about 30% of my photos, where I cannot get both near and far fully in focus, and I'll try this technique out. There is a third category where I can get far and near in focus, but the middle ground of the image stubbornly refuses to get sharp, as it is in a different plane to the plane of focus. So far, I have simply relied on small apertures, f/45 or f/64, to guesstimate my way through, with mixed results. I'm now wondering if it would be better to get near and middle in focus, and then stop down to f/22 or f/32 to get the far in focus.

[Robert A. Zeichner]

In response to that last post, the situation you described where near and far are in focus, but middleground is not is one of those instances where the apparent subject distance and the true optical distance are different. Think of it this way: By tilting you've established a plane of focus that is no longer parallel to the film. As such, objects in front of or behind that plane of focus may in fact be above or below it! Take the example of a scene where you have a small bush in the foreground and a mountain in the background. The ground between is for purposes of this illustration, flat and level. The nearest and furthest objects in the scene that you wish to have sharp are the bush and the mountain. So, you tilt the rear standard backward to establish a focus plane that intersects these two objects. But where along the vertical rise of the bush and mountain do you place this plane? If you were to pick a point about one third of the way down from the tops of each and use those to points to anchor your new focus plane, you will also be establishing a one third forward (think above) and two thirds behind (think below) relationship that an appropriate amount of depth of field will work for you to bring everything else above and below in acceptable focus. So, where is the optically furthest point from the film? If you guessed the base of the mountain, you've got it. It may seem like the middleground, but it's really the new "background". I hope all this makes sense. It's so difficult to draw this diagram with words alone.

[Glenn Kroeger]

Robert just took the words out of my mouth (or keyboard). The "far" isn't always the farthest thing from the camera location. By the way, I think the numbers in Roberts table are cm not mm, since 0.7mm is a pretty small focus shift!