DOF: A restatement of simplified testing for your needs

Since I brought up the matter recently and, due to a number of helpful answers from members, pursued further reading, calculating, and testing, I’ll offer here a few notes from the experience, for any who may find them useful.

Summary statement: Like so many things, this issue really boils down, as the saying goes, to what you, personally, are looking for in your final presentation. My own interest is just one example.

The context of my concern arose from seeing various, different values for total DOF for a given lens and aperture in a given format. The value given in any such calculation is affected by several variables, and the issues swirling about circles of confusion, resolution line-pairs, human eye resolution, and so on, can appear intimidating to mathophobes like me. I had been familiar with these considerations but dug in with a bit more determination this time, in order to answer a solve a specific problem.

Portraiture with my 4x5 is my primary consideration. My lenses, 210 and 135, are sharp but not late-model super-apochromats with aspherical lens elements, record-breaking modulation transfer function, etc. My interest is character, not skin pores. (I don’t say this to criticize anyone else’s approach, just to identify my own.) My film is not Tmax 100 but HP5+. My prints are silver-gelatin (not scanned and sharpened), made on semi-matte paper not larger than 11x14 and rarely with greater magnification than 3.5x when cropped. I don’t necessarily expect viewers to be getting up close to see details; in any case, a viewing distance of about 24 inches for an 11x14, rather than 17 or less, is more likely. Therefore, the limits of what is perceptible as sharp are for my work less stringent than for someone whose work demands critical sharpness at large magnifications.

After finishing with numbers research so to speak, I performed the test often mentioned by others and which I have performed with other formats: set up a yardstick that has good contrast between numbers and base material (mine is an ordinary wood one with black printing) at a 45-degree angle (halfway between vertical and horizontal), and, with camera standards vertical and zeroed, focus head-on from whatever distance you like on a chosen point in the middle. Careful focus can be important here. Stop down to whatever aperture or series of apertures you choose and make your exposure(s). Enlarge the resulting negative(s) to the size(s) you’re interested in judging, develop and dry the print(s), and then view from the distance you consider normal or think is necessary. This is, in a sense, the reality test, showing a result of all the variables of your equipment, processes, and expectations.

With a yardstick at a 45-degree angle, every inch (or other unit of measure) will recede by less than that actual amount. If my math isn’t failing me here, it will be approximately 0.7 rather than 1; about ¾.

In my test, I chose 6 ft. as my distance, since this is in the range of my typical framing for an upper-body portrait with my 210. I used f/11, because I wanted to confirm whether or not I could get about 6-7 inches of DOF at that distance, rather than the roughly 4 inches indicated by one calculation, which is a bit shallow for my purposes. It would mean the difference between being able to use f/11 and f/16 in situations where the larger opening could be important respecting possible shutter speed.

I printed two strips of the yardstick, one about 15% larger than 8x10, another about 15% larger than 11x14, to allow for cropping in either case. Viewing the dry prints at what I consider average viewing distances for the kinds of compositions I make, I am satisfied that 9 inches along the yardstick -- multiplied by 0.7 = 6.3 — appears adequately sharp for what I want.

As a result of this simple test, I was able to create new values for my reference chart for the bedspread focusing technique (https://www.largeformatphotography.info/fstop.html), values a bit more liberal than even the “admissible f/ stop“ ones given there. My admissible is not necessarily yours.

You can also introduce creative sharpness and blur with a little tilt or swing. At your close range for a portrait not sure how much more/less DOF you would really get.

Watched this yesterday, I will not be using it, but it is interesting what this guy did

Increase Blur to Sharpen Better? - Photoshop Trick

The simplest thing I have ever done to determine depth of field is to put high contrast focusing targets in the composition and then at the ground glass to see if they are in focus. If not I manipulate the camera's movements till they ar . . . .or decide that I cannot creat the image in my head on film.

The targets I have used in the past were bar code panels clipped from product boxes, because they are cheap. You might want to use something more "professional" looking and costly.

I am sure there is an app for that now.