Depth of Field calculation in the field

[Leonard Evens]

I recommend the focus spread method in one of its variants, as noted by Emil and Eric. It avoids thinking about the distances in the scene, which are really irrelevant for most purposes if you know what is happening on the rail.

The trouble with many methods is that they don't make the dependence on the circle of confusion explicit. The calculator or table makes a choice for you, and that determines the result. But you may wish to use a different value, based on your requirements.

Often, for distant subject, the only thing you really need to know is the hyperfocal distance. If you focus on the hyper focal distance, everything from infinity down to half the hyperfocal distance will be in focus. The basic formula for that is

hyperfocal distance = square of focal length divided by the product of the f-number and the coc

(Focal length and coc should be measured in the same units and the answer is in those units.)

For 4 x 5 a common choice for coc is 0.1 mm, but many people prefer a smaller value. As an example suppose you are using a 90 mm lens and you choose a coc of size 0.1 mm. Then the hyperfocal distance at f/16 is

90 x 90 / (16 x .1) = 8100 x 10/16 = 81000/16 = 5062.5 mm ~ 5 meters

Note that dividing by 0.1 amounts to multiplying by 10, which simplifies the arithmetic. This is easy to find in the field with a simple calculator, but it is easier to calculate the hyperfocal distance for each of your lenses for one particular f-number such as f/16 and store the result in a simple table. Then to find the hyperfocal distance for another f-number, just multiply by the ratio of the f-numbers. If you remember that successive f-number ratios are always the square root of 2 (about 1.4), this is easy to do in the field, either with a calculator or mentally. For example, if I wanted to know the hyperfocal distance in the above example but at f/22, I would add 40 percent to the value to get 5 + 2 = 7 meters. If I wanted to know the hyperfocal distance at f/32, I would multiply by 2 to get 10 meters.

If you need it in feet, the converstion factor to go from meters is about 3.28.

If you need to find the near and far DOf limits, for distant subjects, here is how you do it.

First find the product of the hyperfocal distance and the focusing distance.

For the nearest distance in focus divide that product by the sum of the hyperfocal distance and the focusing distance

For the farthest distance in focus divde that product by the difference of the hyperfocal distance and the focusing distance. But if the focusing distance is greater than the hyperfocal distance or equal to it, the farthest distance is infinity.

For example, suppose the hyper focal distance as above is 5 meters, and you are focusing at 3 meters. First multiply the two to get 15. The near dof limit would be 15/(5 + 3) = 1.875 meters, and the far dof limit would be 15 /(5 - 3) = 7.5 meters. Everything from 1.875 to 7.5 meters would be in focus if you focus at 3 meters. It works the same in feet, but you need to give the hyperfocal distance in feet.

These calculations can be done with a simple calculator. Some of us old timers who were taught mental arithmetic in the days before calculators can even do the arithmetic closely enough for practical purposes in our heads.

For near subjects, it is better to use a method based on image magnification rather than distance to the subject. That is also not too difficult, but I will leave it for another day.

[Ellis Vener]

I'll also endorse the Rodenstock DOF/Schempflug calculator. I'm a very satisfied customer of this fast to use and accurate tool. unlike a mere calculation program on a Palm or similar device it helps you to previsualize the effects of different apertures and how cshoosing different different standards of what is considered sharp for different size formats (AKA "circles of confusion") will effect the final outcome.

[kreig]

Kodak used to publish a small book that had a wealth of information inside about films, exposures, color balance, etc., and a page containing dials for depth of field calculations for wide, normal, and telephoto lenses, for formats from 35mm to 8x10. Very nice and handy. Not sure if they publish these any more, but you might be able to locate on the used market. Since I am currently overseas, I dont have direct access to this booklet and the title, my memory escapes me.

[Michael Kadillak]

I used to concern myself about hyperfocal tables and DOF calculations probably because I am en engineer . Then a seasoned photographer asked me what the ground glass was telling me when I focused and forced me to stopped while watching the image on various critical points. All of a sudden I realized that the ground glass is like good truth serum. If it ain't on the glass, it ain't there - period. Since then I did not waste any valuable time looking for a table or screwing with calculations or estimating anything. If you are in a warm studio without Mother Nature to deal with then have at it. If you are in the field then efficiency is the name of the game and I would much rather use a visual criteria for having an image dialed in than rely upon any set of empirical algorithms.

Stopping down and just doing the basics like watching the ground glass I feel must be a lost art when I see folks talking about designing a F2 or F4 large format lens. Highly unlikely you would be shooting at these apertures and if you are going to do your focus at f45 or f64, whats the point?

Just my $0.02.

[Dan Fromm]

Michael, I'm with you about 95% on the need for fast LF lenses.

On the one hand, I normally shoot 2x3, which some would say isn't really LF, at f/11 or smaller. On the other, I have a couple of fastish normal or longer lenses (4"/2, 12"/4) that I use on a 2x3 Speed Graphic. I love their brightness for focusing. Against this, a moderately fast longer lens, e.g., my 14"/5.6 Aviar, is just too heavy to carry any distance at all.

I'm also with you about 95% on the need to look at the GG rather than tables or a calculator. But only 95% because there are, believe it or not, blind photographers. I bought that 4"/2 from an Englishman who had a long career as a professional photographer and who was legally blind. Of course, he wasn't entirely sightless.

Cheers,

Dan

[jantman]

I'm a fan of Bob Wheeler's Vade Mecum. I had it on my TI-89 calculator, and now have it on my Kyocera Smartphone (Palm-based). It's a great program. I have to keep a separate text note to remind me of what all the abbreviations mean.