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This is Just Another Web Site dealing with the photography terms of Circle of Confusion and Depth of Field. There are plenty of web pages out there on these subjects - I hope this one actually helps you understand something.

Depth Of Field, Circle of Confusion, and other Calculations useful to the Photographer

I fooled around for a week or so trying to modify an HTML on-line DOF calculator to include different film formats and a methodology to compute Circle of Confusion (CoC) that reflected real world experience. The calculator I was trying to modify had problems, was limited in scope and in general proved unsatisfactory for what I was trying to provide.

Then I found f/Calc written by Warren Young

Problem solved !

f/Calc is a downloadable program that offers :

f number arithmetic

Depth of Field

Hyperfocal distance

Field of View

Angle of View

Magnification

Spot meter angles

Help screens and formulas

the film formats include :

APS - 35mm - 645 - 6x6 - 6x7 - 6x9 - 4x5 - 5x7 - 8x10

One of the nicest features of f/Calc is the ability to change the CoC values to ones more suited to your style of photography. Warren's calculator suggests values of CoC, but you are free to change them as you need or see fit. If you want to see one approach to determining CoC, read the following :

How to derive CoC and some suggested values

With my sincere thanks to Bob Wheeler for all his help and patience - any errors are my own. I encourage you to visit Bob's site and read his notes on the subject.

It is generally accepted that a normal human eye can distinguish 5 lines per millimeter at a distance of 25 cm.

If you accept the proposition that a minimum resolution of 5 lines/mm is needed at the normal viewing distance on an 8 x 10 contact print, the circle of confusion for 8 x 10 is equal to the reciprocal of 5 lines/mm or 0.200 mm. Any smaller film format enlarged proportionally to a print size that corresponds to the 8 x 10 resolution means some degree of magnification is required. This magnification means the film and lens used must be capable of a corresponding finer resolution if the viewed image is to be as acceptable as the 8 x 10 contact print. The circles of confusion indicated below are for use when such a proportional enlargement is done.

To determine the proportions, the diagonal measurement for each format was divided into the 8 x 10 diagonal to determine the proportion and then multiplied by the requisite resolution of 5 lines/mm to determine the resolution factor needed.

Resolution Factor = 5(325/d) or simplified to : 1625/d

[ Where 325mm = 8x10 diagonal and d = diagonal of your film format in mm]

Note : Some sources use 6 l/mm and still others use 8 l/mm - it's your choice ( See below )

The reciprocal of the resolution factor then equals the circle of confusion (CoC) figure for that particular film format. The largest print size for each format with no loss of resolution is shown in the table - it does not include any cropping. In practice, images would be sized to fit existing standard paper stock.

CoC = 1/Resolution Factor

A Point to Ponder

If your camera body allows you to change film formats, you should consider the effect the DOF and CoC will have on your enlargements. Examples : interchangeable backs on medium format cameras and 35mm/APS cameras that allow you to switch to various "panorama" prints.

Determining the resolution needed

The first thing you will need is a chart with accurate scales that shows various ranges of lines/mm. The standard lens test chart is the USAF 1951 lens test chart and it is available from a variety of souces. The benefit of getting one of these charts is that you will be able to use it for testing your lens as well. One source is Edmund Scientific, but you can do a web search. Once you have the chart in hand, follow the following steps.

1. Tack the chart on a wall and illuminate it as well and as equally as possible without glare.

2. You need to determine the largest size of print which you intend to make from your negative. The following chart shows some common enlargement sizes and their diagonal measurements in millimeters and in inches. Measurements are rounded to nearest .5" and inches measurement is given only for convenience in case you don't have a metric ruler. Why the diagonal measurement ? The common practice is that a print should be viewed from a distance approximately equal to the diagonal measurement of the print. You can agree or disagree with the practice as you see fit - I'm not going to argue with or even respond to those who disagree.

Now you know the approximate viewing distance from the chart based on your anticipated largest print size. Measure a distance equal to the diagonal distance from the chart to your eye. Observe the closest spacing (most lines/mm) where you can easily distinguish separation between the lines. Write that number down.

3. Multiply the number (lines/mm) that you got in step 2 above times the diagonal metric measurement in the chart for step 2. Write that number down.

4. Go to the chart below and find your film format and the corresponding diagonal measurement of the on-film image for that format.

Divide the number derived in step 3 by the diagonal measurement for your film format. This is your resolution factor. Write this number down.

5. Take the reciprocal of this number (1/resolution factor) - This is your customized value for CoC.

6. With your customized CoC value in hand, you can now go to f/Calc and do the calculations you need for DOF.

O.K. - It's a long and tedious process. It is also a repeatable method, something which hasn't been really explained in plain English or available anywhere I have looked. It follows a logical path instead of intuition or guesswork. It customizes your DOF selection to the results you want, on the print size you want, using your camera and your lens and your film and ... you can just ignore the whole damn thing if you want to.

Email if you see something wrong.