TrueDoF-Pro Optical Science

As is the case with OptimumCS-Pro, there’s a great deal of lovely optical science behind TrueDoF-Pro. And, again, you may feel free to ignore it — and your creativity won’t suffer. If you’re interested in how the relevant quantities are calculated, though, here’s a very brief discussion.

Derivation

Much of what one can say here takes its starting point from the information presented on the workings of OptimumCS-Pro (see OptimumCS-Pro Optical Science). In particular, one should consult the works of Hansma, Peterson and Conrad that are cited on that webpage.

In any standard treatment of depth of field (as presented in, say, the first few pages of Conrad’s document), one derives formulae to calculate depth of field, using purely geometrical methods, from the following inputs:

•Focal length
•Focus distance
•f/D ratio
•Diameter of circle of confusion

The approach taken here, however, is to recognise that image blur results from two components — a purely geometrical one (the defocus blur, i.e. the circle of confusion), and one due to diffraction. The total blur size can be determined by the methods discussed in OptimumCS-Pro Optical Science.

For use in a depth of field calculator, we specify the diameter of the blur spot and the f-number, and solve for the focus spread. Once we know the focus spread, and having specified a focal length and focus distance, it is a simple matter to calculate the distances to the objects at the nearest and furthest edges of our zone of acceptable sharpness. We thus have our calculator’s outputs.

Other Matters

There is one other, small twist: Circle of confusion or, in our case, blur spot diameter is not easy for the user to visualise in relation to image quality. TrueDoF-Pro therefore offers the option for photographers to specify image quality in terms of print resolution, expressed in line pairs per mm. This can much easier to visualise and permits confidence when changing its value. (The inputs — sensor size, print width and print resolution — are converted to a blur diameter value for use in calculations.)