Pixels and Sensors

I was recently offered the following equation to calculate depth of field: 
TDF = 2 * C * F * (M+1) / (M*M)
Here TDF is total depth of field (both sides of plane of best focus).
F is the nominal F-stop
M is the magnification
C is the size of the "circle of confusion".

A suggested value for the circle of confusion value is 1/500 of the sensor diagonal dimension. 

Full frame sensor 24mm by 36mm	(diagonal is 43.3mm)
Canon APS-C sensor 14.9mm by 22.3mm (diagonal is 26.82mm)
A value of 0.02 is suggested (10/500), which yields (for the MPE-65 lens at 5x and f/4) 
TDF = 2 * .02 * 4 * (5+1) / 25 = .0384 (38 microns)
This is a good starting guideline for focus stacking steps.

There is more on this topic in the thread:

Also take a look at:

This review of the Canon 5Dii has a table of sensor pixel sizes and DLA values:

Some important values: 
Canon 7D   has 4.3 micron pixels and a diffraction limited aperture of f/6.9
Canon 5Dii has 6.4 micron pixels and a diffraction limited aperture of f/10.2
Canon 20D  has 6.4 micron pixels and a diffraction limited aperture of f/10.2
Canon 5D   has 8.2 micron pixels and a diffraction limited aperture of f/13.2
A couple of observations on these values. First, the Canon 20D and the 5Dii have exactly the same size pixels, the 5Dii just has a lot more of them. Secondly, the Canon 5Dii has bigger pixels than the current Canon cameras with a "crop factor" (the Canon 7D is chosen as a representative sample of these). This means that the 5Dii is more forgiving as far as the diffraction limit is concerned. For purposes of focus stacking, this means that with the 5Dii, I can probably use bigger focus stacking steps than I could with a camera like the 7D.

This raises the question of whether a sensor with small pixels like the 7D is "better" as far as capturing more resolution. The answer is that it could be with a lens capable of delivering images that were no already limited by lens issues. In general though, the entire system (lens and sensor) would need to be evaluated.

Feedback? Questions? Drop me a line!

Tom's Mineralogy Info / tom@mmto.org