Circle Of Confusion computations ... confuses me!

[11august]

Hello!

I'm actually in the process of building a camera database and would like to include in my next update a fixed value for what I would call the "minimum circle of confusion" for each camera.

I've found so much different and even contradictory computation methods that I'm not even sure if it's finally possible to put a "fixed value" for the CoC.

In the forum here, there was in 2015 a discuss about this matter and Phil said:

The calculation of the circle of confusion is somewhat subjective.  ExifTool uses D/1440, but some other utilities use D/1750 or something else

The problem is calculating the focal plane size (the "D" in D/1440).  This is calculated from the FocalPlaneResolution tags and the image pixel size.  The FocalPlaneResolution tags should really be scaled if the image is resized, but I know of no image editor that does this.

Alternatively, it may be calculated from FocalLength and FocalLengthIn35mmFormat, but only if both tags are written to the image.  You can probably add FocalLengthIn35mmFormat to your image to fix this.

I have some questions about the choice of the values.

1- Why the choice of 1440 especially? It could be 1450 or 1430 for instance. I mean do you know from where this specific value came from?
2- Do you have the exact formulas that gives "D" using the aforementioned tags?

Thank you!

[Phil Harvey]

You can see the formulae used by ExifTool in the source code.

The value of 1440 is completely arbitrary, and I chose this value only because it was the most common value I saw at the time I wrote this code.

- Phil