In “Depth of Field Repealed,” FDTimes called for comments on “The difference in depth of field between Large Format and S35, comparing lenses with the same angle of view.”
Klaus and Uwe Eckerl, Managing Director and Optical Designer of IB/E Optics in Bavaria, write:
Yes, Circle of confusion really causes some confusion.
In brief, We agree with ARRI (1 stop difference).
A longer explanation:
The circle of confusion scales with the crop factor.
We have to talk about “the same unsharpness in the field of view” – as this is what we are looking for: comparing the depth of fields (in object space!) of two images of the very same object taken with different formats.
It is important to mention these assumptions even if this seem to be trivial.
Assuming we are talking of a crop factor of ~ 1.4 (between Full Format and Super35), this means there are two arguments:
– circle of confusion stays the same, no matter what format => depth of field changes by c^2 (2 stops)
– circle of confusion scales with crop factor => depth of focus changes by c (1 stop)
Klaus and I did a small sketch to make this point more clear. (see above)
Conclusion: comparing ALEXA LF (Full Format approx 36x24m) with ALEXA SXT (Super35 approx 24x18mm), we say that we will have 1 stop difference when crop factor = 1.4.
Or – in general – depth of field goes linearly with the crop factor.
Klaus and Uwe Eckerl
IB/E OPTICS Eckerl GmbH
Passauer Str. 13
T +49 (0)8551 917 65 05
F +49 (0)8551 917 65 06