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EXCERPT page containing first few paragraphs. 2017-04-30 15:46:25

Format-Equivalent Depth of Field and F-Stop

The “look” of medium format images vs 35mm DSLR images is often cited as a reason to shoot medium format. But with respect to geometric parameters of optics there is no difference attributable to format size in the “look” of an image.

When the entrance pupil diameter and position (distance to the subject) remain the same, there is no difference in the “look” of an image by format size. The entrance pupil thus fixed in diameter and position determines the depth of field and the “look”, not the format size. The corollary is that focal length must change (for the same field of view on the format), and therefore the f-stop also changes by the definition of f-stop (entrance pupil diameter / focal length).

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Lens diaphragm: the entrance and exit pupils can be seen by looking through the front or rear of the lens