Depth Of Field

After pouring through a forum request at Luminous Landscape for an explanation of the different depth of field with small sensors (it's more which didn't make sense to the questioner since he knew he had to 'enlarge' the image from the small sensor more), and encountering more wrong answers and irrelevent side tracks, here's my answer which is based on optical formulae, not guesses, assumptions or 'experience'.

Depth of field is simple. The mathematical formula for final sharpness in the print has depth of field inversely proportional to magnification from sensor to print (you make bigger prints, you have less depth of field, you use smaller sensor to make same size print, you have less depth of field), BUT, depth of field is inversely proportional to the SQUARE of the focal length, thus changes in the focal length of the lens used (the actual focal length, not some fudged 35 mm. equivalrent) have a much larger effect on depth of field.

This applies no matter what sensor size you have, whether it's film or digital, contact print or enlargement, sharpness fanatic or prebyopic baby boomer who's lost his reading glasses.

Next time I will show you the formula for hyperfocal distance - that distance you set your camera to have everything in focus from half that distance to infinity - still useful even in these days of digital, autofocus and no distance scales on some lenses.