When is 50mm NOT 50mm?

Now *that* is something I hadn't considered (how much deeper the DOF is and whether there's any consistent multiplier).  For fun, I tried the two options at 10' and f/1.8 in dofmaster and the DOF values were different by a factor of 1.5X.  But at 50' and f/4 it's 1.75X so I don't think you can rely on that.

BTW, if you're interested in some technical bits, the way I understand is as follows:

If you were to compare 50/1.8 versus 75/1.8 both on FF, the shorter lens gives you greater depth of field for a fixed focus distance.  That means that depth of field is greater at the sensor plane.

A 50/1.8 on FF at 10' gives you 1.29'

A 75/1.7 on FF at 10' gives you .57'

Now let's consider the 50/1.8 on APS-C.  It's projecting an image onto the sensor with the same DOF at the sensor plane as it does on FF.  Imagine some tiny little details close to the plane of focus.  Everything at the sensor plane looks identical to either the APS-C or the FF sensor.  However, when you make an 8x10 print from the two images, you're effectively enlarging all the details from the APS-C image to a larger degree relative to the size that they were projected onto the sensor.  That means that something that's almost-but-not-quite in focus, but close enough to be considered sharp (in the DOF range) on FF, is a bit more out of focus when enlarged from the APS-C image.  This degree of enlargement difference is expressed as the "circle of confusion" in DOF calculations, and varies with the film/sensor size and the assumptions you make about the size of the image you're viewing and the distance at which you're viewing it.

So the increase in DOF you see on APS-C with a 50/1.8 versus a 75/1.8 on FF is the increased DOF you get with the shorter lens offset to some degree by the greater degree of magnification needed from the smaller sensor.

- Dennis

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