FX lens on DX body - impact on aperture not just field of view

[William W]

However, if you use the exact same lens on both bodies, same aperture and same focus point, will it not project the exact same light (i.e. image) onto both sensors? The only difference being, some of the light being "missed" and not recorded around the edges of the smaller APS-C sensor. How can the DoF be different? Or is that not how it works?

No. that is not how it works.

Maybe this will help:

Just forget for a moment all about using the same lens on different cameras and forget about using the same aperture etc.

Just consider ONLY that you have two different cameras in your hand.

Now think about the basic principle for the DoF Calculation. The calculation is predicated upon getting a number (being the circle of confusion), by looking at a “standard’ enlarged image and viewing that image from a ‘standard distance’.

Now think about a10x8 print as we look at from 18". Obviously how sharp it needs to be to “look in acceptably sharp focus”, will differ from person to person, but a typical value might that a ‘dot’ must be be 250 microns - 1/100"

So for our purposes we will make a 250 micron ‘dot’ is equal to an ‘acceptably sharp point’ in the 10x8 print.

Now think about the size this represents in the original negative or the digital sensor.

Obviously it depends on how much we have to magnify our original image in order to get to the 10 x 8 print.

This the whole base of the matter: if we enlarge a 35mm negative or sensor image (24mm x 36mm) to a 10 x 8 print we have to magnify it by a factor of about 8 times. So the size on the negative, that would give a 250 micron spot on the print is 250/8 microns. And that is how we calculate the Circle of Confusion for a particular format or negative size.

BUT - when we look at 10x8 print made from an APS-C camera, we will have to magnify it by about 13 times and so the CoC for an APS-C sensor is about 250/13 micons.

SO - the CoC gets SMALLER as the sensor or negative size gets smaller.

SO - when we make COMPARATIVE DoF calculations - to COMPARE different the DoF for different sensor sizes: we must keep all the OTHER parameters the same. That is to say we must assume the print will always be 10x8 and the viewing distance will be 18” and all of have the exact same eyesight.

WW

[D L]

Thanks Bill.
Thinking from the opposite end of the process makes more sense.

[William W]

Good. You're welcome.

[tclune]

I am happy to just accept the concept and go on taking pictures, but now the cogs in my mind won't stop spinning.

Welcome to my world...