Originally posted by Raylon FF cameras have less depth of field.

continuing on:

Comparing FF & APS-C: to get the same AOV with the same lens you would need to move 50% further away with APS-C compared to FF.

The total number of photons falling on each sensor is different due to the different sensor sizes. Hence a 1.3 stops better S/N ratio for FF due to its bigger sensor area if both sensors are using the same level of technology.

Now consider this:

100mm FL, f/4, subject distance 10m.

APS-C camera (Pentax K20D) DOF = 1.59m

FF camera (Nikon D3) DOF = 2.41m

Where the reduction in DOF occurs with FF is when we try for the same AOV at the same distance.

Take: 100mm FL, f/4, subject distance 10m.

See AOV calculator here:

http://www.tawbaware.com/maxlyons/calc.htm
AOV (horiz.) on FF = 20.4 degrees.

AOV (horiz.) on APS-C (1.5x) = 13.7 degrees.

To get the same AOV on the FF as on the APS-C (13.7 degrees), both at 10m distance from the subject, we need to use a 150mm FL lens. But now the DOF on the FF is only 1.05m. So we have the same AOV on both systems but, in doing this, we have reduced the DOF on the FF.

To get back the DOF, we close down the FF's aperture to f/6.3 (1.3 stops down from f/4) =

150mm FL, f/6.3, 10m = 1.59m

We're back to the same DOF as the APS-C with

100mm FL, f/4, 10m = 1.59m

So, to get the same AOV & DOF at the same subject distance, we require 1.3 more stops of aperture reduction (f/6.3) on the FF 150mm FL system compared to the APS-C 100mm FL system (f/4)

**which nullifies the sensitivity advantage of FF**.

So, depending on how you qualify it, you could also claim that "

**FF has the same DOF as APS-C**".

This is the concept of "equivalence" in the two systems, FF & APS-C, i.e. trying as best we can to compare apples with apples, as expounded here:

Equivalence
Dan.