compare IQ/DOF with different sensor size

Started May 2, 2013 | Questions thread
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Christof21
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compare IQ/DOF with different sensor size
May 2, 2013

Hi,

The question may seem very frequent but I would like to have a precise answer, if possible. No need to read my question if you are not interested in the theory.

My question is:

Lot's of articles pretend that having  a smaller sensor, for exemple m4/3 compared to full frame could be an advantage because you have greater DOF. To get the same DOF, you would have to stop down with your FF and consequently loose quality (by compensating with ISO for instance). Even with my APS-C, I have to stop down a lot, which would not be the case with a compact.

May a compact be an advantage in some situations where DOF is important ?

I have a conclusion which concludes that this is not the case:

To compare 2 sensors, we must take the same picture with both cameras. If we take the same picture with the FF  as the smaller sensor:

- it is taken at the same distance from the subject

- with focale multiplied by the crop factor

- with f-stop multiplied by the crop factor (stop down)

In this case, we take the same picture with same field of view, same DOF, very similar indeed.  If we had to stop down, in terms of aperture (diameter of the opening=f/N), it is in fact the same ! So the diffraction effect will be the same in both pictures.

The last point I am not 100% sure:

The quantity of light captured by the larger sensor will be the same as the smaller sensor: the sensor is further from the lens but proportionnaly it is bigger in surface. So the amplification needed for both sensors will be the same (both pictures will have the same grain)

Again both situations seem to be completely equivalent. The only difference is that the larger sensor captures the same amount of light on a bigger surface, so it is better.

So if my reasonning is correct, larger sensors are always better in ALL situations, and this is not correct to list "better DOF" for smaller sensors.

Am I correct ?,

Christophe

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