Just had a thought - yes, the sensor size does affect the camera's field of view, but does it also have an impact on its depth of field?
Let's take a 35mm camera (such as the Canon 5D MKII) which has a full frame sensor. If we stick on an 80mm lens set with an aperture of f/4, then the diameter of the iris at this setting would be 80mm / 4 = 20mm. However, with a cropped sensor of 1.6, the same effective field of view would be obtained from a 50mm lens (i.e. 80mm / 1.6 = 50mm). Therefore, with this, the diameter of the iris, again at f/4 would only be 12.5mm, i.e. smaller than for the uncropped sensor. So, to obtain the same aperture diameter of 20mm, the f number would need to be 50mm / 20mm = f/2.5 which is more than one f stop larger.
Looking at it another way, if the aperture of 80mm lens on the 35mm camera was set to 12.5mm (i.e. the calculated diameter of the 50mm lens on the cropped sensor), its f number would only need to be 80 / 12.5 = 6.4.
Or putting it another way, the equivalent f number rises or falls by a factor equal to the SQUARE of crop factor:
f/4 on 1.6 cropped sensor = f/2.5 on full frame
f4 on full frame sensor = f6.4 on 1.6 cropped sensor.
This is very interesting as it means that to get the same depth of field that say a 50mm f1.4 lens would give on a full frame sensor, one would need a 31mm lens with an aperture of approx. f0.55 (1.4 / 1.6^2)!
It can therefore be seen that using a full frame camera yields not only a change in the field of view, but also a decrease in the depth of field.
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