Light gathering in M43 vs FF

Started Nov 25, 2012 | Discussions thread
SphericalAberration Senior Member • Posts: 1,052
Re: F/1.4 = F/1.4 on all cameras

tt321 wrote:

Peng Bian wrote:

Just Having Fun wrote:

You will see some slight variations, but if you set your FF camera to F/1.4, ISO100 and 1/60th shutter speed, for the same exposure on an APS, M43, Nikon1, etc, you will need F/1.4, ISO100 and 1/60th.

Ignore the arguments about noise for now because that is a different subject.

I do understand a bit about signal to noise, I think "light gathering" is a poor choice of words. Thanks for the clarification.

There is light gathering per unit area, and there is the total light gathered for the purpose of constructing the images. For the former, yes 1.4 is 1.4 regardless of sensor size and the exposure is the same given the same shutter speed. However, imagine you are going to eventually show the image in the same way, e.g. enlarged to A4 size for both systems. One A4 print will have been the result of 4x the total original quantity of light compared with the other.

The size of the pupils of the lenses are different when both are at 1.4, with the 50mm lens having a pupil four times the area as the 25mm one. So at the pupil one allows in 4x the amount of total light during the exposure, given the same shutter speed.

If your eventual display image from FF is 4x the area of that from M4/3 (same enlargement ratio in both cases) then 1.4 is 1.4 across both systems and there is no advantage/disadvantage either way.

There is one application where physical aperture i.e. pupil really matters :

If you take images of point sources e.g. stars at night, then for a given field of view a dim star will appear brighter with a  50/1.4 lens on a 35mm full frame camera than with the 25/1.4 lens on m43.

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