Crop Factor, Low Light and Aperture with m4/3 lenses?

Started Jun 15, 2014 | Discussions thread
tokumeino Senior Member • Posts: 2,534
A page worth reading

Photograpic equivalence is well defined here http://www.josephjamesphotography.com/equivalence/. A video (just forget about the guy speaking) was linked in the begining of the thread : https://www.youtube.com/watch?v=DtDotqLx6nA

The idea about all that is :

  • ISO and f-numbers are easy to manipulate when you don't change sensor size
  • They are misleading measures when comparing formats
  • The important thing is light : the amount of light falling on the sensor, regardless of its size

The last point is the important one : you won't get equivalent pictures if you don't collect the same amount of light. And the point becomes : what parameters are required to collect the same amount of light with FF or M43 ? You can do the maths : if you apply the crop factor to the FL to get an equivalent FOV, you have to apply it to f-numbers as well. Indeed, a FF Xmm f/N lens is no way equivalent to a M43 (X/2)mm f/N one, since only the quarter of the light is collected by time period. To collect the same amount of light, a M43 sensor requires a (X/2)mm f/(N/2) lens. And since you have f/(N/2), to keep the same exposure, you can lower the sensitivity and divide it by 4. There's no free lunch !

That's why it is completely meaningless to compare different sensors formats at same ISO :
a M43 has to be compared to a FF at ISOx4, and to an APSC at ISOx2. So, it is right to compare

  • OM-D at ISO 800 to A7 at ISO 3200, or
  • OM-D at ISO 1600 to A7 at ISO 6400, or
  • OM-D at ISO 3200 to A7 at ISO 12800

Somehow, sensor size does not matter that much : all that really matters is the lens. And to let a lot of light come in, it has to be large, whatever the sensor behind (M43 or FF).

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