Is my thinking about equivalence right?

Started 3 months ago | Questions thread
Mark Ransom
Mark Ransom Veteran Member • Posts: 7,669
Re: Is my thinking about equivalence right?
8

Thomas Traub wrote:

First: equivalence comes from "equivalent" which means "equal" - but equal in which aspect? 99 % of the "equivalence-dwarfs" only compare DOF. And yea: DOF is different between FF and MFT. MFT has more DOF - that can be good or bad. Good if you want more DOF like a street-photographer. Bad if you want to make portraits. But this difference is not good or bad for itself, it is only different. And YOU decide, whether this is and advantage or a disadvantage for you. (But sometimes it is the only argument why FF is better)

Don't compare a system only based on one fact.

This is wrong. Equivalence is not just about DOF, it's also about noise. Truly equivalent systems will have the same amount of noise.

Second: if you catch the light of one scene with a specific angle of view (xxx mm in FF or yyy mm in MFT) than the whole light of this scene/angle of view comes into your lens on your sensor. It depends on lens+sensor (they have to work together) but the whole light of the scene is projected on the sensor. If you have a FF-Sensor the whole light is projected on a 4-times bigger surface than on a MFT-sensor. So the bigger surface get's the same light that a MFT-sensor will get - so it is brighter on the smaller sensor But the pixels on the smaller sensor are much smaller and the pixels on the bigger sensor are much bigger - and the bigger pixels are more capable - but the smaller pixels get more light .......... So you have bigger pixels that are more capable that get less light (each pixel) on the bigger sensor and smaller pixels (they are less capable) that get more light (each pixel) on the smaller sensor. That results in a smaller advantage in dark light because the more capable bigger sensors can make a slightly better work in low light.

To get the same field of view on the mft sensor you must use a lens with half the FL. The aperture diameter is proportional to the FL, so the mft gets less total light. This exactly counteracts the higher concentration on the smaller sensor, so you get the same light density unless you use a larger aperture (lower F-stop).

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