I finally 'get' equivalence

Started May 22, 2012 | Discussions thread
texinwien Veteran Member • Posts: 3,326
This doesn't look right

Thanks for the graphic, but I think it's incorrect. As a thought exercise, consider using the same exact 24mm prime lens at the same exact aperture value on both cameras - one FF and one 4/3.

On the FF, this is a 24mm lens. On the 4/3, this is a 48mm equivalent. On the 4/3, at the same aperture, only the central quarter of the light stream that would have hit the FF sensor is going to hit the 4/3 sensor (that's why it's an equivalent - not all of the light that would have hit the FF sensor will hit the 4/3 sensor from the same lens at the same aperture).

Now, if you open up the aperture by two stops, you quadruple the amount of light that's let in. You'd need to do this on the 4/3 camera in order to get the same amount of light onto the sensor that you got on the FF sensor at the original aperture value.

This is why you get two stops of extra DOF on a 4/3, and also why shallow DOF is more limited on a 4/3 body, and also why the 4/3 gets as much light per sensor area as the FF sensor when using the same lens at the same aperture value.

The 'problem' for 4/3 comes at both ends of the spectrum. On the wide open end, you can only open the aperture so wide. Let's say 0.95 is the theoretical maximum - that's the same theoretical maximum whether the lens is on a FF or 4/3 camera, but the FF will get 4X as much light as the 4/3 at this (or any) aperture.

4/3 suffers on the other end because its photoreceptors / pixels are smaller than an FF's, considering two sensors with the same number of pixels. Larger photoreceptors lead to less noise and higher DR.

So, in the middle, 4/3 and FF are going to be closer to equivalent, but at the ends is where you'll notice the difference.

Fygaren wrote:

Light gathering is based on FOV and aperture, and 4x light does not hit a full frame sensor compared to 4/3 as the OP said. At the same aperture the exact same amount of light hits the sensor, that is why the aperture area of a 4/3 lens is 1/4 of the area of a FF-sensor(same FOV). So in fact, 4x more light hit a 4/3 sensor when you compare it with the area of FF (with same aperture and FOV). Ill try to illustrate this:

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