“Full Frame Equivalence” and Why It Doesn’t Matter

Started Feb 4, 2013 | Discussions thread
Macx Senior Member • Posts: 1,433
Re: “Full Frame Equivalence” and Why It Doesn’t Matter

The reason you should care about total light is because it helps you understand why it's easier to make relatively noise-free cameras with large sensors than with small sensors.

At its heart, it's simple physics, telling us that if you keep the light per area the same (in other words, the exposure) the total signal to noise ratio (SNR) will be twice as good on a 135 format sensor than a 4:3 sensor. That translates into more detail and clearer colours. Having four times as much light is the same as opening the aperture two stops. Look up shot noise for an explanation behind this.

This is complicated by the fact that some sensors are more efficient than others and that there are other sources of noise as well, so it's more of an ideal relationship than a real one. In the OP's article, it is claimed that it's more likely that the SNR on a 135 format sensor behaves as if it has a one and two-thirds advantage, compared to a 4:3, than two.

If we trust the article and go with the the one and two-thirds stop advantage of a 135 format sensor, it tells us that if we take two shots (with identical shutter time), one with a 135 format camera set at f/8 and ISO 400, and one with a 4:3 camera set at f/4.5 and ISO 125, we should get pictures with roughly the same amount of noise.

If we use the "ideal" relationship of two stops, it becomes easier to do the mental arithmetic. In this case, ideally a 4:3 camera using f/4 and ISO 100 gives you the same SNR as a 135 camera using f/8 and ISO 400.

Why might this not matter? Because, the linked article claims: People buy into the system realising that 135 format cameras have this noise advantage already. This is not a unreasonable claim at all, I think, but the thing is that this two-stop advantage plays well into the other relationships between formats:

The reason why, is because of the well known differences in depth of field and diffraction for a 4:3 camera compared to 135 one. If we use the example from above, the f/4 ISO 100 shot on the 4:3 camera doesn't only have about the same amount of noise as the f/8 ISO 400 shot of a 135 camera, it also has about the same depth of field and diffraction, if we keep the framing similar. In other words, for such shots there is no difference or advantage to either the 135 format or the 4:3 format. Is this a case of equivalency mattering or not?

In fact, it seems to me, that equivalency helps us understand that there is no physical disadvantage in using a 4:3 camera as long as you use it inside its "comfort zone". It's when you go outside this zone that the advantages of the 135 format appears: If you want a SNR like the ISO 100 and 200 on the 135 cameras, there is as of yet no equivalent in the 4:3 world. If you want lenses with the same field of view and speed as e.g. 35/1.4, 50/1.4 or 85/1.4 there are no equivalent lenses for 4:3, yet. In other words, 135 cameras allow you a wider "gamut" of exposure than 4:3.

If this isn't needed, there is no disadvantage in using a 4:3 sensor, instead you get the advantage of the smaller size and bulk.

And remember, there are no laws prohibiting you from using the right tools for the job: A 135 camera for the shots that are outside the 4:3 camera's comfort zone, and a 4:3 camera for all the shots that is, where you don't want to deal with the extra weight, size and conspicuousness of the larger system.

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