A few words on Equivalence and comparing systems

Started Apr 2, 2013 | Discussions thread
Macx Senior Member • Posts: 1,433
Re: A few words on Equivalence and comparing systems

Anders W wrote:

Macx wrote:

texinwien wrote:

olliess wrote:

texinwien wrote:

Just another Canon shooter wrote:

texinwien wrote:

This is your mistake. You think a smaller equivalent f-number equals better performance. That is simply not the case, unless shallow DOF is your measure of performance.

More light is not?


"Didn't want that light anyway."

It's the sensor not the lens, and JACS is talking about measures of lens performance w/r/t equivalence.

But I'd wager you knew this and were just trying to be witty. Unfortunately, you just ended up sounding obtuse.


Keep the sensor out of it. The sensor size is interesting because the FOV of a particular lens is dependant on it, but for the same FOV and the same physical aperture the systems are equivalent and they get the same amount of light. If we had lenses with the same FOV and the same physical aperture for micro four-thirds as we have for 135, there would be no innate shot noise advantage of the larger sensor.

In other words, it's the lenses, not the sensor, and if we had a 12-35/2 our exposure envelope or gamut would be equal to having a 24-70/4 for our 135. The loss of a stop of total light when going from a 24-70/4 on a 135 to a 12-35/2.8 on M43 is dependant on the latter being f/2.8 and not f/2.


It seems that your point (FoV instead of FL, absolute size of the aperture rather than relative size) is a pedagogical one. Your idea, if I understand you right, is that it simplifies things to use FoV and absolute aperture size because you can state equivalence in terms of equalities.

Yes, and because these are optical qualities that define what we want in a lens. It's field of view and it's depth of field and/or diffraction properties. We've learned to make the mental arithmetic needed to translate focal length and focal ratio into these values, but why the need for such translation when we could use these "proper" values as the basis for discussing how a lens behaves?

I see that point but I am not sure I agree that this is the more pedagogical solution all things considered. For example, FL and max relative aperture (max f-stop) are known properties of a lens and independent of the sensor to which the lens is fitted. FoV and max absolute aperture are properties that have to be derived (calculated) from those already known and the first of them (FoV) varies depending on the size of the sensor the lens is mated with.

The known properties are the focal length and the maximum absolute (virtual) aperture. The maximum f-stop is derived from these, not the other way around. It is true that to calculate the FoV we need to know what sensor the lens is used for. Even so, at least we would know it, instead of just having an intuitive understanding about the field of view of a 19 mm lens on a 135, for example.

I don't propose that we keep focal length or the f-stops secret. But these values aren't what is interesting about how our lens behaves on our camera, and especially when discussing lens properties across systems it seems to me to be much more pertinent to concentrate on these values instead of having to work with "equivalent focal lengths" and other thought-experiments.

And from a performance standpoint, isn't the performance of a lens at a certain absolute aperture more interesting that what it is at a relative aperture? I think so, at least.

Both ways of thinking about the matter are of course factually correct. It's just a matter of conceptualization. But in the end, I am not sure that a shift of attention away from FL and relative aperture to FoV and absolute aperture makes things any better from a pedagogical point of view.

Maybe not. The lens manufacturers isn't making it easy for us, that's for sure.

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