Why don't comparisons of aperture take sensor size into account?

Question

When one compares focal length, many times we use the 35mm equivalent length. A 50mm lens on an APS-C sensor camera(1.6x) would be a 80mm equiv. length on a full frame camera.

But when we state the aperture of a lens, I do not typically see the aperture given in terms of the sensor size. I recently read an article that did take the sensor size into account Sony DSC RX100 Hands-on (see the sensor comparison chart).

For example if one is looking at a point and shoot lens they will say that it has a f/2.0 lens, when clearly this is not the same depth of field as a f/2.0 lens on a full frame DSLR. Is there a reason that it does not make sense to regularly compare the aperture range of a camera or lens while taking the sensor into account?

Answer 1

The problem is there are two factors determined by the aperture - depth of field and exposure. Sensor size affects depth of field but not exposure.

With focal length it makes sense to talk equivalents as there is exactly one factor involved, angle of view.

To be correct you'd have to say that a 30mm f/1.4 mounted on a 1.5x crop camera had a 45mm equivalent field of view, an f/2.1 equivalent DOF-aperture and an f/1.4 equivalent exposure-aperture. Which would just wind up confusing every body.

Also focal length is much more important to most amateur photographers who might not understand the influence of aperture but do understand zoom.

Answer 2

Take a look at Can a smaller sensor's "crop factor" be used to calculate the exact increase in depth of field?, where the background is explored in detail. It's also roughly the case that larger sensors allow one to use higher ISOs without as much noise penalty, simply because of the intrinsic physics of having more light-gathering area. And, since multiplying the aperture by the crop factor cancels out the relative terms used in the f-stop, it gives a number more useful for considering when sharpness is diffraction limited. (That is, it comes into play at lower f-stops with smaller formats, because the real aperture size is smaller.)

In short, I think you're basically right that they should include this information — for many purposes, an apples-to-apples comparison includes multiplying maximum aperture by the crop factor.

As to why not: I think it's because marketers know that fast lenses are a selling point, and don't want to downplay that. But for focal length, long lenses ("more reach!") is the selling point, so it gets pushed. In the case of the Sony RX100, saying "28-100mm (equivalent) zoom lens!" sounds good, while saying "f/4.8-f/13 (equivalent) max aperture!" somewhat less so.

Additionally, the equivalent focal length is a directly-useful tool in comparing the framing a certain lens will give. But the aperture stop number is useful in its original form for exposure, whereas the converted equivalence is useful for more obscure things and for technical comparison — but less so directly, because then you have to get into arguing whether ISO ought to also be corrected for sensor size, at which point all of your terms become "equivalent" rather than reality, and everyone's head starts spinning even more than it was at the beginning of the conversation. (And then that gets even more confused, because in the past few years, technology generation is arguably more important than sensor area.) But I don't think this confusion is why it's not used — if something sounds good to marketing but introduces more confusion, they'll do it anyway and leave us to sort it out.

So, while I understand why manufacturers and reviewers don't push this equivalence, I think it is a very useful thing to take into account when comparing gear — at least when comparing gear for a purpose where it's meaningful. It's obviously not meaningful when actually figuring exposure settings.

Answer 3

Aperture stays the same and depth of field too, just imagine if you are using film camera and place mask in front of the film (that way you change "sensor" size) the ISO remains the same and exposure remains the same regardless of "sensor" size, it is only controlled by curtains and lens aperture, the crop factor only useful when connected to the resolution of the "film", we had these problems when we were using small 16mm film Kodak cameras, higher ISO, grainier image, the best print output was 3x5 inches prints.