Originally posted by falconeye What a discussion ... but maybe, it has to happen some time. So, why not here while waiting for K-3 news
I don't have much to add anymore. Just a few remarks ...
1. Equivalence is ugly if used inconsistently. Otherwise, it is an elegant simplification. Focal length and 35mm-equivalent focal length are two separate terms. And using 35mm-equivalent focal length but NOT 35mm-equivalent fstop or NOT 35mm-equivalent iso is inconsistent too. I do not oppose to discard 35mm equivalency (or any equivalency). I only oppose to use it inconsistently, i.e., to apply it to focal but not fstop and iso. Which is what the mobile phone industry does to hide their mediocre equivalent fstop and equivalent iso specs. I strongly oppose a mobile phone be labelled e.g. 2.8/28mm (equiv.) rather than 14/28mm (equiv.) or its iso be labelled 100 when it is 2400 (equiv.). The latter gives us an immediate artistic understanding of what photos it will take.
2. Post-processing crop or digital zoom or panorama stitching: equivalence is very important here as it allows us to understand what is going on. E.g., Nokia 808 pureview uses its 41 MP to digitally zoom. Equivalence let's us understand how this makes the equivalent fstop slow at the tele end. While Nokia marketing tried to make us think otherwise like it had a constant aperture. Digital zoom creates a constant aperture zoom, but no constant equivalent-aperture zoom. And it is the equivalent-aperture which matters photographically.
3. Sensor size is NOT introduced everywhere when using equivalence. The elegance is that 3 simple normalizations remove it everwhere and it won't pop up elsewhere. Yes, it does stop there.
4. It is correct that lenses which can be mount for different sensor sizes can't be labeled in an equivalent way. So what?
5. Lenses of the kind we expect to emerge in the future, i.e., with an embedded sensor (digital lens cap, GXR style), are best labelled in an equivalent notation only. As we may mix a dozen sensor sizes with a single body. Which is the ultimate reason why I am convinced that equivalent labelling of lenses and cameras will become the norm. Maybe not tomorrow, but within 20 years.
6. Angle as FoV... I don't like it. Focal in mm describes better than an angle how perspective becomes strange as we approach a 0mm focal. Moreover, 35mm-equivalent terms can be used just like we are used to, like in DoF calculators, exposure meters etc. If angles were more practical, they would have replaced the focal length already.
1. You can insist it's elegant, and I will still think it's ugly.
I'll say again, you're opposing the manufacturers using a standard (ISO) way of computing the sensitivity, instead of making up some fake numbers, and you are opposed.to the metric system and the basic optics notions... a thing I will never agree with.
2. Who cares, when one can simply look through the screen and see what happens? You want to force them to think about equivalent sensitivities and apertures, but people just don't care about it. What they care about is just how much of the scenery fits into the frame.
3. Oh, but it is. Everywhere. The focal length would be normalized with the crop factor, the aperture and ISO, normalized as well. You would be unable to tell (unless you can revert the equivalence calculations) anything about the image as projected on the sensor's/film's plane.
4. You have no problem with such inconsistencies?
5. With point&shoots and fixed lens+sensor modules, it sort of makes sense (because they're too limited anyway). But try to use a lens on multiple formats... and watch your theory failing. Behold, the prime lens whose focal length changes in an instant! (and the funniest part, you'd be able to do it in post). An extension tube would magically change length in the same manner, defying physics and the metric system.
6. You don't like the angle of view concept?
It never was intended as an alternative to the focal length, and I don't get it why you think it's competing with it.
Perspective is only influenced by your position in relation to the subject; not by focal length.
By the way, what's the "equivalent" focal length of a 50mm lens which is unmounted, sitting on your desk?
(please note that I'm not telling the format for which the lens was designed, nor on which camera(s) it was used in the past).