>>3455232>So you're now claiming that transmission is affected by working distance?No, of course not. I'm not talking about light transmission at all. At a given f/number, any lens transmits the same amount of light regardless of focal length or format (give or take, because t/stops, but that's not relevant to this argument).
I'm talking about "Ability to shoot in low light" and "depth of field". I.e., the reasons you would WANT a wider aperture.
An f/1.7 lens on a m4/3 camera transmits the same amount of light per unit area as an f/1.7 lens on crop or an f/1.7 lens on full frame.
But the f/1.7 lens on m4/3 does not give you the same ability to shoot in low light as the f/1.7 lens on the larger formats, because the larger formats have less noise at higher ISOs. So every step up in ISO that the larger format gives you can be matched with a step down in aperture.
So yeah, if you shoot f/1.7 at ISO 6400 on m4/3 and FF, you'll get the same exposure, but the m4/3 will have as much noise as the FF at ISO 1600. If you want to match the amount of noise, you'd need to drop the ISO down two stops, and that means you'd need an aperture two stops wider (keeping shutter speed the same).
(Again, more or less--m4/3 sensors tend to do a little better than a full two-stop drop in noise performance, so it's not exact. But it's a decent enough approximation for the argument)
And conveniently, depth of field scales the same with crop factor. I hope I don't have to justify that one, too, but I can do so if you really need me to.
So in terms of LOW LIGHT SHOOTING ABILITY (but NOT actual light transmission; i.e., the ability to shoot in a given amount of light with a given noisiness in your image) and DEPTH OF FIELD, a m4/3 system is roughly equivalent to a full frame system with a lens 2x slower.
