Originally posted by falconeye RawheaD, I very much welcome your post. It perfectly explains why a f/1.0 lens for PK mount is so hard to make. So hard that it actually make little sense to try.
However, for the sake of completeness of your argument, let me add that retrofocus designs can escape your limit.
Here, again, my understanding of optics is limited, so I have nothing definitive to say; however, I can perfectly understand how the retrofocus design will allow you to have a lens with a focal length that is shorter than the register distance (indeed, you *need* a retrofocus design for that), I fail to see how it can overcome the limitation I presented. Here's my reasoning:
With a retrofocus design, you have a converging lens element at the rear, which is more or less stationary near the mount plane (with a certain amount of protrusion, as I mentioned above).
Whatever amount of light that is gathered in front of that element
has no bearing on the f-stop of that lens. This is an important concept, because the f-stop is not dicated by the "amount" of light that passes through––if that were the case, any lens will become a super fast f0.1 lens by pointing it at the sun ;-)––but the degree to which it can take the light in front of it, and "concentrate" it on the focal plane.
So let's say you have a ginormous primary element, which gathers tons and tons of light, and the light travels through the barrel of the lens and it hits the rear element of your retrofocus lens. At that point, regardless of how much light has been brought there, it now lies squarely on that very final element to cocentrate that light on the focal plane, and the f-stop of that lens is dictated, again, by that very simple formula
f=F/D
Where the F is going to be a number close to the register distance
sans the protrusion and D will be its diameter. So, again, the very last element and its diameter is going to be the bottleneck, and, in turn, the diameter of the rear element is constrained to the diameter of the mount.
I mean, think about it, if a retrofocus design makes it
easier to achieve fast lenses, (a) wouldn't everybody make all their lenses retrofocus, even longer lenses, and (b) shouldn't we have more 30/1.4s and 24/1.2s, etc?