[quote name='Plochmann' timestamp='1333774268' post='17352']
... but I don't think technically answers my question.
Why is it that a smaller imaging circle allows for a lower f-stop number?
[/quote]
1) Lenses with a focal length relatively near to the register distance can be build
without great afford even with larger focal lengths. Typical SLR cameras have
register distances around 40-45mm ... that is why you find a cheap 50/1.4, and that
is also why a 24/1.4 or a 85/1.4 already costs more.
2) The limitations for fast lenses in the telerange are mainly given by the increasing
diameter of the first optical group. A 500mm f/4 already needs a diameter of 130mm.
The optical construction of a 500mm f/2 is not much harder, but results in a lens that
is 4 times heavier. This is obviously more demanding for the mechanical construction
as well as on the stability of the single glass elements.
3) The story is a bit different on the wide side, where the keyfactor is the increasing
angle of view. And here comes the main difference ... a 17mm lens for MFT catches a
much smaller angle of view than the 17mm lens for FF does. The larger angle of view
is one penalty ... the other is the larger register distance of SLRs compared to MFT
which requires a different construction of the lens. With that, you still could build a
17mm f/0.95 for FF, but it would be a very mediocre lens. Nobody would want a lens, that
is not sharp at all (not even in the image center) wide open.
just my 2cts ... Rainer
... but I don't think technically answers my question.
Why is it that a smaller imaging circle allows for a lower f-stop number?
[/quote]
1) Lenses with a focal length relatively near to the register distance can be build
without great afford even with larger focal lengths. Typical SLR cameras have
register distances around 40-45mm ... that is why you find a cheap 50/1.4, and that
is also why a 24/1.4 or a 85/1.4 already costs more.
2) The limitations for fast lenses in the telerange are mainly given by the increasing
diameter of the first optical group. A 500mm f/4 already needs a diameter of 130mm.
The optical construction of a 500mm f/2 is not much harder, but results in a lens that
is 4 times heavier. This is obviously more demanding for the mechanical construction
as well as on the stability of the single glass elements.
3) The story is a bit different on the wide side, where the keyfactor is the increasing
angle of view. And here comes the main difference ... a 17mm lens for MFT catches a
much smaller angle of view than the 17mm lens for FF does. The larger angle of view
is one penalty ... the other is the larger register distance of SLRs compared to MFT
which requires a different construction of the lens. With that, you still could build a
17mm f/0.95 for FF, but it would be a very mediocre lens. Nobody would want a lens, that
is not sharp at all (not even in the image center) wide open.
just my 2cts ... Rainer