I've often heard it said that a 100mm f/2.8 lens (for example) designed
for a 35mm camera if put on a smaller CCD sensor will become a 160mm
f/2.8 i.e. the 'effective' focal length gets multiplied by some factor
(1.6) in my example and the aperture remains constant. I'm not so
convinced the latter is true.

I make a few observations.

1) A 100mm f/2.8 lens put on a small digital sensor remains a 100mm
f/2.8 lens. The lens remains the same.

2) The smaller CCD sensor means the focal length (as compared to 35mm)
is longer, as everyone agrees.

3) The aperture whilst still f/2.8 is "effectively" larger, as much of
the light is thrown away, missing the sides of the sensor. So the
viewfinder will be darker than if fitted with a f/2.8 lens which filled
the sensor and no more.

At first the digital format would seem to allow long focal, fast
telephotos. i.e. my 70-200 f/2.8 would become a 112-320 f/2.8, which
would be a very nice fast lens indeed. But I'm not so sure the lens
would have the light gathering power of a real f/2.8 lens, but instead
be effectively an f3.5 (I think). I suspect if the focal length is
multipled by 1.6, the apeture will be multipled by sqrt(1.6), although I
might be wrong on the exact calculation.


PS,
Does anyone know if Nikon are developing a full frame (35mm) digital SLR
like Canon and Kodak?? It seems such a move would have a lot of
technical advantages (lower noise) and people with expensive 35mm lenses
would get the full benefit, and not throw much of the light away, which
is what I think would happen now.