Aperture fixed when 35mm lenses used on small CCD's??
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. |
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