I've often heard it said that a 100mm f/2.8 lens (for example) designed\nfor a 35mm camera if put on a smaller CCD sensor will become a 160mm\nf/2.8 i.e. the 'effective' focal length gets multiplied by some factor\n(1.6) in my example and the aperture remains constant. I'm not so\nconvinced the latter is true.\n\nI make a few observations.\n\n1) A 100mm f/2.8 lens put on a small digital sensor remains a 100mm\nf/2.8 lens. The lens remains the same.\n\n2) The smaller CCD sensor means the focal length (as compared to 35mm)\nis longer, as everyone agrees.\n\n3) The aperture whilst still f/2.8 is "effectively" larger, as much of\nthe light is thrown away, missing the sides of the sensor. So the\nviewfinder will be darker than if fitted with a f/2.8 lens which filled\nthe sensor and no more.\n\nAt first the digital format would seem to allow long focal, fast\ntelephotos. i.e. my 70-200 f/2.8 would become a 112-320 f/2.8, which\nwould be a very nice fast lens indeed. But I'm not so sure the lens\nwould have the light gathering power of a real f/2.8 lens, but instead\nbe effectively an f3.5 (I think). I suspect if the focal length is\nmultipled by 1.6, the apeture will be multipled by sqrt(1.6), although I\nmight be wrong on the exact calculation.\n\n\nPS,\nDoes anyone know if Nikon are developing a full frame (35mm) digital SLR\nlike Canon and Kodak?? It seems such a move would have a lot of\ntechnical advantages (lower noise) and people with expensive 35mm lenses\nwould get the full benefit, and not throw much of the light away, which\nis what I think would happen now.