For years I have believed the mantra that smaller sensors automatically multiply the focal length of the lens compared to the same lens on a full frame camera. As I contemplate the arrival of my 1Ds MKIII, and ponder adding a 40D, I wonder if that is as simple as it sounds.
If I put my 50mm lens on the 1Ds, I get (or will get) a 21MP image. If I put the same lens on my 1D MkIII I get a 10MP image, and the same if I had a 40D. The argument goes that the image on the 1D and the 40D are respectively equivalent to a 65mm and 80mm on the 1Ds.
But, if I crop a 10MP image out of the image from my 1Ds, so that it is the same number of pixels as the FOVCF images, I would actually have an image equivalent to having a 70mm lens. Thus I effectively have a longer focal length lens than I would on the 1D, and just a little shorter lens than on the 40D.
I think that the real determinant of effective focal length is the pixel size. For the 1Ds MKIII it is 6.4 micro meters, compared to 7.2 and 5.7 for the 1D and 40D respectively. Thus the real focal length multiplier for the 1D is 0.89 and for the 40D 1.125.
Based on this, there is really no advantage in having a smaller sensor if you are interested in shooting with really long focal length lenses, unless it also has very tiny pixels, and then you suffer all the potential noise problems that go along with them.
Or am I completely missing something?
White Balance so easy, even our 5 year old can do it.- Melissa Strickland
You are right, I think. This is the same reason a normal lens for 35mm film is 50mm, and 90mm for 6x7. If you were to put a 50mm lens on a 6x7 camera and crop the image to 35mm dimensions, the image would look the same as a 50mm lens on a 35mm camera.
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