I've been wondering why there is no converter that does the opposite of a tele-converter; that is, demagnify the image so that a standard lens image gets shrunk to fit a cropped sensor. I'm not referring to wide-converters that are attached to the end of the lens, but a converter that sits between body and lens. Doing this would not make sense for full frame cameras as the image circle will become smaller than the sensor, but for an APS-size sensor this should allow one to capture the full field of view for the nominal focal length of the lens. At the same time you would gain one F-stop for a 1.4 fold decropping and a bit more for 1.5x or 1.6x decrop on Nikon or Canon lenses, respectively.
I expect there is a simple technical or practical reason that this can't be done but I just can't figure why a 1.4x demagnification would be more difficult to accomplish than the 1.4x magnification of a teleconverter.
Any ideas?
Bart
