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| p.2 #8 · 70-200f4L, is IS worth an additional $ 500.00? |
You're confusing magnification of the image at the sensor, which is based solely on lens focal length and distance to subject, with apparent sharpness which is assessed at the presentation size, not at the sensor size. Magnification of the image from capture size on the sensor/film to the presentation size is a critical parameter when considering apparent sharpness.
For example, compare image "originals" taken with the same lens of the same subject, at the same distance, on a FF camera (sensor 36mm x 24mm) and a 1.6x CF camera (sensor 22.3mm x 14.9mm). If you make 8" x 12" prints of both images, the presentation magnification for the FF image is 12"/36mm = 8.53x, and that for the 1.6x CF image is 12"/22.3mm = 13.67x. This effect is illustrated by the attached figure, which shows the same magnification at the sensors for both cameras, but different magnification at a common presentation size.
Clearly, any slight unclarity or unsharpness at the original image size will be magnified significantly more on the 1.6x crop image than the FF image. The difference in magnification between different sensor/film sizes to a common presentation size also affects the depth of field and bokeh.
This magnification effect is often incorrectly called "longer reach", where, "longer reach" is actually a function of pixel density or pixels per duck (ppd), not sensor size. The quality or usability of real "longer reach" between cameras with different sensor density is affected by sensor image quality (IQ) at the per-pixel level. When comparing cameras of about the same vintage, sensors with lower-density (i.e. fewer ppd) usually have higher per-pixel IQ than sensors with higher ppd.
The diameter of the circle of confusion (COC) does indeed vary according to the size of the sensor or film format, for example see http://en.wikipedia.org/wiki/Circle_of_confusion
Of course, the COC concept is somewhat irrelevant when evaluating apparent sharpness at the sensor magnification. In this case, things like "IQ per pixel" and pixel density become complicating factors.
Back to the original point: whatever shutter speed you decide is your personal "best above" speed on a particular camera format, then it should indeed be increased or decreased in proportion to the size of other camera formats, assuming that you're judging acceptable sharpness at a common presentation size, which is typically a print or an 'unzoomed' image on a computer display.
P.S. to further complicate matters, many people (self included) think that apparent sharpness is the preceived combination of resolution and contrast.
P.P.S. I had breakfast while Ian was typing.