Although the 17-55mm F2.8 IS is the full frame equivalent of 27-88mm F4 IS
The 17-40mm is 27-64mm F4 on crop
Just to avoid some confusion:
The 17-55mm f/2.8 IS is the full frame equivalent of 27-88mm f/4.5 IS.
The 17-40mm f/4 is equivalent to 27-64mm f/6.4 on crop.
You don't lose a stop and a half by cropping. You must mean 'as it pertains to DOF'. Just to avoid confusion.
Furthermore, I think this example can't be quite right for even DOF. Note that the o.p. says:
The 17-55 is the full frame equivalent of 27-88 f/4.5 IS; AND
the 17-40 f/4 is equivalent to 27-64mm f/6.4 on crop
but how can the equivalent f/stop for DOF purposes increase both when you move to full frame and when you move to crop? Or am I missing something?
Oh sorry! I wanted to use the same words as tomscott. He certainly understands the conversion, I think he had juste typed too fast
. A longer statement would be:
The 17-55mm f/2.8 IS is the full frame equivalent of 27-88mm f/4.5 IS on crop (I ignored the "on crop" in the previous post because the 17-55mm is a lens for crop bodies).
The 17-40mm f/4 is the full frame equivalent to 27-64mm f/6.4 on crop.
@3kramd5: thanks for the added detail, yes, I meant DoF equivalent.