Hi,
One of the reasons that the 1.4 is more expensive then the 1.8 is the extra half stop of aperture. There is no reason that an 1:1.4 would be better than an 1:1.8 at medium apertures.
Larger aperture needs bigger lenses which actually also are a constraint on design. Higher price may make it possible to use more expensive glass which may be good for performance.
Everything in life is a compromise...
Best regards
Erik
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It's true that all good lenses tend have similar performance at F8 and I would not expect to see any significant differences between the Canon 50/1.8 and 50/1.4 at apertures from F5.6 to F11.
However, when shallow DoF is required, the smaller format needs to be used at wider apertures. If the lenses that one happens to use for that smaller format all tend to have rather poor performance wide open, then one might tend to draw the false conclusion that the larger format, of similar pixel count, (the DB) is directly responsible for that significantly sharper result including this elusive sense of 3-dimensionality, simply because it's larger.
This principle can be demonstrated very graphically by the following MTF 50 charts I've copied from Photozone. They use an APS-C cropped format camera for testing purposes (the 8mp 350D I believe), so centre resolution is very relevant to the results one would get using a 1Ds3 which has the same pixel pitch as a 350D. However, the border resolution, as shown in magenta on the charts, would be much worse on the 1Ds3.
The really interesting points one can glean from these charts, are:
(1) The 50/1.4 at F1.8 and F2.8 is significantly sharper than the 50/1.8 at F1.8 and F2.8.
(2) The TS-E 90/2.8 (at F2.
is significantly sharper than the 50/1.4 at F1.4.
Now, the significant point here is, if I were in a studio using my 5D to shoot glamour pics, using the TS-E 90 at F2.8, I would probably be fairly pleased with the results.
However, if I then switched to the 40D and placed the camera on the same tripod, shooting the same model from approximately the same distance, I would need to use the 50/1.4 at full aperture to get the same DoF.
I predict that the results would be awful with regard to sharpness of eyelashes (although the model might be pleased that her complexion imperfections were suitably disguised).
We have a situation here where, not only is the lens on the 5D significantly sharper at the aperture used, than the equivalent lens on the 40D at the equivalent aperture, but such a lens (the TS-E 90mm) would still give better results on the 5D even if it were
not sharper, because the 5D has wider pixel spacing.
I can predict the results of such a comparison. However, I need to actually make the comparison in practice to get a handle on the
significance of such differences on prints.
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