Ken Durling wrote:
> Just what is it that gives a lens "contrast?" And why should there be
> a difference between primes and zooms? I'm not doubting, I'm curious.
> Sharpness my lay mind can understand, because of the relative
> stability of the elements (at least that's what I think) but
> contrast?
To a large extent, what we consider sharpness in a lens (after its ability to
focus) is actually contrast: the ability of the lens to resolve differences
between light and dark areas. Tests that measure the resolution of lines per
millimeter are in essence tests of contrast (or the ability to resolve contrast
differences). One reason (I suspect) that the 28-70L appears to have such good
contrast is its aspherical front element, which helps focus all the rays of
light at a particular point (or plane)�in this instance "sharpness" and
"contrast" go together. Some soft-focus lenses work by increasing the amount of
spherical aberation present, which softens focus and reduces contrast.
No doubt someone on the list can give a more technically correct explanation of
all this.
What we typically think of as contrast in photos�that is, the differences
between (comparatively large) areas of light and dark�is something rather
different. As I understand it, flare or parasitical light can reduce both kinds
of contrast, and although the kind of loss of contrast we see most easily will
be that which causes an image to look washed out, I believe that flare will also
reduce the lens's ability to resolve fine detail, which is the other kind of
contrast.
fcc
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