At 14:19 1/6/02, Joel Wilcox wrote:
[snip]
The thing we have to remember is that distinctions between an A and even a
C rating in Gary's tests are subtle for most of us mere mortals. We talk
about these results as though they are things seen and known, but most of
us will not see them easily or at all.
Joel has a good point to which I'll add a caveat:
How much worry should be spent about "MTF" grading depends on film
granularity and level of enlargement. Even if Gary did the testing
aerially with the lenses dismounted to measure a pure lens MTF, that would
be the lens alone. In a photograph, it's the entire *system* MTF that
counts. This includes confounding by the effect of *film* MTF. If making
prints, it is further confunded by enlarger lens and print granularity! In
projecting to a screen, the projector lens likewise has an effect. [This,
BTW, is a good reason for having an excellent projector lens; it *makes* a
difference!] When using anything but extremely fine-grain film, the
difference from A to C likely cannot be detected even when looking at the
film itself (transparency or negative; not a print which further confounds
lens MTF).
I don't worry down to about the B ratings. At B- and below I know it may
be a little softer; a slight loss of extremely fine detail. Reason? I'm
often pushing limits to create images capable of enlargement to 8x12 and
11x16 prints from the finest grained ISO 50 to ISO 100 films. However, it
also requires tightening up all possible sources that degrade image sharpness:
- using a tripod
- tripping shutter in a manner that doesn't induce vibration
- being more exacting about DoF boundaries,
then selecting aperture and setting focus appropriately
- selecting labs that convey maximum sharpness from film to prints
.
.
.
When making images I know are destined for small prints I don't worry
nearly that much about system MTF other than using a good lab.
Instead we'll be looking at a facial expression, something that indicates
a tripod should have been used, the rendering of the the blue of the sky,
etc. (which is as it should be).
Joel W.
This is the other half of what makes or breaks a good photograph, and for
most it's the first thing evaluated! A weak photograph with maximized
system MTF is a highly resolved, high contrast, weak photograph.
-- John
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