Why might this be so?
1. There has been a great deal of progress in lens materials, such as
improved glasses, design, with computer aid, and manufacturing
techniques, such as asymetric surfaces, over the whole period of the
development of photography.
2. Lenses have been getting faster overall over that period.
3. The economics of the business are such that considerably more has
been spent on quality in the premium lenses in any manufacturers line,
so that they are not only faster, their most obvious characteristic
(other than price!), but include other more expensive design,
manufacturing and material features that improve performance.
So in the real world, slower lenses are on average older and/or cheaper
designs than faster lenses and the generality from Photosig is often
accurate.
As Dizel has pointed out with several examples, this is not always true
in practice. It is certainly not true in theory if each lens is the best
that can be made with the same technology. However, in the practical
world of commercial camera lenses, it may be true enough to perhaps
justify it as a general rule subject to individual confirmation.
In the Zuiko world, it seems to be true in some cases. The 28/3.5 was
replaced by the 28/2.8, which was augmented with the 28/2 and, at least
in Gary's tests, the generality seems to hold. Although the focal
lengths don't exactly coincide, the faster Zuiko telephotos seem to
outperform their more ordinary siblings in all respects in Gary's tests
and comments on the list. Another factor in overall performance is the
floating element(s) that move during focus to improve close focus
quality in many of the f2 series of Zuikos.
It is not always true. In a post some time ago, I compared 50mm Zuikos
based on Gary's tests:
---------------------
I know these comparisons aren't really valid at the 1/3 grade level, but
just for fun... I converted all the resolution grades to numbers, A+=9,
C-=1 and the contrast grades to numbers, VH=5, ML=1. I then added up the
two resolution scores for each f-stop for each lens and selected a
winner based on high score. I then added the contrast scores to the
totals. Since I use only one contrast score and the contrast numbers are
lower, this total is still heavily resolution rated.
Here are the winners based on the exercise:
f-stop Resol. R & C
f/1.4 f1.4 f1.4
f/2 f1.4 f1.4
f/2.8 f1.4 f1.4
f/4 f1.8 f1.8 (scores of all lenses basically a tie)
f/5.6 f3.5 f1.8
f/8 f2.0 f2.0+f3.5
f/11 f3.5 f3.5
f/16 f3.5 f3.5
f/22 f3.5 f3.5
Now, some of the scores are too close to be meaningful, but I think you
see the point, the f1.4 is the best general use lens.
--------------------------
Of course, the 50/3.5 is not a cheap, slow lens, but an example of what
happens when a slow lens is built to top quality. (Gary has said later
that he doesn't think the results for the 50/2 at f2 & 2.8 in his test
are up to the performance of most 50/2s.)
Perhaps a better statement would be. "A faster lens is usually sharper
at the same aperture than a slower lens at its widest 1 or 2 stops."
Moose
Albert wrote:
"Almost any lens is sharper stopped-down than it is wide-open.
True. Notice it says "Almost".
So besides focusing better and more accurately, a fast lens is usually
sharper at the same aperture as a slower lens, simply because it's
been stopped-down more."
That's from an article on photosig.
I don't think that's true. It's easier to make a sharp slow lens. A
50mm f3.5 @ f8 I wouldn't be surprised if it was as sharp or SHARPER
then a 50mm f1.4 @ f8.
Am I wrong, or is the article's statement wrong??
Albert
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