"Almost any lens is sharper stopped-down than it is wide-open. 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.
It is easier to build slower lens of the same optical quality. When you
make a lens one stop brighter, all aberrations increase 3-6 times. To
battle with them, you have to add more elements or add aspheric elements
or split existing elements in two.. Tessar is a 3 group 4 element lens,
created in 1902 as far as I remember. Because of its small aperture
(6.8, later opened to 3.5 then 2.8 by Willy Merte (as far as I remember)
it gives fairly good image, especially when stopped down 2 stops (wide
open, it exhibits noticeable astigmatism). It has just enough variables
(curvatures of lenses, type of glass and group spacings) to fully
correct all aberrations visible at this opening. But wider lenses, just
one - one and half stop brighter (Sonnar, Planar, Xenon, Ernostar..)
exhibited severe second-order aberrations (secondary chromatic and
spherical aberration..) and so constructors had to use more elements to
give them possibility to correct those aberrations to enough degree to
make useful lens. Sonnar is a kind of compromise - derived from Tessar
it has small number of air-glass interfaces, so it works OK without
coatings, but also gives less possibilities to correct aberrations.
Planar had a lot of them and worked better than Sonnar - but it was
practically useless without coatings because of flare. BTW Planar is
older than Tessar :). Sonnar wide open is just OK - you have to close it
down substantially to get Tessar performance. Planar is better - but
still you have to close it down a bit. Closing down reduces some
aberrations - like spherical aberration - and covers/hides some of them
(like field curvature).
Now back to the question :) lens with fewer elements and with the same
coating should transmit contrast better. As film has some threshold of
contrast below which it can't record small differences of contrast,
slower lens with fewer elements can, and in many examples will give
sharper image. It is the case of 1.2, 1.4 and 1.8 50mm Zuikos - also
amnual Pentax SMC lenses, MD Minoltas.. Also Zeiss Planars for Contax
SLRs exhibit similar behaviour - slower 1.7 one is a little bit sharper
than 1.4.
Different situation is with e.g. Canon EOS 50mm lenses - 1.8 is built
CHEAPER and far worse than 1.4 - so it is worse and there is lot of
lens-to-lens variation between them. Then 1.4 is best of the pack, a 'L'
lens - 1.2/50 is a lot worse than 1.4, regardless of f/stop.
Am I wrong, or is the article's statement wrong??
Article is wrong and misleading. F/stop where lens is sharpest depends
of the lens construction, quality of manufacture, even coating used and
subject distance! 'Two stops' is just a rule of thumb - I know some
lenses made purposely so that they are very sharp at full opening and
further closing them give just a modest improvement - like Contax T*
Makro Planar 120/4, Leica APO Summicron 2/90, Zuiko 3.5/28.. :) and
there are lenses purposely made to be used stopped down - like
macro/micro or enlarger lenses. And there are crappy zooms, which you
can close down to 16 and still have blurry image..
Every lens has 'sweet spot'. For Zuiko 2/85 it is at f/4. For 2/35 it is
at f/5.6. For 1.4/50 it is at f/8. For 1.8/50 it is at f/5.6.
And so on :) Polly is tired typing :)
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St.
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