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Roy asks:
I was reading a lens review in Pop Photo and it said the lens was sharpest
two stops down. Is the old adage 2 stops down is sharpest still true like
an optical law. To get the sharpest image at f/5.6 should I buy a f/2.8
lens?
it was a general guide and often proved accurate but the only real way is
to test.
I discovered the accuracy of this in the same place I leanred most about
photography, in the darkroom - racking an enlarger up to full height and
test printing at all apertures you'll discover some lenses only produce
truly sharp images at a single aperture and while often it was a couple of stops
down, others might be sharpest wide open and others near fully
closed. In this case the Fujitsu range surprised me, they were often
sharp across 3 stops. very good lenses.
I recall a couple of 50mm lenses in the Pentax range (possibly discussed
here before) that should have been faster than they were but they had fixed
aperture disks inside the lenses making their widest aperture something like
f2.8 - if you pulled these lenses apart to remove the disk you could make them
1.8 lenses - it led me to guess Pentax had designed them as 1.8's but
discovering them to be very soft, they'd restricted the lenses to ensure no one
had access to using the sub-standard wider speeds. lens fault sure,
but they overcame it to make a good lens.. I guess they figured they
were better off making sharper lenses than playing the fast lens game as other
makers were doing.
having said this, soft lenses aren't necessarily bad, but consumers were
less likely to make generally good images with soft lenses and it would have
hurt the company.. it's better for them to aim for something that's please most
consumers rather than giving them lenses which require the user to learn how to
best use them
Old camera magazines often listed sharpness characteristics or lines per mm
resolving power across the aperture range for lenses they tested - this became a
fixation for some, getting the sharpest lenses and dismissing anything that
didn't hit the big numbers - but the snare in this was they were only testing a
single lens. Quality control might be good but it's not so good that all
lenses will be equal.
A long time back I wrote up my results testing some Canon macro lenses, I
had FL and FD 50mm's and 100mm's, the FDs sold for more at the time being newer
but the FL's were the same quality or better so I sold the FDs for a tidy profit
and kept the FLs. (From memory both were at their sharpest at 2 apertures
4 and 5.6 ) Later I tested an ugly little Russian macro
whcih was sharp, much sharper than any of the Canons I tested across a
whole 4 stops from wide open to f8 - ths lens I kept ! putting
up with disaproving stares from other photographers was a small price to pay for
sharper images.
Again this is all with the caveat that 'sharp' has some
meaning. I've often used a certain print in classes (warning:
contains boob) http://members.iinet.net.au/~shahjen/images/quickwash.jpg -
the print looks sharp because the negative was developed with high accutance
devloper , it was printed sharply using the sharpest aperture on the enlarger
and by taking the time during enlarging to allow the neg to take it's final
heated position in the carrier before the paper was exposed..
it was one I used to show students a number of things, that
printing sharply was something that took skill and technique and you couldn't
just expect the enlarger to do it for you, and it also demonstrated that when
printed sharp, even a badly out of focus image can look sharp (and good).
And even with all that sharpness and grain the image , the image can still
appear soft (coming from a from a soft lens) Basically, you
can make good (subjective) images from even chipped, damaged garbage soft lenses
if you know how best to use them. I don't think I'd have
gotten as good an image from a good lens in this case.
Other things that contributing to sharpness beyond the aperture picked is
the steadiness of the camera and often forgotten, the lens contrast. Even
poor sharpness lenses with masking lens shades like you see in cinema use
can produce excellent images, often better than unshaded 'sharp' lenses when
non-image forming light is excluded from the frame.
The above example is just a pictorial image though, for technical images
I'd have reached for my sharpest lens. I'd only know which apertures are
sharp by testing the specific lens.
k
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