On Thu, 8 May 2003 23:19:54 +0100, David Littlewood
<david@nospam.demon.co.uk> wrote:
>In article
<5779d5d6.0305081215.2df1d183@posting.google.com>, Chris
>Wilkins <chris.wilkins@virgin.net> writes
>>Are lenses better sharper at one aperture setting
compared to another?
[...]
>"Sharpness" is a slightly imprecise term.
There is:
>
>(1) Resolution - the ability to distinguish two very
close objects
>(points, lines etc.).
>
>(2) Contrast - the differentiation between areas of
different tone.
>
>(3) Acutance - the sharpness of the boundary between
areas of different
>tonality.
>
>Acutance is largely a function of the film and the
developer used to
>process it; the lens has little effect on this. A
negative or
>transparency of very high acutance may look sharper than
one of low
>acutance even if the latter has a higher resolution,
until you look very
>closely through a magnifier or a microscope. Since it is
not much
>affected by the lens, I assume this is not what you are
interested in.
>
>Contrast is affected by both lens and film/processing.
Again, a high
>contrast result may look superficially sharper than one
of low contrast
>which has higher resolution.
>
>Resolution is the parameter which people normally refer
to when talking
>of "sharpness", even though the other two may
actually be more important
>in many kinds of image.
>
>Resolution is affected by both lens and film/processing.
It is usually
>measured in lp/mm - line pairs per mm (the finest
pattern of black and
>white lines which can be distinguished on the film).
Just looking at the
>effect of the lens, this is a balance of two factors:
>
>First, the resolution of the lens can never exceed the
diffraction
>limit. This is determined entirely by the effective lens
aperture and
>nothing else (well, actually, by the wavelength of the
light as well -
>but I assume we are talking about white light here). The
resolution is
>approximately 1600/effective f-number of the lens in
lp/mm. ("Effective"
>lens aperture means that if the lens is being used for
macro work the
>reduction in aperture must be taken into account - for
instance, at 1:1,
>the effective aperture of f/8 becomes f/16). [Note some
people use
>1500/f-number or 2000/f-number - there are different
views about what
>can be "seen" to be resolved.]
>
>For example, a lens at f/8 has a diffraction limit of
1600/8 = 200
>lp/mm. Used in macro at 1:1 it would be 1600/16 = 100
lp/mm. Used wide
>open at say f/2 it would be 1600/2 = 800 (but of course
the film would
>not cope - see below).
>
>If this were the only story, all lenses would be best
wide open.
>However, real lenses have aberrations - deviations from
theoretically
>perfect behaviour. There are several of these, and most
of the ones
>which reduce resolution (spherical aberration, coma, astigmatism,
and
>longitudinal colour) are reduced by using a smaller lens
aperture (i.e.
>a higher f-number). Only lateral chromatic aberration
reduces resolution
>and is not reduced by stopping down, but it only causes
serious problems
>in very long large-aperture lenses.
>
>So, a perfect lens would give the best resolution wide
open. A real lens
>for normal photography will have its aberrations reduced
by stopping
>down, and in most cases the improvement is, up to some
intermediate
>aperture, more than the deterioration from diffraction.
A really bad
>lens may need to be stopped down to f/16 before the
aberrations are
>controlled and the performance deteriorates from
diffraction. A really
>good lens will typically work best at about 2 stops down
from maximum,
>maybe 3. Thus f/5.6 or f/8 may well be the aperture
giving the highest
>resolution in many cases for very good 35mm lenses, f/11
for poorer
>ones. This accounts for the resolution chart graphs you
sometimes see in
>magazines, with performance rising to a peak somewhere
in the middle.
>
>There are lenses for which performance is best wide
open; however, these
>are usually limited in some other aspect of their
design, such as having
>a very narrow field of view (many scanning lenses) or to
work only at
>one set of conjugate distances (most microscope lenses).
The very best
>enlarger lenses give best results 1-1.5 stops down.
>
>Of course, as suggested above, lens resolution is only
half the story;
>film resolution (plus the resolution of whatever you use
to "see" the
>film - loupe, scanner, enlarger, projector or whatever).
The interaction
>between the two is complex, but the overall result will
always be worse
>than any of the component parts.
>
>...And this glosses over the interaction between
resolution and
>contrast, and modulation transfer functions. If you want
to go into this
>further I suggest you get hold of a book on optical
theory.
>
>I don't know the answer to your questions about
particular lenses, but I
>would guess about f/8-f/11.
Great answer!