[pure-silver] Re: lens resolution
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Sat, 12 Jan 2008 19:26:55 -0800
----- Original Message -----
From: "Vick Ko" <vick.ko@xxxxxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Saturday, January 12, 2008 6:13 PM
Subject: [pure-silver] Re: lens resolution
It was true in the 80's. Max aperture was degraded by
spherical aberations, minimum aperture was degraded by
diffraction. Sweet spot was in the middle.
Now, with modern computations and aspherical lens
generation, maximum performation can be had at max
aperture (e.g. all those Leica ASPH lenses). Minimum
aperture still comes with diffration effects.
How times change.
As for f64, f64 on a huge lens (e.g. LF lens) can still be
away from the point where diffraction degrades the image.
...Vick
Aspherical surfaces can improve lens correction
especially for spherical aberration and can simplify the
lens since any aspherical surface can be duplicated by two
or more spherical surfaces. However, the fact remains that
several aberrations are proportional to the stop. Spherical,
and its relation oblique spherical, and coma for instance.
Also, unless the lens is designed so that there is no
mechanical vignetting from the mount the uniformity of
illumination will still improve as it is stopped down from
maximum. There are some other factors such as astigmatism.
While astigmatism is not affected by the stop the increased
depth of focus (I mean focus) is increased by stopping down
which tends to mitigate this problem.
Its easier to make lenses now that have minimal
spherical when wide open but most lenses will still benefit
from being stopped down part way. Typical lenses have their
best performance when stopped down from 2 to 3 stops from
maximum aperture but this is not a rigid rule.
Diffraction at the aperture stop can limit resolution.
The limit is dependant on the f/stop and the image angle and
also is different for the radial and tangential fields. The
difference between these has to do with the eliptical shape
of the stop at an angle from the optical axis. The
resolution limit due to diffraction is larger as the stop
becomes larger so that when the stop is not round the
resoluton is greater for lines in the longer dimension. The
places on the lens that the light rays enters determines
whether they are part of the radial or tangential field. In
practice it means that the lens will have greater resolution
for lines that lie along a tangent, i.e., a circle than for
those which lie on a radius. This is a similar effect to
astigmatism but comes from a different source.
The reason Ansel Adams and Edward Weston pictures are
sharp is because most of them were photographed on large
format film. Also, very few were made at f/64 even through
the famous group was named the f/64 group. All lenses will
exhibit a noticable diffraction blure at f/64. Probably the
smallest stop that can be used for large format lenses
without noticable blur is f/45 and most are "optimum" at
around f/22. Note that for lenses with very wide coverage
like the Goerz Dagor or any wide angle lens, the "optimum"
depends on the field angle the lens is intended to cover. A
Dagor working at "normal" focal length, that is where the
focal length is about the same as the format diagonal, will
be optimum at around f/22. When used at its maximum coverage
angle, around 87 degrees (claimed) it must be stopped down
to f/45 to eliminate blur at the margins. Optimum usually
means the stop where increase in resolution from reduction
of aberrations is equalled by loss of resolution by
diffraction blur. For many large format lenses of around
f/4.5 to f/6.8 its around f/22. For lenses of he sort use in
smaller cameras it is generally larger.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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