[rollei_list] Re: Tell the tales of Triotars
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Sun, 5 May 2013 19:03:44 -0700
----- Original Message -----
From: "Sanders McNew" <sanders@xxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Sunday, May 05, 2013 2:20 PM
Subject: [rollei_list] Re: Tell the tales of Triotars
So, Marc and Richard, help me out a bit here.
I have it in my head that when you stop a lens down even one
stop from full aperture, its performance improves
substantially. I think I read somewhere that it had
something to do with not using the extreme periphery of the
lens, though that's probably wrong. For this reason, over
the years I've always tried to stop down from full aperture
when light permitted -- to stop a 2.8E down to f/4 (or
smaller), for example, whenever possible.
Is that correct? And if it is correct, then wouldn't one
expect an f/3.5 Triotar, stopped down to f/4.5, to provide
visibly better results than an f/4.5 Triotar at full
aperture? Maybe the advantage of working the Triotar design
to a larger aperture was, in part, to improve the visual
acuity of the lens at a given working aperture. Or is that
a stupid conclusion built on false assumptions?
Sanders
To Sanders and also to address Eric. Some lens
aberrations vary with the stop. In particular spherical
aberration and coma. Coma varies with the image angle,
getting larger as the angle increases from the center of the
image. Spherical aberration occurs throughout the image.
Both of these are due in part to the angle with which the
light rays transiting the lens strike the various surfaces.
The greater the angle the greater the effect leading to the
aberration. So, using a small aperture reduces the cone of
light and the angles of the rays. There is an "optimum"
which depends also on the coverage angle expected from the
lens. Since coma varies with image angle it gets worse as
the coverage gets larger. So, to get the margins or corners
sharp a smaller stop is needed as the angle increases. Now,
there is another factor working the other way, namely the
loss of sharpness due to diffraction at the iris. The
smaller the iris the greater the blur from diffraction. So,
at some point as one stops down the increase in sharpness to
reducing the aberrations is countered by the loss of
sharpness from diffraction. That is the "optimum". This
optimum is not a fixed ratio from the maximum stop but
depends on the amount of aberration in the lens and its
character and the angle of coverage needed. Now, all lenses
have another important aberration called astigmatism. This
is tricky to explain, it is not the same as what is called
astigmatism in the human eye. If one measures the distance
at which a point of light at infinity (but can actually be
at any distance) but offset somewhat from the center of the
image focuses, one finds that as the lens is moved three
points of focus are found. At one the point focus to a line
in one axis, then it will focus to a round blur spot, then
as the lens is moved further it focuses to a line in the
other axis. At the center of the image this effect does not
exist. It becomes greater as we move away from the axis
until it reaches some maximum and then, near the margin of
the coverage, the two points of line focus coincide and we
have a single round spot. If we graph the focus we find we
have two separate fields, sometimes called radial and
tangential or sometimes sagital. The distance between them
determines the sharpness focus possible. Astigmatism is not
affected by the stop size but it is mitigated by increased
depth of focus. Also, the point where the two fields
coincide at the margin and what happens to the two curves
beyond it are important because that is the main
determinator of how large the coverage of the lens can be.
Two lenses with rather opposite behavior are the Dagor and
the four element air spaced type sometimes called a Dyalite,
the Goerz Dogmar and Apochromatic Artar being examples. In
the Dyalite the two fields deviate very rapidly after coming
together (the point is called the stigmatic node) so the
image quality beyond this point becomes very blurred
regardless of the stop. In the Dagor the fields deviate very
slowly beyond the stigmatic node so the lens can cover
tremendous angles if stopped down enough. In the case of
the Dagor coverage will extend to close to 90 degrees at
f/45, but of course, the resolution is much poorer than it
would be for say a 55 degree "normal" coverage at a much
larger stop.
I am sure this is all much more than anyone wants to
know but is still not complete for the effects of all
aberrations.
--
Richard Knoppow
Los Angeles
WB6KBL
dickburk@xxxxxxxxxxxxx
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