[rollei_list] Re: Tell the tales of Triotars
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Mon, 6 May 2013 16:51:36 -0700
----- Original Message -----
From: "Eric Goldstein" <egoldste@xxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Monday, May 06, 2013 1:23 PM
Subject: [rollei_list] Re: Tell the tales of Triotars
Hi Sanders -
OK, got it. You are not looking for a practical answer,
but a theoretical
one...
So, you have this backwards. The lens that is closer to
it's optimal design
speed should, all things being equal, be "sharper" (your
original question)
than the faster version stopped down; it should have less
flare and better
resolution. So the slower lens should be superior to the
faster lens
stopped down to the slower lens' full aperture speed...
Eric Goldstein
If lens resolution were limited _only_ by diffraction
the faster the lens the better. However, practical lenses
have many limiting factors and many get much harder to
correct as the lens becomes faster. Its not just a matter of
math because computers have taken care of the calculation
and modeling but a computer can design a lens which can't be
made. Modern computer optimization programs have places to
insert all sorts of limits to avoid this problem. The
program must also calculate the sensitivity of the elements
and mounting to slight variations and avoid glass types that
may be very expensive or unstable.
Its interesting to study some basic optical design.
Most lenses are made from spherical surfaces for the simple
reason that they are the easiest to calculate and generate.
But, a spherical surface can not produce a sharp image. Take
an ordinary biconvex magnifying glass and look at the image
that comes from it. In order to use spherical surfaces a
combination of positive and negative surfaces must be used
to approximate a surface of curvature that _will_ produce an
image. Its very much making a silk purse from a sow's ear.
While aspherical surfaces can simplify some lens designs its
not magic and until rather recently aspherical surfaces had
to be hand figured and were extremely expensive. Computer
controlled machines have eliminated that but asphers are
still expensive to make and mount.
--
Richard Knoppow
Los Angeles
WB6KBL
dickburk@xxxxxxxxxxxxx
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