[rollei_list] Re: Tell the tales of Triotars
- From: Don Williams <dwilli10@xxxxxxx>
- To: rollei_list@xxxxxxxxxxxxx
- Date: Mon, 06 May 2013 19:08:44 -0500
At 06:51 PM 5/6/2013, Richard wrote:
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.
Somewhere I have some charts for at least one Planar and I would
expect some folks on this list have charts for several lenses. I'm
reasonably sure they are based on measurements.
It would be interesting to see how these "factory tests" compare.
One thing that has always confused me is that the company that makes
Questar telescopes used to, perhaps still does, claim their systems
beat the theoretical resolution for their apertures.
DAW
Other related posts:
