[rollei_list] Re: Tell the tales of Triotars
- From: Eric Goldstein <egoldste@xxxxxxxxx>
- To: "rollei_list@xxxxxxxxxxxxx" <rollei_list@xxxxxxxxxxxxx>
- Date: Mon, 6 May 2013 20:29:38 -0400
----- Original Message ----- From: "Eric Goldstein" <egoldste@xxxxxxxxx>
>
>
> 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...
>>
>>
On Mon, May 6, 2013 at 7:51 PM, Richard Knoppow <dickburk@xxxxxxxxxxxxx>
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.
>
To which I will add that lenses that must cover MF or LF are much for
difficult to correct than 35 or 35 mm motion picture lenses
Eric Goldstein
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