[rollei_list] Re: Tell the tales of Triotars
- From: Sanders McNew <sanders@xxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Mon, 6 May 2013 15:51:17 -0400
Eric Goldstein wrote:
> Sanders, the problem with your question is that these cameras have enormous
> individual variation. There was lens variation right out of the factory,
> and now do to age there is condition variation. So while we might be able
> to test batches of cameras and draw conclusions with
> statistical significance, the probability that any individual camera will
> conform to these aggregates is not high...
>
Eric, my friend, I'm not buying that response! You yourself were able to state
this hypothesis:
>> I'm remembering that in the 1950s the optimum design speed of an MF Tessar
>> was f/6.3. This means that a well-designed f/6.3 Tessar would perform best
>> wide open and that it would provide superior corrections compared with an
>> f/3.8 or f/4.5 Tessar stopped down to f/6.3. This is because of the
>> compromises needed to make these latter two lenses faster than the optimal
>> design speed lens.
I'm asking an equally-theoretical question, to test your hypothesis. Is it not
true that taking a faster lens
and closing it one stop will produce a superior image (as respects resolution)
compared to the lens shot
at its full aperture? I am not an optics engineer -- I'm just repeating what
I've read elsewhere. If -- if --
that proposition is correct, then are you saying that whatever improvement is
obtained by stopping the
lens down, is more than offset by the superior design of the lens with the
smaller maximum aperture?
(My question assumes that the smaller aperture will be the "optimal" aperture
for the design, but that
seems a logical assumption since designers are usually pushing to produce
faster lenses.)
I appreciate that an f/3.5 Triotar design will necessarily differ from its
f/4.5 counterpart, and I assume that
to be true if one were to substitute Planars for Triotars, and compare the
f/2.8 Planar with an f/3.5 Planar.
So, I am willing to be told that the faster and slower lenses, though they
share the same design and name,
are still different lenses, and that one cannot generalize like this. But I
see no reason why we cannot at
least test your claim that the slower Tessar will render an image with greater
resolution than would a
faster Tessar stopped down to the slower model's full aperture. Begging off on
sample variation blows
a hole in your own assertion, doesn't it?
Sanders
On May 6, 2013, at 12:23 PM, Sanders wrote:
> So, which of us is correct, do you suppose?
>
> For those just tuning in, here's the question:
>
> You shoot a portrait with two Rolleicords, each at f/4.5.
> One has the f/3.5 Triotar, the other has the f/4.5 Triotar.
> With no other variations, which produces the sharper image?
>
> And why?
>
> Sanders
>
>
> Eric Goldstein wrote:
>
>> I'm not Richard or Mark but will jump in with this...
>>
>> I'm remembering that in the 1950s the optimum design speed of an MF Tessar
>> was f/6.3. This means that a well-designed f/6.3 Tessar would perform best
>> wide open and that it would provide superior corrections compared with an
>> f/3.8 or f/4.5 Tessar stopped down to f/6.3. This is because of the
>> compromises needed to make these latter two lenses faster than the optimal
>> design speed lens.
>>
>> I do not know what the optimal design speed for a MF Triplet (Triotar)
>> would be; my guess it it would not have been f/6.3 in the 50s because with
>> only 6 surfaces to work with and 3 glass types, you might need more speed
>> to achieve optimal than with a tessar-type
>>
>> Today with superior glass types available, it is possible that the optimal
>> design speed for these lenses could be a bit faster. Using asherical
>> surfaces would probably get you another bump faster...
>>
>>
>> Eric Goldstein
>>
>> --
>>
>>
>>
>> On Sun, May 5, 2013 at 5:20 PM, Sanders McNew <sanders@xxxxxxxxx> wrote:
>>
>> > 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?
>
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