[rollei_list] Re: Tell the tales of Triotars
- From: Sanders McNew <sanders@xxxxxxxxx>
- To: "<rollei_list@xxxxxxxxxxxxx>" <rollei_list@xxxxxxxxxxxxx>
- Date: Mon, 6 May 2013 16:23:44 +0000
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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