Re: [ATM] Re: Aero Ektar Coma Corrector?

[Woodchuck <djv@bedford.net>]

On Thu, 27 Jan 2005, M/S Heald wrote:

> Do you have the prescription or some other information about the original
> lens?
>
>
>
>
>
> No, I'd have to do some research to see if its available.  I imagine I'd
> have to reduce the info from actual photographs, though.

I have a vague recollection that the Aero-Ektars were of the Tessar
design, which resembles a Cooke triplet with a cemented third element.
(But this is not the origin of the Tessar design).  Other Kodak Ektars
were Tessars.  Try a patent search if you've got a few years free :-).

Jacking a Tessar up to f:2.5 is possible, but you have seen the
result.  They're adequate at f:2.8 in shorter fl's, an 80mm Tessar
will cover a 6x6cm negative well. (Used in Rollei and other cameras
in this format, such as the Praktika).

> The reason I am curious is that most 4x5 lenses are f5.6, with an occasional
> one thrown in at f4 - nothing at f2.5.  Even purchasing a new lens at f4.0

Right.  One cannot simply scale up a Double Gauss (typical design
for a 50mm F:2 for 35mm cameras) or other fast small format lens
-- the aberrations scale up, too, and for some of the designs the
amount of glass is prodigious, may not even be available in the
diameters and thicknesses required.  (Picture a 200mm Summicron).

> or f5.6 would not guarantee that it would not need to be stopped down to
> become sharp.  F5.6 would likely be too slow to use filters for B&W as well.
> Color emulsions are a problem in 4x5 as well.

Many aero lenses were intended to be used with a light yellow filter,
and were actually designed to work best in this "minus-blue" light,
so using a filter (K1 or K2 Wratten) might improve not only the color
correction, but some other aberrations. I seem to recall that the
thickness of the glass filter, in addition to its spectral effect,
were part of the design in some cases.

I seem to recall (no real definite memory) that this lens was intended
for less than 4x5 coverage, as part of some roll-film aero camera.

What *is* the field of good coverage, wide open?  I'm guessing about
10-12 cm.

> It may not be worth the effort to try and make a coma corrector, but it
> might be an interesting project, and if it were to pan out, make one heck of
> a wide field lens and open up a lot of imaging options.

When last I looked at 4x5 cameras, a good lens in this f.l. was the
Schneider "Symmar", a sort of asymmetrical Gauss design, which would
at F=180mm cover more than 4x5 (possibly 5x7), wide open, which would
be f:5.6.

As for f:4 lenses for 4x5 cameras, they are scarce, as you say.
Something interesting is the old "soft-focus" portrait lenses; these
were deliberately undercorrected for spherical, and also had a
deliberate curavature of field so that the subjects nose and ears
were in more or less focus at the same time as the eyes. (talk about
specialized lens design).  They were of "Petzval" design.  Perhaps,
with all the spherical present, this would make designing a corrector
for a wider field *easier*?  just a stab in the dark.  A session
with OSLO would give answers.  These portrait lenses were in the
10-12" fl range, though.

The lenses we all want are the ones on the Keyhole satellites, but
they're doubtless some sort of exotic aspherics.

One problem with a coma corrector will be introducing color.

Do keep us posted on developments!

Dave
-- 
        In each of us, there burns a soul of a woodchuck.
        In every generation a few are chosen to prove it.

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