"Jean-David Beyer" <jeandavid8@[EMAIL PROTECTED]
> wrote in message
news:rbfVj.1212$OT1.79@[EMAIL PROTECTED]
> Richard Knoppow wrote:
>
>> That's quite interesting. I suspect the Rodagon and
>> Componon designs are
>> quite similar although I don't have the actual
>> prescriptions. Since its
>> likely all the relatively modern lenses were designed
>> with the aid of
>> computers I suspect the performance should be much alike.
>> I think the
>> Focotar is an older design. One of the characteristics of
>> the generic
>> Plasmat type, which is what the Rodagon and Componon are,
>> is that they
>> can be unusually well corrected for astigmatism. Forgoing
>> an explanation
>> of what exactly that is (different in a camera lens than
>> in opthalmic
>> lenses) it leads to being able to get a very flat field.
>> While both
>> manufacturers claim superiority I suspect its pretty much
>> a draw. Some
>> think the Rodagon is mechanically superior to the
>> Componon. Do your
>> lenses have metal or plastic iris blades?
>>
> When I first got into 4x5" work, in the mid 1970s, I got a
> Schneider
> Componon-S f/5.6 to f/45 180mm enlarging lens, # 11 973
> xxx. It works fine.
> Its diaphragm has lots of blades (about 19 of them),
> enough to make the
> aperture look round, and they appear to be metal. At least
> 10 years later, I
> got a Componon-S f/5.6 to f/45 150mm lens, # 14 588 yyy.
> It has only 5
> blades, so the aperture looks approximately like a
> pentagon (except the
> edges are not quite straght. I cannot tell if they are
> metal or plastic; the
> 180 blades are ****nier than the 150 and darker, reminding
> me of blackened
> brass (but I do not know what they are for sure), and the
> 150 blades are
> duller, but slightly lighter in color and rougher,
> reminding me of anodized
> aluminum (but I very much doubt they would actually be
> aluminum). Perhaps
> that is what plastic blades look like.
>
> For normal photograph use, is there any benefit to having
> a round aperture?
> I know in half-tone work with a sealed half-tone screen,
> there is a benefit
> to having a square aperture, but round holes work OK --
> you just get a
> little bit of a different transfer function from the
> original to the half-tone.
>
> Unless you use the lens in the hot sun or something, there
> might even be a
> slight benefit to having plastic iris blades: less likely
> to rust or corrode.
>
> --
> .~. Jean-David Beyer Registered Linux User
> 85642.
> /V\ PGP-Key: 9A2FC99A Registered Machine
> 241939.
> /( )\ Shrewsbury, New Jersey http://counter.li.org
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> 4.19, 4.16, 4.08
It appears that the shape of the iris affects out of
focus areas of the image. This is perhaps part of the effect
called bokeh by the Japanese. In any case bright points
which are not sharply focused are rendered in the shape of
the iris. This may not be as noticable for enlarging where a
flat surface is imaged onto another flat surface.
In making half-tone plates the iris is imaged by the
half tone screen as an array of spots or dots. By using a
square aperture the intestices of the dots are at the
corners so the variation is smoother. For color work each of
the images is photographed using an iris with a
lozenge-shaped aperture at a different angle. I can't
remember now if this is to prevent moir but I think it is.
In any case there is an optimum set of angles for the
apertures. These apertures are usually in the form of
Waterhouse stops and is the reason process lenses usually
have a slot in the side. My barrel mounted Apo-Artars have
the slot but the shutter mounted one does not. The barrel
mounted Artars also have 20 blade irises and a very nearly
perfectly round hole.
--
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@[EMAIL PROTECTED]
|
