[LargeFormat] Goerz Artars

[Karl Wolz]

Don't know the answer on theta, but I can help on the other two questions.

1) Open the shutter of a lens and look through it at an oblique angle with
the lens stopped down and gradually open it up.  As you fiddle around with
different viewing angles, you'll see that you can see the entire lens
opening when stopped down, but as you open the diaphragm, it'll reach a
point where the edge of the lens mount interferes with the opening.  At this
point, though some light is falling on the groundglass, the edges of the
image will begin to degrade.  This is one of the reasons that many
groundlesses have clipped corners - to make looking in and checking the lens
iris visually.

2) This one's probably easier.  Light comes out of the back of the lens in a
cone.  In macro work, you extend the bellows more, so you are dealing with a
larger section of that come of light.  This, incidentally is why the lens
needs to be opened more to allow for "bellows factor".  There is only so
much light available for any given f:stop, so as you move further back on
that expanding cone of light, the image gets dimmer.

Guy; did you take any pictures of the pinhole setup in Montreal?  I'd be
interested in seeing them.  My email is wolzphoto@worldnet.att.net.

Thanks,

Karl
----- Original Message -----
From: "Guy Glorieux" <guy.glorieux@sympatico.ca>
To: <largeformat@f32.net>
Sent: Monday, May 27, 2002 3:17 AM
Subject: Re: [LargeFormat] Goerz Artars


> Michael, Ken,
>
> Thanks for the information.  I should be getting the lens in the not too
> distant future and will be reporting on what I get.
>
> Michael, you had a formula for working out coverage, but I didn't quite
> understand.  What is the "theta" of a lens?  Is this information easy to
> find for any lens?  Then in the formual D = 2 f tan (theta / 2), is it a
> straight multiplication of all factors?
>
> Also, why would the coverage increase with stopping down the lens.  I
> seem to recall having read this somewhere, but I don't quite understand
> how this would work.
>
> Finall, why would the coverage increase when shooting on a 1:1 ratio,
> versus landscape?
>
> Thanks for helping me in my education.
>
> Guy
>
> P.S.  Interesting reference you make to pinhole, Michael.  There is a
> rule of thumb in pinhole photography that the useful coverage is about
> 3.5 times the Focal.  This is due to light falling out as the focal
> increases on a flat film plane as the distance increase away from the
> center of the image - the effective aperture thus closes down.  There is
> also the fact that the shape of the pinhole turns into an oval as it is
> considered from the edges and, finally, there is loss of light too due
> to the thickness of the material in which the pinhole is cut.  Not
> terribly scientific, I'm afraid, but useful to know.
>
> ----- Original Message -----
> From: "Michael Briggs" <MichaelBriggs@Earthlink.net>
> To: <largeformat@f32.net>
> Sent: Saturday, May 25, 2002 5:23 PM
> Subject: Re: [LargeFormat] Goerz Artars
>
>
> >
> > On 25-May-02 Michael Briggs wrote:
> > >
> > > The diagonal of 11x14 is 17.8 inches, so the lens probably won't
> fully cover.
> >
> > My calculation is based on the coverage typically cited by
> manufacturers' of
> > this lens design.  Since you are probably making contact prints, your
> needs for
> > resolution may not be the same as in the original application of the
> lenses,
> > and the coverage may be somewhat larger by stopping down.
> >
> > --Michael
> >
> >
> > P.S.  If one stops down far enough, the lens design hardly comes into
> play and
> > one has a pinhole camera.
>
>
>
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