[LargeFormat] Goerz Artars

[Michael Briggs]

On 27-May-02 Guy Glorieux wrote:
> 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.

Yes, it would be nice to hear your report and compare it to the various
predictions.

> 
> 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?

Theta is the angle of coverage.   For the Artar it is the 46 degrees that I
cited.   The angle of coverage is a characteristic of the lens design.  

Yes, the formula is a straight multiplication of factors.  To check your
results, for theta = 46 degrees and f = 19 inches, D = 16.130 inches.   Neither
theta nor D is defined extremely precisely -- the ".130" part of the D value
doesn't really have any meaning but could help verify your use of the equation.

There is no simple way to know the value of theta for a particular lens design. 
It is a characteristic of the lens design, so if you know theta for another
focal length of the same series, or even for a lens of the same design made by a
different manufacturer, then you can use that value as a good guess for the
lens you are interested in.  In the case of the Artar, several manufacturers
that have made lenses of the design in the past few decades have specified
theta as 46 degrees.  (Sometimes a little smaller for really long focal
lengths.)


> 
> 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.

The abberations of a lens improve as the aperture is decreased--it is much
easier to design an f16 lens than an f1.4l lens.   With most lenses, the
coverage improves substantially as one stops down two or three stops.  With
most lens designs, further stopping down doesn't significantly improve the
coverage.   With a few designs, further stopping down does keep increasing the
coverage -- Dagors and Angulons are said to belong to this group, but I have no
experience with these lenses.   If you stop down far enough, say f128 or
farther, you are practically in the regieme of the pinhole camera and coverage
will probably increase, but at the expense of decreased sharpness.


> Finall, why would the coverage increase when shooting on a 1:1 ratio,
> versus landscape?

As Karl already explained, lenses project a cone of quality image.  As you
focus on closer objects, the film is move farther from the lens and the
intersection of the cone and the plane of a film becomes a larger diameter
circle.    The equation I gave about can be generalized by replacing "f" with
"d_i", where d_i is the distance between the lens and the film.    There are
two special cases: 1) when focused on infinity, d_i = f and the equation is as
given above; 2) when focused to obtain a lifesize image, d_i = 2 f and the
coverage is double the coverage when focused at infinity.

--Michael