[pure-silver] Re: Focusing a Verito
- From: Chauncey Walden <clwaldeniii@xxxxxxxxxxx>
- To: pure-silver@xxxxxxxxxxxxx
- Date: Mon, 10 Nov 2008 11:11:33 -0700
Richard Knoppow wrote:
You may be able to tell what it is by counting the reflections on
each half. A R-R lens has two cemented elements on each side so it will
have two bright reflections and one dim one. Most, but not all, R-R
lenses are symmetrical, there were some made with different focal
lengths for each half. The R-R is corrected for spherical aberration and
coma but not for astigtmatism. Astigmatism in a photographic lens is not
the same thing as astigmatism in the human eye. The latter is a slightly
cylinderical surface which has a different focal length for two crossed
axis, astigmatism in a photographic lens is a difference in focal length
for light from a point entering the lens along a radius and light
entering along a line defining the circumrerence of a circle. This
results in two focal planes, one for the radial surfaces and one for the
tangential surfaces. These are the same at the center but, in genral,
become more separated as one moves out from the optical axis. In
non-astigmatic lenses the astigmatism is often mitigated by introducing
some curvature of field so that the two stigmatic fields are disposed
about equally on either side of the focal plane. While the stop has no
effect on astigmatism the increase in depth of focus with stopping tends
to also mitigate its effects.
The effect of astigmatism is to create a soft focus effect which is
greater as one moves away from the center of the image. Also, the focus
of linear objects will depend on whether than are closer to lying along
a radial or longitudinal line. That is, the spokes and the rim of a
waggon wheel can not be brought into focus at the same time.
This may be what is causing the soft focus effect of this lens, or,
perhaps, it has considerable uncorrected spherical even though that is
not typical of a R-R. Spherical is the same all over, that is, it does
not vary with image angle as to some other aberrations, so the softness
is evenly disposed over the entire image.
Both SA and astigmatism can give the effect of greater depth of
field, especially SA. In fact, the fixed focus lenses in box cameras
were designed to have enough SA to exagerate this effect. A lens with SA
has, in effect, many different focal lengths around the principal FL.
Since the distance in focus tends to be brighter than out of focus areas
each distance along the range where the lens is nearly in focus will
result in an image with some amount of sharpness and the effect is of a
moderately sharp image with great depth of field. A single meniscus
lens such as used on most box cameras is adjusted for minimum SA and
minimum coma by bending the lens and choice of stop, they can be made to
perform quite surprizingly well for such a simple optical device.
It is certainly an RR by construction, and since it is a slower f/11,
probably of the type they called "portable". I don't think the effect is
due to astigmatism as it is too uniform across the field. After reading
the data on the Veritar, I think it may well be a combination of SA and
CA. I am going to have to look at the negs carefully and see if the
color of the subjects affects the softness.
Chauncey
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