Goerz 19" Red Dot Image Circle

I've been looking on the web but have not found out what the coverage
for a Goerz Red Dot 19" APO Artar is. I have a Deardorff 8x10.

Thanks.

"Ron Gans" wrote in message
oups.com...
I've been looking on the web but have not found out what
the coverage
for a Goerz Red Dot 19" APO Artar is. I have a Deardorff
8x10.

Thanks.

In general, Artars will cover an image circle equal in
diameter to their focal length at infinity focus.
Officially, they have a coverage angle of about 48 degrees,
which is quite narrow and is typical of this type of lens.
This is equivalent to about a 17 inch image circle for a 19"
lens but it will do a bit better for pictorial purposes.
Artars are optimized for 1:1 or near it although some of
the Red Dots in shutters were adjusted for greater object
distances. The corrections of this type are rather stable
with object distance, the main aberration which appears at
infinity is coma, which is eliminated by stopping down a
bit.
19" is the most common focal length for the Apo Artar.
These are extremely sharp lenses even at infinity focus.


--
---
Richard Knoppow
Los Angeles, CA, USA

Richard Knoppow wrote:
"Ron Gans" wrote in message
oups.com...
I've been looking on the web but have not found out what
the coverage
for a Goerz Red Dot 19" APO Artar is. I have a Deardorff
8x10.

Thanks.

In general, Artars will cover an image circle equal in
diameter to their focal length at infinity focus.
Officially, they have a coverage angle of about 48 degrees,
which is quite narrow and is typical of this type of lens.
This is equivalent to about a 17 inch image circle for a 19"
lens but it will do a bit better for pictorial purposes.
Artars are optimized for 1:1 or near it although some of
the Red Dots in shutters were adjusted for greater object
distances. The corrections of this type are rather stable
with object distance, the main aberration which appears at
infinity is coma, which is eliminated by stopping down a
bit.
19" is the most common focal length for the Apo Artar.
These are extremely sharp lenses even at infinity focus.


--
---
Richard Knoppow
Los Angeles, CA, USA


THANK YOU! As I would use it only for close-up stills, it would be seem
to be very good.

RON

THANK YOU! As I would use it only for close-up stills, it would be seem
to be very good.

RON

Not so fast!

As I understand it, Artars are apochromatic lenses corrected at 1:1 for
flat fields - in other words, they're corrected for reproduction or
copy work.

Maybe Richard can amplify on this.

Bob G wrote:

THANK YOU! As I would use it only for close-up stills, it would be seem
to be very good.

RON

Not so fast!

As I understand it, Artars are apochromatic lenses corrected at 1:1 for
flat fields - in other words, they're corrected for reproduction or
copy work.

Maybe Richard can amplify on this.

This is correct. All lenses with fixed position elements can be
corrected for certain aberrations at one object distance only. At
others some of the aberration creeps in. In general, this is due to the
chang in angle of the light rays inside the lens. For slow lenses and
for those with fairly narrow coverage angles, where the curvature of
the elements is shallow, the difference is not very great. The dialyte
type lens, that is four element air spaced lenses where the elements
are not meniscus lenses (if they are it becomes a double Gauss lens)
have corrections which are quite stable with change in object distance.

The Apochromatic Artar, to give it its full name, was designed for
three color process work, that is, for making three color printing
places for ink on paper printing. This requires that all three colors
be in focus at the same time and be of exactly the same size, requiring
very good correction for both longitudinal and lateral chromatic
aberration. The lateral chromatic is automatically corrected by
symmetry and the Artar is a symmetrical lens.
All lenses are designed to be flat field but some are better than
others. For copy or process work it is imperative that the field be
flat, i.e., a flat object is imaged onto a flat surface. This requires
both that what is called the Petzval sum be small and that the lens be
well corrected for astigmatism, the Artar is both.
Astigmatism is a peculiar aberration where light comes to a
different focus depending on whether it is coming into the lens along a
section of a radius or a segment of a circle. In an ideal lens the
point of focus would be the same. When a lens has astigmatism it isn't.
The practical results are than when a point is imaged two points of
best focus will be found. At one the point is imaged as a line in the
direction of a radius, in the other it is a line in the direction of a
circle, i.e. tangential. Somewhere inbetween one finds a place where
the point is imaged as a round blur spot. The better the correction for
astigmatism the smaller this blur spot will be. Where a lens has
astigmatism stopping down will help. Even though the aberration is not
proportional to the stop increasing depth of focus will tend to
mitagate it.
When used at infinity lenses like the Artar begin to exhibit
another aberration known as Coma. Coma is automatically cancelled in
symmetrical lenses where the entire optical system is symmetrical,
i.e., where image and object distance are the same. Coma is
proportional to the distance of the image from the center. It looks
like a tear drop shaped blur. Since it is proportional to the stop
stopping down reduces or eliminates it. The Arter shows a little coma
in the corners when used at infinity focus but stopping down about two
stops will get rid of it even at slightly larger coverage angles than
the lens is supposed to do.
One problem with lenses of this sort is that stopping down does not
much increase the coverage angle of the lens. This traces from
astigmatism. The two fields, radial and tangential, can be made to be
very close up to some image angle, where they are made to cross, but
after than they depart rapidly and the image quality becomes very poor
very quickly.
Artars work very well for infinity coverage provided the fairly
narrow coverage is kept in mind. Since at closer distances the coverage
becomes larger (until at 1:1 it is twice the diameter it is at
infinity) the lens is somewhat less limited for table top work.
Officially, the Arter is best over a magnification range of -5 to +5,
but again, it works well at infinity focus.
A very brief answer is that being corrected for process or copy
work does not automatically disqualify a lens from being good for
normal pictorial work.

Richard Knoppow
Los Angeles, CA, USA