a humbler and new question

"brian" wrote in message
m...
"jjs" wrote in message
...
The Humbler - Last week was one of those times where so much went wrong.
A
real humbling experience. I got a consistent light leak in two good film
holders when working in bright sunlight with the new-used Sinar. See it
on
the left of this
image:http://course1.winona.edu/jstafford/farmers-1-sm.gif
(That's a work print, not worth trying to print properly due to the
leak.)
The leak is almost certainly in the back itself rather than the bellows
because Sinar bellows are so well cinched.

The Question - I am experimenting with some very large (7") glass
filters
which are _very_ thick. I'll make thinner ones if someone can affirm
that
the thickness of the glass with WA lenses contributes to light fall-off
or
lack of sharpness at the very edges. Affirm? No?

A plane-parallel block of glass in collimated light can't introduce
aberration. If you are shooting at or near infinity, then you could
have a filter a meter thick, and you would get no aberration. Of
course, thats assuming the glass is perfectly homogeneous and the
surfaces really are flat. It also assumes that you are placing the
filter in front where the numerical aperture is smallest.

Any coatings on the filter will be less effective off-axis, which
might introduce a minute amount of light falloff. The path length is
also greater off-axis, so if the filter has any absorption then that
will also contribute to light falloff.

If the filter is plane-parallel, homogeneous, and has low absorption,
then it won't cause the problems you describe.

Brian
www.caldwellphotographic.com

I would have thought that shooting at any focused distance, light from an
object away from the lens axis will be coming through the filter at an angle
and therefore be displaced and dispersed by refraction. Is that not the
case?

In article gw2Hc.45216$WB5.26540@pd7tw2no, (John
Hendry) says...

I would have thought that shooting at any focused distance, light from an
object away from the lens axis will be coming through the filter at an angle
and therefore be displaced and dispersed by refraction. Is that not the
case?

A flat pane of glass will change the focal point of the lens by about
1/3 the thickness of the glass. If the glass is thick enough, I suppose
the longer path through the glass at an oblique angle might affect the
flatness of the focal plane. Given the natural depth of field of wide
angle lenses, I doubt it would be a problem.


--
http://home.teleport.com/~larryc

In article gw2Hc.45216$WB5.26540@pd7tw2no, (John
Hendry) says...

I would have thought that shooting at any focused distance, light from an
object away from the lens axis will be coming through the filter at an angle
and therefore be displaced and dispersed by refraction. Is that not the
case?

A flat pane of glass will change the focal point of the lens by about
1/3 the thickness of the glass. If the glass is thick enough, I suppose
the longer path through the glass at an oblique angle might affect the
flatness of the focal plane. Given the natural depth of field of wide
angle lenses, I doubt it would be a problem.


--
http://home.teleport.com/~larryc

"Larry Caldwell" wrote in message
k.net...

A flat pane of glass will change the focal point of the lens by about
1/3 the thickness of the glass. If the glass is thick enough, I suppose
the longer path through the glass at an oblique angle might affect the
flatness of the focal plane. Given the natural depth of field of wide
angle lenses, I doubt it would be a problem.

To answer an earlier, related question: The glass is 9.7mm thick, and the
lens focal length is 76mm. Methinks the filter is a bit too, ah, robust.

"Larry Caldwell" wrote in message
k.net...

A flat pane of glass will change the focal point of the lens by about
1/3 the thickness of the glass. If the glass is thick enough, I suppose
the longer path through the glass at an oblique angle might affect the
flatness of the focal plane. Given the natural depth of field of wide
angle lenses, I doubt it would be a problem.

To answer an earlier, related question: The glass is 9.7mm thick, and the
lens focal length is 76mm. Methinks the filter is a bit too, ah, robust.

In article ,
"jjs" wrote:

To answer an earlier, related question: The glass is 9.7mm thick, and the
lens focal length is 76mm. Methinks the filter is a bit too, ah, robust.

Is it optical grade glass ? At that thickness almost a cm, if not optical
grade glass, you maybe correct. Even Optical Grade glass may diffract
but 76mm lens is not hugely wide.

"Wise Ass Poaster" wrote in message
...
In article ,
"jjs" wrote:

To answer an earlier, related question: The glass is 9.7mm thick, and
the
lens focal length is 76mm. Methinks the filter is a bit too, ah,
robust.

Is it optical grade glass ? At that thickness almost a cm, if not optical
grade glass, you maybe correct. Even Optical Grade glass may diffract
but 76mm lens is not hugely wide.

Yes, it is the very best optical glass the US government could buy at the
time (the sixties). Good stuff.

"Wise Ass Poaster" wrote in message
...
In article ,
"jjs" wrote:

To answer an earlier, related question: The glass is 9.7mm thick, and
the
lens focal length is 76mm. Methinks the filter is a bit too, ah,
robust.

Is it optical grade glass ? At that thickness almost a cm, if not optical
grade glass, you maybe correct. Even Optical Grade glass may diffract
but 76mm lens is not hugely wide.

Yes, it is the very best optical glass the US government could buy at the
time (the sixties). Good stuff.

"Wise Ass Poaster" wrote in message
...

[...] Even Optical Grade glass may diffract but 76mm lens is not hugely
wide.

(Hit Enter too soon).

That depends upon the attempted image circle, no? 76mm is wide if it is to
cover 5" square.

"Wise Ass Poaster" wrote in message
...

[...] Even Optical Grade glass may diffract but 76mm lens is not hugely
wide.

(Hit Enter too soon).

That depends upon the attempted image circle, no? 76mm is wide if it is to
cover 5" square.