a humbler and new question

In article ,
(jjs) says...
"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.

Hmmm. 76 mm is 3" What is the coverage of the lens? If you are
shooting 5x7, that would be what, about 55 degrees center to corner?
(just guessing here). That would make the difference in focal distance
from the center to corner about

[9.7/cos(55)-9.7]/3 = 2.4mm (hairball guess)

That's a pretty substantial edge curl, but an ultra wide like that ought
to be able to handle it, particularly if you stop down a bit. Why don't
you try a frame or two and see what results you get?

Out of curiosity, what was the glass originally used for? The only
filters I have seen that were that thick were IR absorptive filters to
keep film from melting when exposed to a heat source, like a 500 watt
slide projector bulb.

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

In article ,
(jjs) says...
"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.

Hmmm. 76 mm is 3" What is the coverage of the lens? If you are
shooting 5x7, that would be what, about 55 degrees center to corner?
(just guessing here). That would make the difference in focal distance
from the center to corner about

[9.7/cos(55)-9.7]/3 = 2.4mm (hairball guess)

That's a pretty substantial edge curl, but an ultra wide like that ought
to be able to handle it, particularly if you stop down a bit. Why don't
you try a frame or two and see what results you get?

Out of curiosity, what was the glass originally used for? The only
filters I have seen that were that thick were IR absorptive filters to
keep film from melting when exposed to a heat source, like a 500 watt
slide projector bulb.

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

"Larry Caldwell" 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.

Hmmm. 76 mm is 3" What is the coverage of the lens?

Coverage is even to 5" square, then drops off dramatically as most
late-design Biogons (that I know of) do.

If you are
shooting 5x7, that would be what, about 55 degrees center to corner?
(just guessing here). That would make the difference in focal distance
from the center to corner about

[9.7/cos(55)-9.7]/3 = 2.4mm (hairball guess)

That's a pretty substantial edge curl, but an ultra wide like that ought
to be able to handle it, particularly if you stop down a bit. Why don't
you try a frame or two and see what results you get?

I have shot only two subjects with that lens so far, and with filters for
each. I'll shoot some more "real soon now" to check edge sharpness without
filters. (Darned day job interferes so much.)

Out of curiosity, what was the glass originally used for?

Aerial photo mapping and recon.

"Larry Caldwell" 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.

Hmmm. 76 mm is 3" What is the coverage of the lens?

Coverage is even to 5" square, then drops off dramatically as most
late-design Biogons (that I know of) do.

If you are
shooting 5x7, that would be what, about 55 degrees center to corner?
(just guessing here). That would make the difference in focal distance
from the center to corner about

[9.7/cos(55)-9.7]/3 = 2.4mm (hairball guess)

That's a pretty substantial edge curl, but an ultra wide like that ought
to be able to handle it, particularly if you stop down a bit. Why don't
you try a frame or two and see what results you get?

I have shot only two subjects with that lens so far, and with filters for
each. I'll shoot some more "real soon now" to check edge sharpness without
filters. (Darned day job interferes so much.)

Out of curiosity, what was the glass originally used for?

Aerial photo mapping and recon.

"brian" wrote in message
m...

A good quality filter won't introduce any aberrations when used on the
object side of the lens for infinity focus. The above field curvature
calculation is wrong. If you're putting the filter on the image side
of the lens then thats a whole different story. If you focus down to
300mm or closer then you *might* start to see some effects, but I
imagine that the filter-induced problems will pale in comparison to
the lens-induced ones. If you focus further than a meter away then
there is just no way that the filter will cause any visible
aberrations.

Many thanks, Brian. You have put my concern to rest.

Any thoughts on how a person could make his own graduated filters? I mean
the physical thing itself, not a photoshop filter application (which I have
already.)

"brian" wrote in message
m...

A good quality filter won't introduce any aberrations when used on the
object side of the lens for infinity focus. The above field curvature
calculation is wrong. If you're putting the filter on the image side
of the lens then thats a whole different story. If you focus down to
300mm or closer then you *might* start to see some effects, but I
imagine that the filter-induced problems will pale in comparison to
the lens-induced ones. If you focus further than a meter away then
there is just no way that the filter will cause any visible
aberrations.

Many thanks, Brian. You have put my concern to rest.

Any thoughts on how a person could make his own graduated filters? I mean
the physical thing itself, not a photoshop filter application (which I have
already.)

In article , brianc1959
@aol.com (brian) says...

The above field curvature
calculation is wrong. If you're putting the filter on the image side
of the lens then thats a whole different story.

Yeah, after I posted it I realized that the filter would have to be
between the lens and film to cause any focus shift. I guess I was
having a senior moment.

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

In article , brianc1959
@aol.com (brian) says...

The above field curvature
calculation is wrong. If you're putting the filter on the image side
of the lens then thats a whole different story.

Yeah, after I posted it I realized that the filter would have to be
between the lens and film to cause any focus shift. I guess I was
having a senior moment.

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

"brian" wrote in message
m...
"jjs" wrote in message
...


Any thoughts on how a person could make his own graduated filters? I
mean
the physical thing itself, not a photoshop filter application (which I
have
already.)

A true graduated filter would be a real challenge to make, unless you
have a vacuum coating facility at home. A good possibility for a
home-built filter is the little-known "butterfly filter" dating all
the way back to the 1850's:
http://caldwellphotographic.com/Butterfly.jpg

This consists of a normal [... snip excellent article ...]

Fascinating! We have no material like that in our university consortium
libraries. Thanks very much, and I look forward to your putting the document
online. I'd be happy to scan it to OCR, PDF, whatever if you like.

An aside: I have a military contrast filter (red) that has an interesting
filter that I presume is a graduating filter. It hasx a mirrored pattern in
a form somewhat like snowflake on the surface. Of course, it's much too
small for my lens. If I can get a scan of it, I'll post it on a server.

"brian" wrote in message
m...
"jjs" wrote in message
...


Any thoughts on how a person could make his own graduated filters? I
mean
the physical thing itself, not a photoshop filter application (which I
have
already.)

A true graduated filter would be a real challenge to make, unless you
have a vacuum coating facility at home. A good possibility for a
home-built filter is the little-known "butterfly filter" dating all
the way back to the 1850's:
http://caldwellphotographic.com/Butterfly.jpg

This consists of a normal [... snip excellent article ...]

Fascinating! We have no material like that in our university consortium
libraries. Thanks very much, and I look forward to your putting the document
online. I'd be happy to scan it to OCR, PDF, whatever if you like.

An aside: I have a military contrast filter (red) that has an interesting
filter that I presume is a graduating filter. It hasx a mirrored pattern in
a form somewhat like snowflake on the surface. Of course, it's much too
small for my lens. If I can get a scan of it, I'll post it on a server.