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Wide Angle film illumination



 
 
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  #1  
Old January 13th 04, 04:53 AM
stacey
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Default Wide Angle film illumination

Jan Brittenson wrote:



Rafe B. wrote:

Right. So why is it necessary on LF lenses and not
on "equivalent" 35 mm or MF lenses?


I use the same center filter on both my 43mm Mamiya7
lens (6x7) as I do with my 75mm lens on 4x5.


But I don't see any serious falloff with a 45mm lens on a 6X6 SLR because of
it's retrofocus design.

--

Stacey
  #2  
Old January 15th 04, 02:46 AM
brian
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Default Wide Angle film illumination

"Richard Knoppow" wrote in message nk.net...
"Rafe B." wrote in message
...
On Sat, 10 Jan 2004 23:27:46 GMT, "Richard Knoppow"
wrote:


The theoretical fall off of a lens of standard design

is
cos^4 theta where theta is the angle of the image point

from
the axis. Some lenses have a little more than this. Some
designs use what is called a "tilting entrance pupil" to
eliminate one factor giving approximately cos^3 theta.

The
only way to further reduce the fall off is to use barral
distortion as in a fish-eye lens.
The usual way of correcting the fall off is by the use

of
a kind of mask called a center filter. This is a neutral
filter which tapers in density from center to edge at the
right rate for a particular lens. They are not quite

generic
although a center filter for one lens may work for

another
of the same focal length and front diameter.
Fall off is not affected by the stop once you get past

the
point where there is mechanical vignetting, usually about
two stops down from maximum opening.
Whether the effect is noticable or not depends on the
scene and to some extent on the film.
Fall off can be compensated for in printing, provided

its
not too great and provided the overall exposure has been
calculated to give sufficient exposure to the margins of

the
image to prevent its being underexposed.
Fall off can be seen even in only moderately wide angle
images, but, since the eye expects the edges of a picture

to
be slightly darker than the center, its often not
objectionable.
For a 90degree coverage lens (theta = 45 degrees) the

edge
illumination is 0.25 of the center so the margins of the
picture are down a full two stops from the center. Its

easy
to figure the fall off using a hand calculator or the
calculator built into most computer operating systems.




Call me confused. I have 35 mm primes and zooms
that go down to 24 and 28 mm respectively. I've not
noticed the light falloff in these, even on chromes.

Now as I understand it, 24 mm would be 90 mm
or so for 4x5,so it seems to me the view angle isn't
quite enough to explain the light falloff all by itself.

I'm still confused. Seems bizarre that in LF one first
needs to put up with a seriously expensive f/8 wide
angle (equiv. to 24 mm on a Nikon,) and then have to
add an expensive center filter that's going to turn it
into an f/16.


rafe b.
http://www.terrapinphoto.com


I can't explain your results. I have a 24mm Nikor for my
Nikon F and find it has quite noticable fall off even in
B&W. This is a good lens with a tilting entrance pupil so
the fall off is less than the cos^4 theta, but its still
noticable. I don't know what the fall-off of this lens is
compared to a Super-Angulon or Rodenstock Grandagon. Both of
those also have tilting entrance pupils but are symmetrical
(or nearly so) lenses. The Nikon lens is a retrofocus so it
_might_ have somewhat less fall off. I suspect the
difference, if there is one, is not significant, but can't
say for certain. Because the inverse square law applying to
the image from the lens to the film is less for a retrofocus
lens than for a standard lens they have somewhat less fall
off. This is because the image appears to be coming from
further away then for a "normal" lens of the same FL so the
difference in distance between the center and corner of the
image, hense the square law loss, is less.


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



Just an added note: in certain extreme reversed telephoto designs its
actually possible to have a brighter image in the corners than in the
center. I've seen this happen both with fisheyes and with completely
distortionless systems. Not your average lenses perhaps, but
nevertheless real systems that get designed and built. With enough
chief ray bending and pupil aberrations just about anything is
possible!

Brian
www.caldwellphotographic.com
  #3  
Old January 15th 04, 04:54 PM
jjs
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Default Wide Angle film illumination


"brian" wrote in message
m...

Just an added note: in certain extreme reversed telephoto designs its
actually possible to have a brighter image in the corners than in the
center. I've seen this happen both with fisheyes and with completely
distortionless systems. Not your average lenses perhaps, but
nevertheless real systems that get designed and built. With enough
chief ray bending and pupil aberrations just about anything is
possible!


I thought that was one case. Thanks. I wish you were closer, Brian so that
you could try one of my new wide lenses. I think you might be pleased, even
though it's not a reversed-tele.


 




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