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Re: [OM] [OT] Center filters

Subject: Re: [OM] [OT] Center filters
From: "Jeff Keller" <om-list@xxxxxxxxxxx>
Date: Mon, 20 Jul 2009 16:55:09 -0700
Besides the physics and modeling explaining the light fall off don't forget 
with large format, lens shift is common. So although a 90mm Super Angulon XL 
may not seem that wide, the center filter works for the 110 degree coverage of 
the entire image circle. I don't know how effective photoshop would be at 
removing vignetting when the image isn't centered.

>From occasional visits to largeformatphotograpy.info it seems that a good 
>number of people still shooting large format are shooting LARGE images and 
>contact printing (11x14 up to about 16x20).

In the end it seems the significant cost of a center filter motivates many to 
do without.

Jeff Keller


-----Original Message-----
From: usher99@xxxxxxx [mailto:usher99@xxxxxxx] 
Sent: Monday, July 20, 2009 3:09 PM
To: olympus@xxxxxxxxxxxxxxxxx
Subject: [OM] [OT] Center filters

Well, not so fast.  The inverse square law accounts for only  one of 
the  cosines in the Cosine**4 law.  I think you are describing the 
Slyusarev effect which lens designers utilize to increases the apparent 
size of one or both pupils for off-axis points and greatly mitigates 
the cosine**4 law.

A second factor that causes light loss off axis is that the lens pupil 
is no longer round but elliptical  when viewed from the periphery.  It 
also strikes at an angle off axis.  There is additional attenuation due 
to the Lambert effect.  I think that makes four, but I am tired after a 
long day and a couple hours left to go and can no longer count.

Mike (protg  of Dr. Focus)


see

http://toothwalker.org/optics/vignetting.html

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