Caveats to dynamic range measurements made with transparent step wedges

Started Apr 28, 2008 | Discussions thread
Bill Janes Senior Member • Posts: 1,983
Re: Caveats to dynamic range measurements made with transparent step wedges

Iliah Borg wrote:
Dear Bill,

I took a shot of my
setup without the wedge in place, using only the background of my
light table. I used the Nikon D3 with a 105 mm macro lens at f/8.

This is a view of the green1 channel in ImageJ (a powerful freeware
program) showing the pixel levels across the field of view. I would
guess that the non-uniform but symmetrical illumination is due to
cos^4 falloff. For more accurate results one would have to make a
correction table.

Yes, cos falloff is playing a very serious role here. Checking the
uniformity of the screen I use with a spectrophotometer I have less
then 1% variation of light. Shooting it with a 400mm lens stopped
down to f/40 I have up to 10% variation on a D3 in DX mode. If I
replace the body with a fim body and measure the variation with the
same spectrophotometer at the film gate, results are 2% only.

We would welcome your article on cos factor on our libraw site if you
have time.

Iliah,

I did perform some cos^4 calculations and performed a vignetting test for the MicroNikkor 105 mm f/2.8 lens.

This table shows cos^4 falloff for various focal length lens as a ratio of corner to center illumination and as fall off in f/stops.

These plots are from green1 channel of a uniformly illuminated surface photographed at various apertures with the D3 and the 105 mm f/2.8 MicroNikkor:

f/2.8

f/8

These are the measured falloff values at various apertures. Wide open, the fall off is mainly from optical vignetting and by f/8 cos^4 falloff is predominant.

Here is a graphical representation of the measured fall off:

As you indicated in your post, cos^4 falloff is most pronounced with wide angle lenses and becomes relatively minor with telephoto lenses and is not affectd by stopping down. Optical vignetting is reduced by stopping down. Interested readers may find this post helpful:

http://bjanes.smugmug.com/photos/297264577_wNo6z-O.gifhttp://www.vanwalree.com/optics/vignetting.html

I would be interested in preparing an article for your web site once I finish a few ongoing projects.

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Bill Janes

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