"Non-Image-Forming Light": What does this mean?

Started Jul 4, 2012 | Discussions thread
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Jarvis393
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"Non-Image-Forming Light": What does this mean?
Jul 4, 2012

On a thread below about furniture photography with two lights, Barrie Davis wrote:
( http://forums.dpreview.com/forums/read.asp?forum=1025&message=41518486 )

"Strictly speaking you should use no more light than is necessary, 1/2 stop overexposure being enough.

Too much non-image-forming light passing through the lens is a bad idea. It flares around between the glass elements and reduces quality."

Barrie, could you or someone else elaborate on what you mean by "non-mage-forming"?

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