CCD & CMOS sensor REFLECTION

Started Sep 20, 2005 | Discussions thread
Thom Hogan Forum Pro • Posts: 13,660
Re: CCD & CMOS sensor REFLECTION

DMillier wrote:

Many people who bought the Kodak 14n were plagued by the great red
spot. Essentially light reflected from the red coloured sensor was
bouncing back off the rear element of certain lenses (mainly
primes) - Fortunately mine doesn't do this!

No. It wasn't just light bouncing back off the rear element of the lenses in question. The lenses in question were of a specific design and what you were seeing in the hot spot was indeed a reflection, but it was aperture-position relative to certain element designs that caused it (the problem was aperture-dependent and generally only showed at small apertures). This problem pre-dated digital. On film cameras in certain conditions you could see a faint (usually blue) ring. But film's reflectivity was low enough and film generally didn't stay flat enough to make the problem highly visible.

The fact that the color of the hot spot was red has nothing to do with the sensor. It has to do with the filters on TOP of the sensor. The same lens used on an S2 Pro, Pro 14n, and D1 showed different colored reflections, and the color was that of the filter array.

This gets back to the original question. Sensors are reflective (as are most things), but not enough to really worry about. Filter arrays over the sensors are usually highly reflective. DSLR users have started noting that using protective (UV, skylight) filters on their lenses produces ghosting flare in certain situations. This is due to the bounce-back reflections that go from light source to filter array to filter back to the sensor. Having two somewhat reflective flat surfaces in an optical path isn't a good idea. One of the common problems I get questions about is shooting at night and seeing ghosts of light sources. My immediate answer is always "were you using a protective filter on your lens," and the usual reply is "yes." Taking the protective lens filter off usually either solves the problem or lessons it.

Lens makers have learned that they need to rethink coatings on surfaces to protect against bounceback reflections. Now filter makers need to.

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Thom Hogan
author, Nikon Field Guide & Nikon Flash Guide
editor, Nikon DSLR Report
author, Complete Guides: D70, D100, D1 series, D2h, S2 Pro
http://www.bythom.com

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