BetterPhoto Q&A
Category: New Questions

Photography Question 
Jeffrey Bernstein

What's wrong with this camera (or my shooting)?

Notice the green phantom of the main highlight in all three of these photos. These were taken with my new Nikon D50 with two different lenses (the first with my old Nikon 75-240 zoom, the second two with the 18-55 kit lens). Is there a problem with my camera or my technique? Thank you!

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6/28/2006 9:25:47 PM

Jon Close
BetterPhoto Member Since: 5/18/2000
  The green spots are mirror reflections of the bright light points in the photos. Typical causes are from using a filter (especially inexpensive non-coated or single coated), or possibly reflections onto an uncoated rear lens element off the glass covering the digital sensor. In each case the effect is exacerbated by overexposure. The first picture of the moon is at ISO 200, f/5.6 and 1.3 seconds. Proper exposure of the moon - which is very brightly lit by sunlight - at ISO 200 would be something like f/8 and 1/400 sec.

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6/29/2006 6:06:54 AM

Alan N. Marcus   Hi Jeffrey,
Sorry you had this experence.

What you have is ghost flare, an internal reflection that made it way to the image plane. Over exposure give emphasis. A helpful countermeasure is a lens hood. Bright object just out of the field of view are sometimes responsible.

The color you see is reflected light from a coated optical surface. The coating was specialized to control the transmission of red and blue light (magenta). Green was reflected as this the color that is opposite green (its complement).

You are finding out that flare is devastating! We wish the camera lens could be constructed with just one glass element. Such as design has minimal flare. Sorry to report, a simple lens has so many aberrations for our needs. When we ask for a camera with expanded picture taking capabilities, we are forced to make the lens complex.

Complex means many glass lens elements, each with a polished glass surface. Some elements cemented together using transparent glue, others set close with an air space in-between. When light strikes a lens surface three things happen.
a. Refraction light enters passes through and its path is altered.
b. Reflection light striking the shinny surface is reflected back and away.
c. Absorption light passing through the material is converted to heat.

About 4 6% of the light is lost due to refection at each interface. Most often it is reflected backwards and hits the next forward element and is thus re-reflected. The bottom line is; all these reflections become stray light and we call it flare light. Most finds its way to the film/chip and intermingles with the image forming rays. Flare is devastating; it takes a terrible toll causing a loss of image contrast.

Lens coating to the rescue:
Opticians were astounded when they discovered that old (antique) lenses pass more light (were faster) than new lens of the same design. This discovery prompted investigation. Seems atmospheric pollution, over time, applies a thin transparent overcoat. This coating somehow made the lens pass more light by reducing surface reflection. In 1935 the technique of artificial lens tarnishing came into being. Today lenses are vacuum plated with silicone dioxide and/or fluoride. So coated, the light loss due to reflection is reduced to about 2%.

The coating thickness is the key:
The thickness must be equal to of the wave length of light. This is a problem because each color has a different wave length so when coated; the reflection retardant effect is centered on a single color. Multi-coatings, each with a different thickness give broader color handling. Typical is three overcoats. Extravagant system can have 7 or even 11 overcoats. Improvements to 99% for a targeted region of the spectrum have been achieved.

Alan Marcus

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6/29/2006 9:04:06 AM

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