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If I shoot directly in to the (sun)light, I get a green reflection in my photos. Is this because of the UV filter? How should I solve this?

Camera: Sony A7ii
Lens: Sony 55mm 1.8
UV filter: Hoya HMC UV (C) Lens (49mm filter) [Amazon link][1]

Image with reflection

Image with reflection

Michael C
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TomCB
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2 Answers2

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This is a type of lens flare called "ghosting". A UV filter can certainly cause it to be strongly visible, although it can happen without as well. A UV filter can be useful protection if you are in a dangerous environment (for example, somewhere with blowing sand), but otherwise it's really better to use a lens hood for protection.

As Michael Clark points out in a comment, his answer to What is the blue circle in this moon image? explains why the artifact shows up as green.

Community
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mattdm
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  • I'm not sure this fully answers the question of why the ghosting is green. My guess would be that the extremely high input dynamic range causes a very small green-bias in the internal image processing to become obvious. – Carl Witthoft Dec 05 '16 at 13:39
  • @CarlWitthoft could the green be caused by the filter coating? – Crazy Dino Dec 05 '16 at 16:32
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    @CrazyDino the UV filter is unlikely to be coated. WHether the internal lens surfaces have a coating which transmits green at severe off-axis input angles (as you'd get from flare or internal reflections) is a question only the manufacturer can answer -- and they usually don't (answer, that is) – Carl Witthoft Dec 05 '16 at 18:24
  • AR coating has a frequency (color) at which it is least effective, which is usually yellow-green. Look at a bright light reflected from the back surface of the filter, or front surface of your lens to see the effect. – Jim Garrison Dec 06 '16 at 00:03
  • As has been explained in several answers to other [ghosting] questions, the color of the reflection is a combination of the coatings on the filter and the lower intensity of the reflected light than the intensity of the primary light. The sun is very yellow/green in color, but it is so bright that often all three color channels are fully saturated and the differences between the channels are lost. With the reflection the intensity is lowered enough to show at least some of those differences. – Michael C Dec 06 '16 at 00:41
  • Please see, for instance, this answer: http://photo.stackexchange.com/a/48964/15871 – Michael C Dec 06 '16 at 00:53
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As you know, the camera lens is fabricated using numerous lens elements, each with two polished glass surfaces. Now polished glass reflects away about 5% of the light. This reflected light is lost so only about 95% of the image forming rays will be presented to the lens. It gets worse; each lens element does the same. The result is that only about 60% of image forming rays make it through.

Lens coating to the rescue. An English optician, Harold Dennis Taylor (1862 - 1943) observed that old shelved lenses of the same design as new ones passed more light. It seems the old lenses were coated with a film of muck and grime, ever present in London’s air pollution from coal burning at the turn of the last century. Taylor experimented using various methods to artificially age (bloom) lenses. He was granted the first patent for lens coating.

Coated lenses mitigate light reflection from the polished glass surfaces. What do you think happens when light rays are reflected from surfaces of internal lens elements? The answer is, these redirected rays bathe the film or sensor with stray light rays. The result is what we call flare. Now flare is devastating because it robs our images of their expected contrast. Additionally, misdirected light play on film or sensor making ghost images.

Optical engineers address this problem by coating each lens surfaces with a thin film of minerals. It is the thickness of the coat that does the trick. The coat must be ¼ of the wavelength of the light to be controlled. Since each color has a specific wavelength, opticians often apply multiple coats to the glass. The color cast you see in the ghost image is due to unchecked internal reelections.

Adding a UV filter makes these conditions worst because you are adding two additional polished glass surfaces. Besides, the UV filter only works to mitigate haze seen in distant landscapes and aerial photography. The UV is redundant in the digital camera as the protective cover over the sensor is also a UV filter. The bottom line for filters, is never mount a filter if the harm outweighs the good.

Alan Marcus
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