Oh, No! The Green Moon of Michigan!!!
Elise’s cousin sent me a few pictures the other day with a strange green “moon” in them. She was concerned that there may be something wrong with her camera. After answering her question I got to thinking that this may be of interest to our photo taking families.
“What is the green moon in these photos? What did I do wrong?!”
That moon is called refraction, which is also mixed in with some lens flare. This can happens when the sun is just outside of you field of view. The lens can still "see" the sun but you don't see it in the view finder. So what happens is that the sun's light (image) is so strong that it ends up bouncing at an angles throughout the lens. This is also how lens flare happens. Remember those 70's movies and TV shows that opened with the sun and the circles moving and the camera panning to the hero most likely walking through a dessert, cue the Kung Fu Tv theme. In those days the circles were more colorful and probably more of them. Modern lens design and coatings have come along way to reduce these effects and control flare. The reason the spot is green has to do with the fact that different wavelengths of light bend at different rates through the lens, the lens coatings may also have an affect. This can happen with other light sources than the sun, street lamps at night and reflections off of glass or mirrors are two easy examples.
Typically you can see the refraction happening in the viewfinder if you look carefully (be careful not to see the sun through the finder or you may burn your eye, similar to a welders eye burn, very painful). As you move the camera so the offending light source is moving away from the central axis of your lens the spots will move off but you may still get lens flare. Lens flare is where the light source bounces through the lens at an oblique angle and produces a colored haze, usually orange and sometimes with green tins.
A matching lens hood from the manufacture can go a long way preventing or reducing lens flare. But the spots can only be removed by changing your angle to the sun. Or placing a block between the front of the lens an the sun. This can take the form of a tree or tree trunk, a building, a light pole, some card board, or even your assistant ( I’ve used Elise a number of times for this). In the movie business this is called a Gobo or Go Between.
Question, Do you have a UV or Haze filter on the front of your lens? If so, sometimes these filters have lower quality coating and optical properties than the lens. If you see flare a lot it may be coming from the filter. Doing a test to make the flare appear with the filter then remove the filter and see if it is as easy to make happen. This is not to say that you should take the filter off, they make insurance for protecting the front of the expensive lens. I am saying that the quality will only be as good as the cheapest / dirtiest part of the optical path.
If any of our readers have similar questions that I can help to answer, send me an image if you have one and the question and I would love to take a crack at answering it.
Rob
