Beware Newton’s Rings

Protection for an optical price

UV/Haze filter (B&H Photo)

However!

Aurora near Edmonton, October 9, 2012, Canon 7D, EF-S 17–55mm f/2.8 (when you are new to aurora, there are powerlines everywhere!)
Yes. There they are. Can’t un-see.

Newtonian optiks?

Why do they only show up in aurora images?

The proof

I thought I used this lens when initially doing the math, but no. The pixel pitch for the sensor is 4300nm per pixel, and the wavelength of the aurora light is 558nm. A few iterations and corrections led to a 24cm radius…
That radius looks about right for the 17–55mm. Light likely bounces between the front element and the UV filter, which admittedly was cheap and had poor or no anti-reflective coating. There is also a relatively small air-gap between them, which may help make the rings more visible

The risky fix

The monster Sigma 14 f/1.8. It doesn’t even have front filter threads, so I guess that is one way of solving the problem of Newton’s rings. (B&H Photo)

The lens protection checklist

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