In April there was a great full moon and I went out to the Charles River to photograph the city with the moon overhead. I had just realized how nice a lens I got with my K1000 when I bought it from eBay. It’s a 135mm f/2.5 and if you look down the barrel it’s all glass – beautiful.
Anyway, apart from my 50mm f/2.0, this is my fastest lens. The zoom lenses I got with my K10D start at f/3.5 or something. Since it was dark and I wanted to keep the exposures as short as possible, I thought I’d use my 135mm.
Unfortunately, when I took some pictures with it, I got the image below and others like it:
The moon shows up twice and the word “Prudential” is also visible twice. From the word “Prudential” it’s clear that this secondary image is upside down and backwards with respect to the original.
Initially, I didn’t know what was going on. At some point, however, I thought it might be related to the UV filter I had on my lens. I took the UV filter off the lens and the weird reflections disappeared. This solved the problem but it still doesn’t explain to me why or how this happened. I can’t think of a physical explanation for what I observed.
If anyone knows what set of reflections could be responsible for upside down and backwards “ghost” images appearing, please let me know.
[Insert Witty Title Here] 
My guess is that the image first reflects off the surface of the sensor, travels back out of the lens to the inside surface of the UV filter and is then reflected back through the lens onto the sensor again, where it’s recorded again.
However, it seems to me that the reflection should be coincide with the original, rather than being upside down and backwards. Both the original image and the reflection originally travel once from the outside, through the lens, hitting the sensor. Then, the reflection travels back out of the lens and hits the UV filter, where it is reflected and travels through the lens a 3rd time. It seems to me that, since the reflection is flipped upside down and backwards twice by traveling through the lens twice between hitting the sensor the first and second times, the net effect should be nothing.
If someone who understands optics could clear this up for me, I would appreciate it.