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Using a long (10 second) exposure I found I was getting an odd "reflection" in the picture, where the brightest object leaves a residue in the opposite part of the frame.

You can see it here: sample of the problem

I was still getting it in 4/5 second exposures as well. NB: Using Canon 550d with a 11-16mm F2.8 Tokina lens, at 16mm.

I assume it is some physical thing like an internal mirror or lens thing, but does anyone know for sure?

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4 Answers 4

up vote 13 down vote accepted

This isn't uncommon to see when you have a UV filter attached to the lens which, generally, a lot of people do because it gets recommended by the camera store as lens protection. If you want a really detailed explanation, there's one on Luminous Landscape showing and explaining the issue.

My take, and it's a personal opinion, is to lose the UV filter if you have one. The "protection" offered is usually minimal, at best, and the impact to certain images is not. So, to my way of thinking, the loss outweighs the gain.

Now, all of that assumes you have a filter on your lens... Looks like it, but you didn't say.

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2  
Ah - I had no idea this would affect it, I do indeed have a UV filter on the front. –  AlastairC May 2 '11 at 18:31
2  
@AlastairC: Awesome. Now, make another big bonfire and try again with the filter off, so we can see if that was indeed it. :) –  mattdm May 2 '11 at 18:34
    
Also, if your filter got a thin film of smoke on it from the fire, that might increase the effect. –  coneslayer May 2 '11 at 18:34
1  
I'll ditch the UV filter, I'd added it assuming it was needed to protect the lens. –  AlastairC May 2 '11 at 18:38
1  
Just adding that I have seen the same problems with UV filters when shooting mostly dark scenes with bright spots, such as candles. Even being aware of it, I'd forget to remove the filter fairly often, so I'm going without now. –  ieure May 2 '11 at 23:13

The reflection is just a form of lens flare, you actually get reflections like this in every image, but the intensity is usually lower than the rest of the incoming light so it's totally invisible. Here the fire is massively overexposed allowing the reflection to show up against a dark background.

If it's not the result of a filter there's not a lot you can do about it except reduce the exposure.

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Hazarding a guess, it looks like you have badly coated element in there, or perhaps a non-coated filter, those are likely to cause such things in some situations. It has to do with brightness striking the glass at just the right angle, so it might not be noticeable in shorter exposures.

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Is it possible that light from the fire is illuminating the smoke? it's possible that in the 10 seconds there was enough smoke to receive enough light to create the anomaly. Does this also occur in other nighttime long exposures without the fire subject?

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an interesting theory, but the odds of seeing the shape of the logs mirrored in the smoke? seems pretty slim to me. –  cabbey May 2 '11 at 19:22
    
The link to 'Luminous Landscape' in John Cavan's answer showed the same effect with spot-lights. My example is unusual because you can see the shapes of the logs in it, but it looks like just the same effect. –  AlastairC May 2 '11 at 23:00

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