Amp Glow?

Boomn4x4 · Nov 9, 2010

I was playing around last night with long exposures, and I keept getting a strange pinkish purple blob on the right side of my shots. It would show up after about 5 mins of exposure and get prgressively worse. When taking the photos I assumed it was either light getting in from my view finder combined with a light on my neighbors house. However after getting the images in for processing, I began second guessing myself. So to test it, I went into a bathroom, set the camera on the vanity, turned off all the lights and started an exposure. Came back 20 mintues later, and the same glow was there even in a completly dark room.

So now it got me into thinking that I have a defect with my camera, so I did some googling. I found a few hits on something called "Amp glow". First of all, does what I have in that shot look like amp glow? Secondly, I did have my noise reduction turned off, in my readings, noise reduction will fix it. Is there anything else I should know or how to avoid it... maybe even what causes it?

Not to inflate anyones ego, but I've come to trust the photography knowledge quite well at this site and would rather hear what you have to say than some flikr discussion board.

Boomn4x4 · Nov 10, 2010

Yes Boomn4x4, that is definatly "amp glow" it is a factor in long exposure shots and is noticable in most all digital camera models, more or less. To correct it you need to turn on Noise Reduction. Noise Reduction will virtually elminate amp glow. One thing you need to know, however, is that before you go out and try again with noise reduction turned on, make sure you have a full battery.

Noise reduction works by taking a second black exposure with the shutter closed and overlaying it on the original. This second exposure is done just as long as the original. The black exposure will have the same amp glow in it as the first, so the camera's processor will take the black image and remove the amp glow from the first image giving you a clean shot.

What this means to you, is that if you take a 20 mintue exposure, then camera will do another 20 minute exposure meaning the total shot will take 40 minutes before the processed image shows up on the screen. The green led on the back will stay lit and in the display it will flash "Job nr".

Shoothing like this is a battery hog and you may not get off more than 2 shots even with a fresh battery.

Also make sure you turn Noise Reduction back off when not doing long exposures as it has no value to short exposure times and only make it take twice as long to get your images, effectivly reducing your camera's max shutter rate of 3.0 fps to 1.5 fps.

Infidel · Nov 10, 2010

Boomn4x4 said:
Is there anything else I should know or how to avoid it... maybe even what causes it?

Sounds like you've already determined that this is related to exposure duration. If you were so inclined, you could figure out the minimum exposure duration required for this effect to disrupt image quality...although this is subject to a number of factors. That said, you could try to keep exposure durations shorter to avoid this problem.

My understanding is that this is a heat-induced artifact. Many higher-end cameras dedicated for astrophotography (also fluorescence/dark field microscopy) incorporate active cooling systems to prevent this. Be careful, because it is possible to damage or destroy the sensor/amplifier with a sufficiently long exposure.