The Mystery of the Purple Blob

[DWHonan]

Time for some rather overdue responses to this thread (it was a busy week at work)...

Originally Posted by BobH

So, seeing the noise in an exposure over 20 minutes long is something I'd somewhat expect. (Question, I know you've done a lot of night shots before, how do they compare?)

This was the first time I had tried really long exposures at ISO 1600, so I'd never really had any prior experience with such noise. However, I've never seen anything like that at ISO 400, which has been virtually clean on the 5D in exposures up to seven and a half minutes.

Originally Posted by BobH

The purple blob is a different story. It looks like that part of the sensor is overheating.

Interesting thought, not something I would have considered.

Originally Posted by Railgeek

As I've read on other forums (FM and POTN,) that kind of noise can be rendered out with noise reduction in camera.

Wish I'd thought of that before sending the unit off to Canon; I would have liked to have run a test with NR on.

Originally Posted by squirl033

23 minutes? what'd you do, tape the shutter button down and go for coffee? <snip> seriously, though, i'd have expected a whole lot brighter image than that for 23 minutes at ISO 1600!

Canon has this neat cable release gizmo with a shutter-lockdown feature... Yeah, I was definitely hoping for a brighter shot, particularly since there was a full moon somewhere above the clouds; the photo I posted for the January contest was taken almost exactly 24 hours prior to this one at 2 minutes and ISO 400, though I guess the clouds must have been much thinner.

Originally Posted by BobH

On the other hand, the sky is so bright you can't really make out the streaks of the train's headlights. In fact, the only way I can tell there was a train is the little bit of bright light on the left side of the bridge and the way the trees change color.

That was exactly the intent of this shot, Bob. Actually, letting the headlight streaks creep into the scene was an accident, as I had hoped to close the shutter before they got onto the bridge. As it turned out, with the sky getting blown out that wouldn't have mattered much, and I liked the way the headlights' side lobes lit up the creek down below, so in the end I wished I had left the shutter open for the entire train's passage. Good thing #8 was riding this guy's blocks, which gave me a perfect opportunity to try a second shot! Maybe I'll try to work with that image one of these days.

[Brianjbean]

I read that with the 5D, you can take a 10 minute exposure at iso100 and expect it to be free of noise or relatively so. At 10 minutes the sensor overheats and that's when you start having the super-noise issue. the further you move away from 100iso the more noise you'll have too. the purple blob is probably a manifestation of that.

[BobH]

That's pretty much what the night photography book I have was saying. Once you get over about 10 minutes things start heating up and getting noisy.

The solution they have is "stacking" exposures to get star trails, where you take a series of shorter shots, and combine them to get the long star trails.

What I don't understand is how the shorter exposures work better. After all, the sensor is still on for long periods of time, but with just a tiny gap between exposures. But that apparently makes the difference.

[DWHonan]

Originally Posted by BobH

The solution they have is "stacking" exposures to get star trails, where you take a series of shorter shots, and combine them to get the long star trails.

I can see how that would help with star trails, but what about when it's quite dark and you're trying to capture enough light to show the whole scene? Sure, you can push it a stop or two when processing the raw file, but the book's solution doesn't really do much for situations like that in which I found myself last weekend.

[BobH]

Dave,

Yes, you're right about that, but in most cases a 10 minute exposure will be adequate. In fact, I'm still not sure how you needed 20 minutes at ISO 1600 with moonlight. What was your aperature? I know it's dark out there, real dark.

[Brianjbean]

Originally Posted by BobH

What I don't understand is how the shorter exposures work better. After all, the sensor is still on for long periods of time, but with just a tiny gap between exposures. But that apparently makes the difference.

yeah i guess the gap between shots gives it enough of a break to dissipate some of the heat. stacking is something i gotta try myself. looks like a great way to solve the problem.