The Mystery of the Purple Blob

[DWHonan]

My beloved 5D is currently in a box, on a truck, bound for the Canon service center in Irvine, CA. Why? Because of this:

That's a 1,396-second exposure from up on Stevens Pass last Saturday night, f/4.0 at ISO 1600, unprocessed but for resizing and a light USM pass. Yes, it's as noisy as it looks, and then there's The Blob. I subsequently did some tests in a controlled environment at home and ran a series of tests to confirm that there's no possible way The Blob was caused by light entering the body:

1. 17-40 f/4L, ISO 200, 20 minutes, no covers
2. 17-40 f/4L, ISO 1600, 10 minutes, viewfinder covered
3. 17-40 f/4L, ISO 1600, 10 minutes, viewfinder covered and lens cap on
4. 28-70 f/2.8L, ISO 1600, 10 minutes, viewfinder covered and lens cap on
5. 28-70 f/2.8L, ISO 1600, 10 minutes, piece of electrical tape over the LED and viewfinder covered and lens cap on, with a bath towel draped over the body/tripod to eliminate any possible external source of light contamination.

I cannot recall any unusual handling (no drops or other impacts) of the camera during the day -- which is not to say that I might have inadvertently done something stupid without noticing -- and the blob is not at all evident on two-minute exposures taken the previous night. Does anyone have any ideas what might have caused this? I'm curious to learn what you folks think before I hear back from Canon.

Thanks,

[JaniceL]

Only one explanation that I can see, poltergeists :-)

[BobH]

Dave,

The noise doesn't surprise me. I just bought a book on night photography, and he mentions that long exposure typically generate a lot of noise, due to the sensor heating up.

In fact, he even talks about a technique for getting star trails where you "layer" a series of shorter images, rather than one long exposures. The entire reason for that is the noise the occurs during long time exposures. This is one area where film has a major advantage, it doesn't heat up during a two hour exposure.

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?)

The purple blob is a different story. It looks like that part of the sensor is overheating. No idea why, or what would have caused it. I'm guessing you never saw that before. As for what to do about it, you've already figured out the only logical course of action, send it to the shop.

[BobH]

David Honan, now specializing in trains during the day and "Twilight" photos at night...

[Railgeek]

As I've read on other forums (FM and POTN,) that kind of noise can be rendered out with noise reduction in camera. Bob has is mostly right- the buildup of electricity around the sensor in the electronics causes some sort of "photonic" (photons) reaction, therefore making the sensor light up. Here is an example with a Nikon D200.

http://www.flickr.com/photos/43765368@N07/

I don't think that Canon will replace anything having to do with the sensor itself, it might replace some PCB around it, or behind it most-likely. Give us an update on what Canon says when you get your repair back.

[BobH]

Wow, I'd never have guessed that would come using in camera noise reduction.

Of course there is one big downside to that. Your exposure time doubles, since it closes the shutter and takes a second image, then uses that to to compare with the first and figure out what is noise and what is actually part of the image.

Not a problem at 1/2 second exposure, but when your shot is 20 minutes long, that's a real hassle!

[squirl033]

23 minutes? what'd you do, tape the shutter button down and go for coffee? if you wanted a daylight shot, why not go there in daytime instead of shooting a 23-minute exposure at night? seriously, though, i'd have expected a whole lot brighter image than that for 23 minutes at ISO 1600!

the 5D is marginal at ISO 1600 - it's just not up to the high-ISO performance of the newer models, and you will definitely get quite a lot of noise in any exposure setting, especially in super-long exposures over 10 minutes.

[BobH]

Originally Posted by squirl033

23 minutes? what'd you do, tape the shutter button down and go for coffee? if you wanted a daylight shot, why not go there in daytime instead of shooting a 23-minute exposure at night? seriously, though, i'd have expected a whole lot brighter image than that for 23 minutes at ISO 1600!

Rocky already knows the answer, he also does night shots. But if you're wondering why Dave would bother, here's the answer. When it works, you get something magical, like his Foss River Bridge shot.

As for the light, I agree, it's pretty dark for 23 minutes at 1600. 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. Just goes to show that when you get away from the city and all the light pollution you find there, the night really is dark.

[Steve Carter]

My guess is it's the sensor failing. If so, that's going to set you back a pretty penny. To bad you don't have another camera. Oh, wait you do, or will have soon. The 50D will do much better at 1600 iso, and certainly not too shabby at 800-1000.

[Railgeek]

I don't know why people say the 5D is marginal at ISO1600- to me it's great. Moving from a 10D where the image was grainy at ISO400- 3200 is like my old 800. Don't let me near a 5D II!