Just a quick post while I finish processing the photos from our Mojave Road wrap-up trip last weekend. Toad and Bullfrog stayed to camp with us at the Kelso Dunes. We missed sunset because we were busy exploring elsewhere, so I decided to play around with some night photography.
I suspended a little LED tent lantern inside the top of the tent, and the light on the ground in front of my tent is from the campfire. It's partially blocked by Toad's jeep which created some nice shadows that look almost like a path leading up to the tent. The last time I tried this I only had the light on for five minutes and it wasn't nearly enough light so I knew I needed to leave it on much longer this time. Unfortunately I didn't take a watch with me, so I just had to guess, and I think it was close to half an hour. It's a little too hot, especially at the top of the tent, so next time I'm going to move the light closer to the middle of the tent and keep it around 20 minutes (and remember to bring a watch).
It was a dark moonless night; there should've been a baby new moon out there somewhere, but I don't think it ever made it over the mountains. It was so dark that I coudn't even see if I had a good composition and I just set focus at the hyperfocal distance. Another note to self that I need to get a much more powerful flashlight for night photography. The ones I have are fine for poking around in caves or navigating the terrain in the dark, but not nearly bright enough to light distant objects sufficiently to allow sharp focusing.
I also just realized that my 20D EXIF doesn't record shutter speeds in excess of 1024 seconds! Don't know why I never realized this before...maybe because I usually note the length of a long exposure using a watch. Anyway, I'm guessing that the total exposure for this image was close to 1.5 hours (maybe two?), f/2.8 at ISO 100.
I use the Long Exposure Noise Reduction feature (C.Fn-02) so the in-camera processing time is equal to the length of the exposure. An astrophotographer explained to me that it is more accurately referred to as dark current noise. Dark current arises from thermal energy leakage within the CCD/CMOS sensor and is affected by high temperatures and long exposures which contributes to the degradation of the image. True astronomical CCDs are cooled to liquid nitrogen temperatures to limit the dark current, but that technology is cost-prohibitive to most of us so we're limited to dark frame subtraction.
The dark current pattern is repeatable given controlled temperatures. So basically, Long Exposure Noise Reduction takes another frame of the same exposure length of 'nothing', the equivalent of shoting a frame with the lens cap on. This creates a dark frame image of nothing but black and the dark current noise, which is then subtracted from the original image. It's not 100% perfect out in the field because it's not a controlled environment and temperature fluctuations affect the dark current emitted, but it does a remarkable job. If your camera does not have a noise reduction feature, it is possible to do it manually; just shoot another frame with the lens cap on with an equal exposure time, and do the dark frame subtraction back home on the computer. It's a lot of work, though.
Long Exposure Noise Reduction really prohibits taking more than one or two shots each night; if you take a one hour exposure, the camera needs another one hour to perform the dark image processing and subtraction. A two hour exposure requires four hours for the complete processing. It also eats batteries. When I finish a two hour exposure at 1:30 in the morning, I usually just let the camera crunch away while I sleep, knowing that it will need a fresh battery in the morning.