Introduction
The Canon EOS 20D digital camera has powerful features for taking shots in low light conditions: a low-noise sensor that can be pushed all the way to the equivalent of ISO3200 film, and the ability to use image-stabilized lenses. I ran some tests on what can be done with these features, and am summarizing the results here, with more details attached to each individual photograph. For those who just want to jump directly to my collection of test shots, they are available in their own image gallery, along with their associated notes. Each image below can be clicked on to take you to the larger gallery image, and from there links are available to the original size, which at 3504x2336 is a bit larger than most screens can comfortably handle, but are available to check zoom quality.
Handheld shots at night
I started without a tripod, just to show what can be done. The following shot was taken without a flash after 10pm at night, in an area that is not well lit; it looks much brighter from the photograph than it does to the human eye. Comparisons can be made to following images to understand exactly how dark it really is. 0.7s is a long time to hold a camera steady, even with a computer stabilizing the lens for you, and it shows, particularly at the card above the fire extinguisher. This is also at the widest setting on this lens (the EF-S 17-85mm), which gets me f/4.0... but zoomed in, I can only get f/5.6.
If you take a shot just of the statue visible at the center, go back down to the equivalent of ISO400, which is the maximum that you can get off of consumer-grade film or compact digital cameras (even good ones such as the Canon Powershot G6), and fire at a much more reasonable 1/8s (0.125s), but at 56mm zoom (which forces f/5.6) you get this:
This isn't very impressive; it looks like it's a dead black image. This isn't quite true, as you can use digital editing software such as The Gimp or Photoshop to raise levels on even a photo like this fairly simply, which gives you back some information, at the cost of horrendous quality:
Update: I went back and tried this recovery again using the raw import plugin in Adobe Photoshop CS to handle exposure, shadows, contrast, and saturation adjustment, then ran the result through Noise Ninja, and got an even more impressive result, though nowhere near the quality (nor ease) of the higher ISO shot that is displayed after this one:
Now for the magic: with a flick of a dial, the sensitivity of the sensor is increased to be effectively ISO3200 film, and the same shot is taken again, still at 1/8s and f/5.6:
This doesn't look spectacularly impressive, but there really is a lot more information in this image, and a mild underexposure can be corrected in a digital editor without the same kind of degradation seen in the extreme example at the beginning:
Not bad for a night shot with no flash. Noise is visible inside the statue, but it's minor, and could probably be corrected with a program such as Noise Ninja.
Flash
A flash won't help you if you're trying to take a night shot of a distant object, but here the object is only some 5m away, so serves to illustrate the difference in power between the built-in flash and an external flash such as the Speedlite 580EX.
With the built-in flash at ISO400, the computer tells me that the best shot will be at 1/60s and f/5.6, which is reasonable, and provides:
The Speedlite 580EX provides much better illumination at the same settings (and for a $400 flash, it had better):
Tripod
With a tripod, of course, night shots become easy even without special features. At ISO400, you can take a f/5.6 shot nicely exposed in 4 seconds, bringing in the background where even the flash couldn't (though for a shot like this, the background may only distract anyway):
If you switch to ISO1600, you can get the same image in four seconds even at f/11.0:
A tripod also lets me show exactly how much light is being added by each ISO jump. We can start with an underexposed image at ISO400, 1 second exposure time at f/5.6:
... see it improve at ISO800:
... and come out just fine at ISO1600:
One last shot at ISO3200 shows that you can get that same shot with only half a second of exposure time, which with the image stabilizer is just barely feasible without a tripod in desperate cases, though the shot here is still taken tripod-mounted:
Conclusion
You can do some fairly impressive things at night with a Canon EOS 20D even with a slow lens, and without a flash or a tripod. The price you pay is a bit of additional noise, but that can often be filtered out in post-production, and just being able to go up to ISO800 or ISO1600 can make an otherwise impossible shot possible.
Raw-imported, noise-filtered version of black shot
I've updated this to show a version of the "total darkness" shot that used a raw import and Noise Ninja to perform a spectacular recovery. Getting the shot right the first time is still better, of course.