This really isn't a Canon or DSLR-specific trick, but for a lack of a better place to talk about it, this is where I'll do my write up.
I have been experimenting this week with extremely long daylight exposures in the urban environment. My goal is to create "ghost city" images, devoid of pedestrians and moving vehicles.

A while back I was experimenting my DSLR and a pinhole lens. If you don't know what a pinhole lens is and what it has to do with photography, I recommend a quick Google search for "Camera Obscura".

One of the effects I was looking forward to with the pinhole lens was the extended time of exposure. In my initial tests I found that I could extend my exposures to up to 30 seconds at ISO100. This was great, but the downside of this was the extreme sharpnesses of the resulting image.

But during my pinhole research I found a fellow named Rob Gardiner who managed to capture Paris with a pinhole with amazingly sharp results and long exposures. Despite my laser-cut pinhole lens I wasn't really happy with the results and the fairly long focal length (about 50mm).

So I gave up the hunt until one day I was curious about how long of an exposure I could take using my glass solar filter from my telescope. So I "mounted" it on my 17-40mm (its was a couple cm's too large to fit snugly) and tried a few exposures in my back yard. So I did some simple time tests: 30 seconds.. nothing... 1 minute.. slight gray skies, still dark foreground.. 2 minutes.. I could just make out my shed.. 4 minutes... AHA!

The square looking white bit on the filter is a reflection of the sky, not a piece of paper!

The thing that I was most surprised to found out was that the white balance was fairly accurate and that the filter was truly a neutral density filter. There were no colors missing from the resulting snapshot. This was good news. There were some issues with light leaks as the inside diameter of the filter is 90mm, much too large for the 17-40.

A few months went by before I was able to get back to my experimenting. Finally, I took the 300d (the only camera I have with a cable release), the tripod and the filter in to work with me, as I could take some photos down town during lunch. My first results were promising, but again, the light leakage issue was very evident. I set up the tripod in Pioneer Square. This was a high traffic area both for people and cars, especially around noon. I decided it would be a good time to make a foam insert for the filter so that it both snugs on better and blocks out any stray light. That evening I cut off some 3/8 inch foam from a sleeping pad for hiking which worked great.

My second results were taken from the roof level of the Macy's parking garage looking down on Third and Pike streets. This was a very high traffic area at lunch time. The results were pretty good this time. With exposures of over 5 minutes, the people completely disappeared. The only problem, really, was the fact that the metro buses tend to stop for extended times along Third avenue and appear ghosted.

My third attempt would be to test just how long of an exposure I could make during lunch. Again, I chose Pioneer Square as my test. This time I went as long as 20 minutes, however stopping cars were cutting my exposures short as I knew that they'd throw the exposure for me. One major difference between this day and the previous two was the fact that it was a bright sunny day compared to the overcast of the previous two tries. The results, to say the least, were interesting. Almost like a Daguerreotype. There were a lot of flares which I have not pinpointed the cause of and the noise levels were nearing the unacceptable. I had to use "despeckle" in the gimp to eliminate them.

In the brief 3 hour lunch breaks I've had to experiment with this project, I've found that the best exposure times seem to be around 7 to 10 minutes, a balance between time and noise. The optimal conditions are overcast, where there are no harsh shadows or light sources to cause strong flaring. I'm going to continue to experiment with this during my lunch breaks when I can, and I also hope to take the filter out to a seashore to experiment with water and waves with long exposures.