Flash unit
 

In addition to video, I am interested in the still photography aspect of the 5D2 and I want to do some timelapse. I have been shooting local fire artists and want to do long exposures of the fire with a single flash to freeze a moment of the person while leaving the trails of fire in perhaps a one-second exposure. What sort of flash should I get for the 5D2 without spending a fortune? I don't use a flash much as I don't like the look but they are useful on occasion so even recommendation of a simple type of flash that I can manually trigger might work. Oh, I already know that flash is daylight-balanced so I plan to put CTO over it to match the firelight.

For the work you have described you'll need a flash unit that allows you to adjust the
" Strobe " time interval....

Most lower end flash units wont allow the strobe interval timing to be adjusted manually...

An example would be if you were to drop a quarter coin from your hand in front of the
camera and you wanted to record the quarter as it dropped... if you don't adjust the strobe timing then when the quarter drops and the camera takes the picture, the picture will show the quarter at a given time during the drop and so you will have a picture that
shows just one quarter and no sense that the quarter is even moving at all.
But lets say that the quarter dropped four inches in a 1 second time frame... now you
set your exposure for one second and fire the flash in " strobe mode" four times/second. Now when you look at the picture that the camera took you will see four quarters, each in 1/4 of a second time interval...you can also adjust the strobe for it to fire even faster during the drop and you would end up with a picture of the quarter being hundreds of quarters blended together during the drop.
now you see what a manually adjusted strobe will do for you...

apply the same to your fire artist and you can play with the strobe timing to get what you
want out of the picture.... if you fire the strobe fast you will get lots of fire in the picture.
In the Canon line of flash units, it would be 580EX

EOS (SLR) Camera Systems - Flashes - Speedlite 580EX II - Canon USA Consumer Products

Actually, Ray, I was hoping for just one flash during the exposure. Your explanation of how strobing flash units work is helpful but that is not what I need for this kind of photography. Is there a flash that is not so pricey that can be set to fire manually once or programmed to go off amidst a long exposure (instead of at the shutter release)? I need an external mic and perhaps a juicedlink more than I need a flash and lens fever has my bank account low.

I think you will find that if you were to set the camera exposure to 1 second and somewhere
in that second of time you fired a flash you'd get a ghosting effect where there would be
some of the picture in correct exposed light and some of the picture blown out of exposure
when the flash took over.

Lets say you were doing the fire artist a magazine cover shoot.. ideally you'd want at least
two flash units... one flash would be remote that you would use to illuminate the artist
so it could be on the ground in front of the artist... that flash would fire and capture the
artist... and you might even have to adjust the flash strobe and the flash power because
your going to be giving some more exposure from the second flash unit also... so its all
about control... controlling the timeing the amount of light and where you place the light.

Thinking this through a little more.. on the less costly side of things.. :-)

You could put a lamp out in front of the artist and illuminate the artist... then set your
exposure for 1 second and set the aperture correctly for the artist exposure...

this should let the capture get the artist in correct exposure and the fire would be a fire
blurr... might work out well??

sounds like a fun shoot... let us know how it goes and show us some pics when its all done.

The problem with shooting fire artists with the 5D2 is not getting sufficient exposure. A fast lens with the great sensitivity of the 5D means that will never be an issue with fire as a light source. The problem is that the fire is so much brighter than the person. With the fire as the subject itself, it needs to be exposed in a way that brings out its detail and colors. The only way to accomplish this is to brightly light the person with a flash for stills and studio lights for motion video. It is easier to just use a flash and take interesting stills and the fire artists don't seem to mind a flash. I will probably convince a few of them to go in front of lights for video, but that is not so easy in the location where they practice.

What you're looking for is called rear- or second-curtain sync... I believe if you connect a speedlight it will have an option under the 'External Speedlight control' menu. Some units have a dedicated button on the flash but I believe even the cheaper ones can be controlled through the camera menu to enable this.