Tuesday, January 22, 2008

Short Tute: Off-Camera Flash

Before we get started with the off-camera flash discussion, let's take a quick look at "on-camera" camera flash. It's usually a good idea to be able to compare one thing with something that most people are familiar. So on-camera flash - what is it and what's it get us and, even more importantly, what might it's short comings be?

On camera flash is convenient. Your on-board flash, if your camera is so equipped, is always there. If you have an external flash, it’s quite easy to mount it on your camera’s hot-shoe and start shooting. In fact, most modern cameras can “talk” to your flash and, between the two of them, figure out how much light should be flashed at your subject to get what the camera thinks is a good exposure. And you often end up with something that looks like this:

This image is fairly well exposed, but there’s no shadows to help the viewer get a sense of depth. Contrary to many people's first thoughts on the matter, shadows, when taken in moderation, are a good thing. The light in the above photo causes it to look very flat - there are almost no clues for the view to help him see the 3D relationships between the various elements of the photo.

Off-Camera Flash can provide a completely different look. The following photos show just some of the possibilities.

There are two different light sources for this photograph. The first (main) is to camera left and is providing an almost backlighting effect (note the brighter areas on the right side of her face) and another light (about 1 stop lower) to camera right and behind the camera that is providing a certain amount of fill (note the gentleman's face and how it's not in deep shadow).

You can also generate some very dramatic light/shadow effects with off-camera lighting that is not possible with on-camera flash:

and this one

I can hear it now, "Those are some stunning photos! But, it's gotta be a lot of work to set up the lights and then have to always adjust the light output so you can get the shots you need/want." OK, maybe no one is actually saying they are stunning. At any rate, off-camera lighting is actually so much easier and more reliable then on-camera flash. Here's one way to make it work:

  • Get some manual flashes (yep, actually works best with manual flash and it less expensive then, for example, Canon Speedlights). In the above examples, I used two Sunpak 622 flashes. You also need the appropriate battery packs.
  • You need something to trigger the flashes. Lots of possibilities here:
    • PocketWizards
    • Optical triggers - you can get fairly cheap items from EBay. These often require line-of-sight between the "master" and the "slaves" units and they can, sometimes, be triggered by the flashes of guests' cameras.
    • Wireless triggers - again, inexpensive items can be found on EBay. The reliability of these is hit or miss. Some have modified them after receiving them, adding longer antennas, etc to improve their reliability and still, sometimes, not attaining 100% success.
    • My poison of choice is the PocketWizards. They are more expensive then the alternatives, but they work and over a considerable distance. You don't need to worry about line-of-sight restrictions. And, the cost of these more than makes up for the savings realized on the purchase of the manual flashes.
  • Get some lightstands; one per flash - You need to get the flashes up above the floor. I like to have them be the highest thing in the room, but lower than any hanging light fixtures or ceiling beams, etc (these can cast some distracting shadows).
  • Put it all together and place one in opposing corners of the room (assuming a rectangular room :D)
  • Find the spot that is approximately half-way between the two flashes. Using a flash light meter, set each light to the desired aperture.
All the above will result in even cross-lighting at the "half-way" point. Subjects closer to one flash then the other will, of course, be lit at different ratios and the exposure from the "closer" light will, of course, be higher then at the half-way point.

You will need to compensate. But, you can't be running around all night with a flash light meter, asking the guests to "hold that thought" while you attempt to get an accurate light reading. So, what to do? Well, You remember that light propagates according to the "inverse-square law", right? If not, check out The Inverse Square Law - what it means to Photographers. When you halve the distance between the subject and the light source, you need to adjust (close down) your aperture 2 stops. Like-wise, when that distance is doubled, you need to open the aperture 2 stops. Simple stuff. Ok, but the guests are not going to be at 1/2, 1/4, 1/8 of your base distance from your light source just to make your life easier. You're supposed to be a professional. The solution is that you have a certain amount of "windage" in the exposure, especially if you are shooting RAW. You estimate the required adjustment (you should be able to get it within 1/2 stop without breaking a sweat) and shoot, correcting the exposure in post. Easy stuff.

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