The test flash button on your flash (usually that little red glowing button on the lower left of the flash back) doesn't get much use. In our flash workshops we've discovered that some folks have never touched that button, and have never test-fired their flash. In normal, every-day shooting there probably is no need to, but the test flash button has some very important functions.
If you use a slave unit or slave flashes, the test flash button on your master flash (usually the flash attached to your camera) will trip the other flashes, allowing you to test your lights and see if everything is assembled correctly and is working. We do this all the time on our Hummingbird Photo Shoots in Arizona, and on our Reptiles of the World shoots we do each year here at Hoot Hollow.
However, when we're doing this we usually have all of our flashes on manual mode, either to control the power ratio (the hummers) and because of the manual studio units we're using (the reptile shoot). In the field, we usually use TTL flash, especially if we're working birds and especially if we're using a tele-flash extender (the Visual Echoes tele-flash). When a tele-flash is mounted, it is possible that the fresnel lens of the tele-flash is not quite aimed properly, ESPECIALLY if you have your flash unit mounted off-camera. Why would you do that? Well, if your subject has reflective eyes mounting the flash off-camera will prevent or reduce the chances of recording red-eye. If you're working a relatively stationary subject (a roosting whip-poor-will or a nest-cavity bird, for example), an off-camera flash or flashes may provide better lighting because of the shadow placement.
When you test your flash to assess whether or not it is aimed properly and will hit your subject, don't test fire the flash when it is on the TTL mode. You won't hurt the camera or flash if you do, but you will drain your batteries. TTL flash determines an exposure from the film plane, not from a separate sensor in the camera's pentaprism. Accordingly, when you test fire a TTL flash, the flash dumps all of its energy as it attempts to acheive an exposure on film, when, in fact, the flash sensor for determining exposure is never exposed! The camera actually has to fire for the TTL to work correctly, but doing so will waste a frame (or several if your aim is off) and may also drain the flash's energy if the subject is distant or if you're using a small f-stop.
By setting your flash unit on Manual Mode, and by dialing in a low power ratio (1/4 will usually work, but in dim lighting situations even 1/16th power will), you can test fire your flash several times and the flash's stored energy will not be depleted. You'll be able to see where the flash is aimed and make adjustments if necessary. At a low power ratio the flash doesn't have to expend much energy, so it will recharge faster. On TTL, conversely, you might find that the flash takes 8 seconds or longer to recycle (unless you are using an supplemental battery pack), and if you have to test fire the flash several times, you might drain your batteries!
Remember, when you're finished and you have determined that the flash is aimed properly (or if you're testing slaves), remember to put your flash back to TTL mode if, indeed, that is the flash mode you will be using for your flash exposures! This tip is just one of the items we cover more thoroughly in our Advanced Nature Photo Course (the next one will be 2004) which is a course dedicated to understanding and applying flash photography, although we also cover the essential points of TTL flash in our Complete Nature Photo Course.
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