Exposure Determination And Bounce Flash

by William Lulow

I wrote a blog post recently about how I use my bounce flash. One of my latest assignments was to document a large space that housed the Westchester Table Tennis Center, owned and operated by the well-known New York Times crossword puzzle editor, Will Shortz. The space was fairly cavernous, so I needed to make sure it was well lit, but I only had time to use my on-camera bounce flash rig. This is what it looks like:

I also had to capture a lot of action. So, I set my ISO at 1000 and wound up shooting most exposures at 1/125th of a second at something like f/5.6. This gave me enough light and speed to catch the action of a table tennis match:

(That’s Mr. Shortz on the far side of the table). The thing is that you don’t really need a super fast shutter speed to capture most motion. You might need 1/1000th of a second if you are really interested in freezing everything, but here the slight blur of the paddle and the ball, add a sense of drama to an otherwise static shot. You also might need a more powerful flash.

It’s interesting to note, and it bears repeating, that exposure is a combination of factors that include: ISO settings, lens aperture and shutter speed as well as, what we call “lamp-to-subject” distance. Also, if your light meter reads f/5.6 at 1/100th of a second with ISO 100, there are any number of combinations of the three settings that will yield the same exposure. For example, if you increased the ISO setting to ISO 200, kept the shutter speed at 1/100th of a second, you would now only need an aperture of f/8 to yield the same exposure. This is another way of saying that, measured in f/stops, every doubling of ISO setting results in a halving of your f/stop setting, keeping the shutter speed the same. It’s an “inverse” ratio. Same goes for each of the three main components of exposure. The rule in the physics of light is that the intensity of the light varies inversely as the square of the distance between the light and the subject. Got that? If you have a flash unit on your camera, get a reading, then move back twice as far from your subject, the light on your subject will decrease by one f/stop. The converse would be true if you moved half the distance closer to the subject. So, the closer your flash is to your subject, the less exposure is necessary.

Most speedlights (the kind you can mount on your camera or on a light stand) do not have as high firing speeds as regular, studio strobe units, mostly because the batteries that power them are not very powerful. There is a way to synch your speedlight to higher shutter speeds in order to freeze most action by using the High Synch Speed mode on your speedlight. Flash manufacturers have built this feature into modern speedlights so that photographers can synch digital cameras with shutters built in to them with their on-camera flash units. In the past, shutters were built into lenses. These days, electronic shutters are built into cameras and are capable of higher synch speeds.

Also, studio strobe units, being more powerful than speedlights are often used to stop action because they fire at speeds of 1/4000 of second not just 1/400th or even 1/100th.

Since I use bounce flash for most of these assignments, I also expect that the flash output will be reduced as well. Adding a bounce card to your on-camera flash will usually reduce its output by at least half. This means that if your exposure was f/8 using the flash directly, bouncing it will require an exposure of at least f/5.6 or more. 

So, if you keep in mind the rule of halves and twos, it should help you get a handle on your flash exposures. Namely,  f/5.6 is twice the exposure of f/8. 1/50th of a second is twice the exposure of 1/100th of a second, etc. Also, increasing your ISO setting from 100 to 200 is also doubling the exposure. Once you get used to these effects on your images, setting your camera on manual will be much easier and your results will improve dramatically.