I had an aha moment today about Flash photography, and for those budding photographers that want to raise their photo IQ by a few points, I'm going to share my insight.

One golden rule of flash photography is "shutter speed controls your ambient exposure, and aperture controls your flash exposure". I find it edifying to understand "why is that exactly", instead of just following the adage blindly. If you think about it, it actually makes a lot of sense, and I'll delve into this, but first it helps to have a basic understanding of photography.

There are three basic dimensions to gathering light from a camera. The first of these is ISO, or sensitivity of the film/sensor. The higher the ISO, the less light is necessary to capture the image, but film that is so sensitive, and electronic sensors that use higher voltages to detect the light, cause more noise or graininess to enter the fray. Most cameras do fairly well at ISO 400-800, so that's where I recommend most people start. Only when you have an abundance or lack of enough light should you resort to lower/higher ISO settings. The adage with ISO is "the higher you go, the greater the exposure, with more noise in the photograph".

The next is aperture. This setting controls the size of the opening of the lens during the exposure. The wider the opening, the more light you let in. In an extreme example, a pinhole camera has a tiny opening, and because light collected from all corners of the photograph come in from the same miniscule hole, nearly the entire photograph is "in focus". Meanwhile, with a wide aperture, your focusing element controls which part of the photo is in focus because the lens will cause (often a pleasant) blur of light that is not in the focused plane because it is commingled with light coming from other parts of the scene which are focused on the same point on the sensor. The adage with aperture is, "the wider you go, the greater the exposure, with more blur in the photograph".

Finally, is shutter speed. This setting controls how long the sensor is exposed to the light. The faster your shutter, the less likely that any movement in the photo will be blurry, but the less light that will be allowed in. With a slow shutter speed, you can let in a LOT of light (most common photographs use exposures that are fractions of a second) but the greater the chance that any movement will cause blur. The adage with shutter speed is, "the slower you go, the greater the exposure, with more motion blur in the photograph".

The photographer that stumbles into "Manual" mode will struggle with these basic principles, and the tradeoffs they represent.

The landscape (pardon the double entendre) changes, however, when you introduce flash photography into the mix, and to figure out why the adage works, we need to distinguish between the nature of ambient light and flash light.

Ambient light is light that is generally pervasive, and it is continuous. In other words, the light from a streetlamp at night, or the light from the sun at sunset doesn't, for all practical purposes, change in intensity or duration during your typical exposure.

Flash light, on the other hand is typically limited in time, not continuous. Even if you have a long shutter speed (say 3 seconds) a flash only lasts a fraction of a second. So a flash during a 1 second exposure isn't going to net you 1 second of flash-exposed light. You could modulate the amount of flash light you let in by setting your shutter speed to a really fast exposure to limit the duration of the flash that is captured by the camera, but this would have the effect of capturing so little ambient light, that only your subject would be visible-- a technique that is useful for blacking out the background and getting a sharp mask around your foreground subject. In general however, this is why your shutter speed will have little to do with the exposure of the flash. That's why the shutter speed should be set to capture the ambient light. Imagine your photo as just the background. How would you set the shutter speed to capture the scenery?

Once you've answered that question, your flash is going to be used to expose your subject in that scene. To set the relative exposure of your subject to the background, your principal choice (assuming a fixed ISO setting) at this point is your aperture. If you want your subject to be brighter relative to the background, choose a wider aperture, and narrow it if your subject already has enough light.

From the above, however, realize that the wider your aperture, the less depth of field (i.e. more blur outside the area of focus) you will get in your photo. If you're OK with giving up this element of control in your photo, you're set. If you want a brighter subject, and want to maintain a high depth of field (more stuff in focus), then you need to begin investigating advanced flash techniques, such as using off-camera flash, reflecting your flash off of umbrellas, walls, ceilings, or using multiple flashes to get the effect you want.

In general, an amateur photographer can capture interesting photos by giving up control over the depth of field, and simply modulating the relative exposure of foreground (by adjusting the aperture) and the background (by adjusting the shutter) using the built-in flash on their consumer-level SLR or point-and-shoot cameras.

One final note before I leave you to experiment with your cameras-- If you have some ISO headroom to explore and would rather make the sacrifice not in depth of field, but in graininess of the photo, you can bump your camera up from ISO 400, say, to ISO 800. Now, with your sensor twice as sensitive, you can get greater depth of field by halving your aperture... In other words, if you were shooting at 1/8th a second to capture ambient light, and had your aperture set to f/2.8 to capture your subject (with flash) at ISO 400 but found the photograph to be too "soft", you can accept greater noise by going to ISO 800, but now you can shoot at 1/8th a second with your aperture set to f/5.6 with the same flash to get a greater depth of field. Repeat again if you want greater depth-- ISO 1600 and 1/8th a second with aperture now at aperture f/11...

Check back later and I'll try to add some example photographs I've taken to demonstrate these principles. As you can see, photography is often the art of making the right tradeoffs and trying to avoid any significant sacrifices by adjusting your environment or your camera to get the desired effect.