Almost all of the entry-level and mid-level digital SLR cameras come with a built-in flash that pops up when needed (Figure 5.1). For many people, this is all the flash they'll ever need. It's always there, never needs additional batteries, and doesn't take up extra space in the bag or pocket. However, a built-in flash is not always a panacea.
Figure 5.1. All entry-level and most mid-level dSLRs come with a built-in flash. It's handy to have and always ready, but doesn't give you the flexibility of an external flash.
The drawbacks to a built-in flash are flexibility and powerbasically, you don't get either with these flashes. Because the flash folds into the camera body, it's small and can't be adjusted to different angles (more on this later). You can see the size difference between a built-in flash and an external flash in Figure 5.2. This relates closely to the second issue: power.
Figure 5.2. Comparing the size of the flash tube to a external flash, it's easy to see why you don't have the range or power you get with the external unit.
Flash strength is measured by guide number, or GN. To determine how close your subject must be to be lit up by the flash, divide the GN number by the aperture, and the resulting number gives you the effective distance of your flash in meters. So if you read a flash strength of GN=13 at ISO 100, that means if you want to shoot at f/4, the flash will reach about 3.3 meters, or 10 feet.
Another issue with the built-in flash is related to drawback number one above. Because the flash is so close to the lens, red-eye is a common problem when using these flashes with people or animals. Most cameras have a red-eye reduction mode to ease this effect, though, so let's take a look at that.
When the light source is too close to the lens, red-eye becomes an issue. It's such a common problem that almost every camera out today has a red-eye reduction mode. This works by firing one or more pre-flashes to prevent the actual flash from capturing the reflection in your subject's eyes (Figure 5.3).
Figure 5.3. Red-eye is a common problem with flash photography, especially when using a built-in flash unit.
Photo by Laurence Chen
It doesn't always eliminate the problem, but typically using red-eye reduction mode will lessen the amount of time you need to spend in your image editor fixing up your friend's eyes so they don't look bloodshot.
Red-eye reduction causes a slight delay in the capture of your image. When you press the shutter release, the flash needs to send these pre-flashes out before the actual image is captured, which can lead to shots with people in a different position than planned.