Almost every modern camera comes with an on-board flash, even the higher-end DSLRs. You may wonder why, since almost every modern photo taken with an onboard flash is ugly: blown out highlights, red eye, harsh shadows and that characteristic look that screams "flash photo!" So why do manufacturers insist on including an onboard flash on cameras that are generally marketed to people who should know better? Let's find out...
The onboard flash does have limited uses. The best use for it is somewhat opposite of what most hobby photographers use their flash for: as a fill light during the day, rather than as a primary light source at night. In very bright outdoor light, your onboard flash can be used in a pinch to fill in those dark shadows, especially on people's faces. It can also help bring out detail in a backlit subject.
The onboard flash can also be used in situations where you can't turn the ISO up high enough (especially if your camera produces grain at high ISOs). But that's where its usefulness ends. Because when your best reason for using it is because "it's better than nothing," you probably ought to mostly avoid using it.
That's why every photographer needs to have an external flash.
Technical Reasons Why External Flashes are Better
Before we get into the primary reasons why external flashes are superior to your onboard flash (because they make for better pictures), there are also some technical reasons why you ought to use an external flash. The first is because you'll save your battery's life, especially if you find yourself taking a lot of flash photos. An external flash has its own set of batteries, so it won't suck down your camera's battery. This also means it has a faster "recycling" time - you can fire the flash more often, without having to wait as long between shots.
An external flash also has more power than an onboard flash (much more power, often as much as 15 times more), which means you can illuminate subjects at a greater distance. You can also illuminate wider shots - an onboard flash has a narrower range than an external one and can sometimes leaves the edges of the frame in darkness.
You've probably heard me suggest that you "bounce" your flash. If this helpful tip has ever been directed at you, it was probably while someone was politely looking at one of your least appealing flash photos. Of course, the term "bounce your flash" doesn't mean a whole lot without a fairly detailed explanation of how to do it.
You can sort of get your onboard flash to bounce by putting a small piece of white card in front of it at a 45 degree angle. This will "bounce" the light off of the ceiling, which will in turn diffuse it and make it softer (you'll probably need to turn up your exposure compensation too, since the light will be less bright and may not illuminate the scene as completely as it would have if you hadn't bounced it). But alas! With your external flash you don't need to settle for such a hokey and limited arrangement. You can aim the light at the ceiling, at the walls, at a large white animal or any other good-sized reflective surface that happens to be in the area. The flexibility of the external flash means you aren't limited to just the ceiling. The external flash also tends to give you nicer results since it is more powerful, and therefore more capable of illuminating the entire room instead of just a small part of it.
Red eye is that somewhat benign term we use to describe photos that feature people who look as if they are possessed by B-movie demons. Red eye happens with the flash reflects off of the retina of a person whose eyes are dilated (which is what happens when you are in a dark room). Even basic post-processing applications usually provide some method for removing red-eye, but no one really wants to sit in front of their computer correcting every photo if they don't have to. Because an external flash is further from the lens and at a different angle than the onboard flash, the light from the unit doesn't reflect directly back at the lens and red eye doesn't become a problem.
Though you can use your onboard flash as a fill flash, an external flash is better for that, too. The basic reason for this is power: the sun is bright, and you need to match the brightness of the sun if you're going to adequately fill in a dark shadow. This means that your flash unit doesn't have to be as close to your subject as your onboard flash would have to be to get the same effect. And the obvious bonus is that you can place your external flash anywhere - off to the left or right, if that's where it's needed, while with your onboard you are limited to straight on.
Almost everything good has some drawbacks, otherwise life would be way too easy. Bigger, more expensive external flashes tend to give better results, and bigger flashes mean you have to have some means to carry them around. Giant camera bag, here you come. And expensive goes without saying - not everyone has the budget for that sort of unit.
There's also some hit or miss involved; you'll need to experiment with different angles and different surfaces to bounce the light off of, plus you'll more than likely also need to make some adjustments to your flash's settings as well as your camera's settings. You'll also need to understand the relationship between your flash, aperture and the distance from your subject (divide the "guide number" of your flash by your selected aperture and that will tell you how far your subject needs to be from the flash). Otherwise you'll get photos that aren't illuminated enough, or are over-illuminated.
Until you've had some practice, you'll be doing this on a trial-and-error basis, so remember that the perfect shot isn't likely to happen immediately, and you may miss some shots while trying.
Sometimes flash is a necessary evil, so an external flash unit is something that every photographer ought to have, especially for those occasions when you don't have a tripod, or when you can't or don't want to bump up your ISO in very low light. With enough practice, you can use your external flash to take photos that don't look like flash photos. And that's definitely a whole lot better than "better than nothing."
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