Built-In versus an External Flash for a dSLR Camera
You can enjoy flash photography with your digital SLR using either the built-in flash (also called a pop-up flash, available on all but the most expensive models) or an external unit. Basically, both types of flash work the same way: they emit a strong pulse of light that briefly illuminates the scene or subject you are shooting. This extra light brightens the subject and helps the camera expose the photo properly.
Despite the similarities, there are big differences between the two approaches. Knowing why you would want to use your camera's built-in flash and when you should invest in an external flash will help you be ready for flash photography, no matter what the situation
Using your camera's built-in flash
You might want to use your camera's built-in flash for several reasons:
You don't need additional equipment. When you buy the camera, you also get the flash. That means you don't need to spend another few hundred dollars on yet another thing that you have to squeeze into your camera bag.
It's always there for you. Because the built-in flash is literally built into the top of your camera, you will always have the flash with you.
Batteries are included. A built-in flash relies on the camera's battery for power. That means you don't have to buy extra batteries of a different type than your camera.
It stows and goes. When you're not using the built-in flash, you can keep it discreetly locked down and out of the way.
It’s less obtrusive. Even when raised, the built-in flash doesn't look so gawkishly geeky that you turn heads.
Upgrading to an external flash
You should consider investing in a good external flash for several reasons:
Variety. Most manufacturers offer three tiers of external flashes for you to choose from: entry level, mid-range, and professional. You can shop for compatible flashes made by another manufacturer.
Power. External flashes are more powerful than their built-in counterparts. When you compare guide numbers (a measure of how strong the flash is at a certain distance and ISO setting), external flashes can be up to three times more powerful.
They turn heads. All but the most basic external flash heads tilt and swivel. This means you can bounce the flash off a ceiling or wall to create indirect lighting.
They don't produce lens shadow. When mounted on the camera's hot shoe, most external flashes sit higher than a built-in flash. The flash clears the lens when it fires, avoiding the dreaded lens shadow in your photos.
You can accessorize them. You can customize external flashes with snoots, diffusers, soft boxes, gels, and other gadgets designed to soften or color the light.
They work at more shutter speeds. External flashes often have a special high-speed sync (HSS) mode, which lets you use much faster shutter speeds than built-in flashes.
Flexibility. You don't have to mount external flashes on your camera's hot shoe if you don't want to. You can place them off-camera on a bracket, a light stand, or a small mini-stand.
They are cordless. Most external flashes can work wirelessly. That means you can not only mount them off-camera, but don't have to use a special cord to hook things up. More expensive external flashes (called the master) can control other external flashes.
They don't discharge your camera's battery. External flashes do not suck camera batteries. That's both a pro and a con. However, keeping the battery power separate means you can take more photos from a single camera battery charge than if you were using the built-in flash.
They offer custom settings. More advanced units have a tremendous number of customizable options. This appeals to professionals who may use the flash in a number of circumstances and need precise control.