Using flash is one of the technical aspects of photography that’s difficult to understand, mainly because it’s something you cannot really see with your own eyes. Working with natural light, you can usually envision how your shot will turn out, and then when you become a bit more skilled, you can even envision how the effects of aperture and shutter speed will affect your images. Working with flash further complicates our visual relationship to what we’re photographing, since it’s an additional level of abstraction we’re adding to the mix.
I won’t argue that working with flash is difficult to master, even after you’ve become a veteran “strobist,” but it is one of those things that becomes easier to use the more you use it. One step toward learning to work with flash, whether it’s your on-camera flash or an external flash, is understanding that there are dedicated modes to help you refine how a subject is depicted when using flash. Much like how there are well-known exposure modes, flash modes exist, too. And mirroring the exposure modes we’re familiar with, these flash modes can be selected depending on the amount of manual control we want, versus automated settings selection, and they can also be chosen to prioritize certain aspects of exposure. For example, you’d choose to work with the shutter-priority mode if you’re trying to freeze fast-moving subjects since you can deliberately choose, or prioritize, a faster shutter speed. Let’s take a look at some of the common flash modes you’ll encounter.
Auto
This one is simple enough, and pretty much will do as expected. Much like an automated mode on a camera, an automated flash mode will select the flash output and timing based on how the camera interprets what you are photographing using distance and exposure measurements. Just like an auto shooting mode, though, this likely won’t be the best option when you’re trying to do something a bit more creative with your exposure, like intentionally darkening the background or adding some blur to a moving subject.
Front Curtain Sync/First Curtain Sync
Despite its potentially confusing name, this is your straightforward flash mode, and might even just be called “On” or “Flash On” depending on your camera. This mode will sync the flash to fire at the beginning of your exposure, which will stop motion. If you’re working in a setting with some ambient light and you continue the exposure after the flash has fired, then the subject will blur from the point where they were in the frame when the flash fired. This effect is called “dragging the shutter.”
Rear Curtain Sync/Second Curtain Sync
Opposite to front curtain sync, rear curtain sync is when the flash fires at the end of an exposure. The easiest way to envision the difference between front and rear curtain sync is to picture a subject moving laterally across a frame; if you photograph this subject with just a long exposure and no flash, this subject will be a single horizontal blur. If you photograph them with a flash using front curtain sync, the subject will be frozen at the beginning of the movement and then the blur would continue in the direction they are moving. If you photograph them with a flash using rear curtain sync, the subject will be frozen at the end of their movement, leaving a blurred trail behind them. These two flash modes can greatly impact how motion and speed are perceived.
Slow Sync
This flash mode isn’t too dissimilar from front or rear curtain sync, but its implications and use applications are. Whereas front and rear curtain sync are intended for working with motion, and are not necessarily used in conjunction with slow shutter speeds, slow sync is a mode that blends flash exposure with longer exposure times to suit working in low-light conditions, specifically when the flash itself cannot adequately illuminate the scene. Depending on the specific situation, this is one of the few flash modes that might need some additional support, like a tripod or monopod, to achieve a perfectly sharp background, but you might also be able to get away with a bit of motion blur if the background is also slightly out of focus, too.
High-Speed Sync
A unique and useful mode, and something worth delving into a bit more for its own sake, high-speed sync is a unique function that lets you photograph with flash using shutter speeds higher than the camera’s maximum flash sync speed. Most cameras have a sync speed of around 1/250-second, 1/180-second, 1/60-second, or something reasonably similar; these speeds are the fastest speeds at which your camera can use flash and still achieve a fully exposed frame. They are the fastest times where the entire sensor or film plane is being exposed at once, and the threshold point where faster exposure times are only exposing a portion of the frame at a time to achieve the faster total shutter speed. In other words, the rear curtain is already beginning to close over the frame prior to the front curtain ever being fully opened.
High-speed sync works in conjunction with this shutter design and essentially fires one longer duration flash (or what ends up being a series of flash pulses) that will last from when the front curtain opens all the way until the rear curtain closes. This sounds great, but there are some drawbacks compared to more traditional flash modes—namely, the flash output power is typically much reduced, which makes this flash mode most suitable for close-up portraits or other nearby shooting situations. It’s not a flash mode you’ll use all the time, but it comes in handy when trying to get shallow-depth-of-field effects and shoot with flash at the same time.
Red-Eye Reduction
Everyone has seen a photo, usually taken at an indoor party or at nighttime, of a group of people in which everyone’s eyes are bright red due to the flash exposure. And everyone pretty much knows that cameras have a dedicated red-eye reduction mode... but how does this setting work? Without getting too deep into the science of why the red-eye effect is a thing, it is helpful to know it is most commonly caused by the flash being close to the same axis as the lens itself, which is why it’s often associated with point-and-shoot cameras and on-camera flash usage. One of the best ways to avoid getting the red-eye effect is to simply move your flash farther away from the lens, but if this isn’t possible for one reason or another, camera manufacturers also feature this specialized red-eye reduction mode to minimize the effect. This flash mode will pre-fire the flash a few times to effectively force subjects’ pupils to close, which will reduce the likelihood of the red-eye effect showing up.
Commander/Remote
While the above modes apply to on-camera, built-in flashes and external flashes alike, commander and remote flash modes specifically imply that you will be working with an external flash. These modes will be named differently depending on the camera brand you’re working with, but they perform the similar function of being able to wirelessly trigger an off-camera flash. Depending on the sophistication of the system, your specific camera, on-camera flash, or built-in flash, these modes offer varying degrees of what you can “command” an external flash to do, such as change its flash output. These systems typically work in conjunction with TTL flash metering and are easy and intuitive ways of developing more complicated lighting schemes.
Hopefully this helps demystify some of the different modes you can use with your flash. Regardless of whether this all makes sense immediately or if it’s still tough to envision, the best way to learn more about flash is by practicing using it and also taking a look at my more general guide to on-camera flash. Just like any other skill, you’ll only improve with experience, and this is even more crucial with flash since it’s a more abstract process. Let us know if you have any specific flash mode questions in the Comments section, below!