Photo Question: Why Would You Use a Flash During the Daytime?

What’s the point of using a flash during the daytime if you can just edit the photo later on?

Though the more experienced photographers may already know the answer, there are lots of folks who don’t understand why you’re supposed to use a flash during the day. I mean, why not just overexpose the image or go into the shadows to shoot the photo? Well, life isn’t always that plain and simple. And if you’re really hellbent on fixing it in post-production, please believe me when I say that using a flash during the day will make it much more manageable. You can probably do this with your on-camera flash, but in most other cases, an off-camera flash will do this the best.

Notice how tough it is to get both the subject and the background perfectly exposed without a flash?

The Science Behind It

There are two ways to do this, and each approach has its own science. The first is HSS–otherwise known as high-speed sync. Different companies call it different things, but it basically lets you shoot a photo with a flash at a crazy fast mechanical shutter speed. The other method is by using a flash that has a quick flash duration. In layman’s terms, what it does is act as a second shutter speed.

High-Speed Sync

Typically speaking, when you use a flash, it will sync up to no more than 1/320th. But with the high-speed sync method, it will often sync up to 1/8000th. With the way that light works when a flash is firing:

  • Shutter speed controls the ambient light
  • ISO controls overall sensitivity
  • Aperture controls how the flash affects the scene depending on TTL or manual modes

This is sometimes called overpowering the sun, and you’re getting just that here. The point is for you to underexpose the ambient light. And to provide some fill on the subject, you use the flash. If you were to otherwise expose the scene, you wouldn’t get the details of the subject and the background: one or the other would be either blown out or underexposed. Photographers usually use this method when they’re photographing an editorial or commercial campaign on location. It’s also very popular with portraiture and weddings.

The way this works is that the flash switches from firing one powerful blast to a bunch of rapid bursts. The flashes happen so fast that the human eye can’t even see them. But one of those blasts is bound to hit the imaging plane (sensor or film) at one point or another. This means that it uses a lot more energy, and you can drain the battery power a lot faster, but you’ll get a much better shot.

To do this, you need a flash that can do high-speed sync with your camera system. Set the flash to high-speed sync, and then just fire at your subject. Be sure to diffuse the light in a softbox, umbrella, octabank, or whatever you prefer.

Flash Duration

The idea behind flash duration is quite a bit more complicated than High-Speed sync. The way that this works is by using a studio strobe or something a bit more powerful than a typical flash. Each flash has what’s called a flash duration. The flash duration is how fast it will pop, and it’s measured in fractions of a second–the same way that shutter speeds are mentioned. The quicker the flash duration, the faster it can act as a second shutter speed. The faster this second shutter speed is, the more it can overpower the sun and kill ambient lighting.

That’s really all that there is to flash duration. All you need is a higher-end light. Check the tech specs; most cheaper flashes don’t have a fast flash duration. The quick flash duration also can stop fast-moving subjects pretty easily, so it’s ideal for sports or anything you want to shoot that’s fast-moving.