THE BASICS

The basics of flash

One of the most overused yet misunderstood pieces of photographic equipment is the flash. Here's a guide to the fundamentals.

Flash (noun): a brief, sudden burst of light The biggest advantage of the flash that's either built into your camera or an external unit such as a Nikon SB910 or a Canon 600 X-RT is the ability to add light to your images, but the biggest disadvantage is the complex nature of flash that puts many off to using it to its full potential. Understanding the nomenclature, the buttons, and dials on the flash itself, and the menus inside, is akin to trying to learn a foreign language in only a week: it's impossible.

What in the world is FEC? What does TTL mean and how do we use it effectively? What is high-speed synchronization? How are we as photographers, most of us non-engineering-types, going to understand the complexities of these systems and put them to effective use in the field? Picking up a manual full of technical jargon is a daunting task, considering what is inside is just as confusing as the flash itself. When you purchase your flash, you usually follow something called the "Guide Number. But what is a guide number? Is it related to subject distance responding to flash output, shutter speed, aperture, and ISO, or is it just a number that gives us a total flash power rating? That's the problem. As modern-day photographers used to picking up a camera and having it do all the work for us, some of the foundations that are used to build a good technical understanding of photography have gone to the wayside. How are we to understand what a "Guide Number" is if we've never had to deal with the concept?

The automation of photography is one of the greatest gifts in photography. Now, anyone can pick up any kind of camera and capture a great image. Along with computer software, even the worst image we've ever taken can be turned into art, worthy of a gallery or the wall of your home. But we really need the lessons of the past to understand the current trends in flash photography, so we can fully begin to understand the flash on or off our cameras.

Let's jump right in here and look at the basics of flash. Let's try to understand the concepts and numbers that cloud our thoughts, and see how we can get great images without spending an excessive amount of time in software: you may find that you're trying to fix images you could have captured properly in the first place.

Guide numbers

First, let's look at "guide numbers." In the old days, in those photos where you see a press photographer holding an ancient Graflex camera with a bulb flash on the side, the photographer had to figure out subject-to-camera distance in order to set a shutter speed and aperture to capture an image his editor would accept for the next edition of the paper. So think about what is going through the photographer's head. In the amount of time it took to read this passage, the photographer figured it all out, set his camera and took a shot, replaced his flash bulb and film, and got ready for another shot: Whew!

As modern-day photographers, we need not concern ourselves with figuring all of that out — or do we? Understanding how guide numbers work will give us a greater understanding of how our flashes work and in what situations. To find the right guide number for your flash situation, you need only to look at simple math:

Guide Number = Distance x Aperture

Initial guide number calculations are based on an ISO of 100. As in all things in photography, if you change the aperture, distance, or ISO up or down the guide number will change reciprocally. But you need to find this initial starting point to figure that out. So if your subject distance is, say, 10ft (3m) and your aperture is f/5.6, then the guide number on your flash should be 56ft (17m) with your flash on full power, using all the power accessible to you. If you stop your aperture down, say, to f/8, then your distance halves to 28ft (8.5m). This is, of course, if you are working with the flash at full power. If you halve the power, then the distance, just like stopping the aperture down, halves to about 14ft (4.25m). These calculations are all based on using your flash with a bare head. The numbers will be affected by a great many variables, including putting a diffuser over the flash head, using an umbrella, or a softbox.

This does get easier. On the back of some flashes, there should be a Guide Number or GN setting. This takes the guesswork out of guide numbers, if you want to use them. On the back of a Nikon SB910, for instance, the GN setting gives you all the information you need. Set to an ISO of 200, it's base ISO setting, and the GN setting gives you the perfect flash output and subject distance. If I set my camera to Program Automatic, letting the camera choose everything, with a 50mm lens I get an ISO of 200, shutter speed of 1/60 sec., and an aperture of f/5. At these settings the flash will automatically tell me that my optimal camera-to-subject distance is 6.6ft (2m) with automatically adjusted flash output of between 5.8ft (1.77m) to 7.4ft (2.25m). In this mode, the camera will do everything for me.

If I swap settings and go to Manual Mode on my camera using the same 50mm lens and choose ISO 200, shutter speed 1/125 sec., and an aperture of f/8, the GN setting on the flash will automatically tell me the camera-to-subject distance of 6.6ft (2m), but the automatically adjusted output for the higher aperture will decrease a little and have an adjusted GN distance of 5ft (1.5m) to just under 7ft (2.1m). By choosing a faster shutter speed and smaller aperture, the overall distance is reduced a bit.

Guide numbers now have been relegated to the category of overall flash output. If you do a search for a new flash for your kit, you'll be thinking about guide numbers a lot. The guide number of a Nissin Di866 is 198ft (60m) at ISO 100, compared to the Nikon SB910, which is 111,5ft (34m) at ISO 100, or the Canon 600 EX-RT, which is 197ft (60m) at ISO 100. The units differ in overall flash output on the highest end of the guide number spectrum, so depending on your camera system or flash, you can shop using guide numbers as your gauge to the most powerful flash at optimal outputs for your needs.

Through the lens

Now that we've covered the basic and classical way of determining your flash-to-subject distance to obtain your optimal flash output for a correct exposure under perfect conditions, let's take a look at what your flash is capable of without any consideration for the math involved in determining your exposure. Everything that determines exposure on your camera — point and shoot or DSLR — is determined Through The Lens or TTL. The acronym TTL confuses a great deal of novice photographers. Through The Lens or TTL only means that the camera has the ability to automatically determine exposure by simply seeing the scene in front of it. All this...