How camera flash photography and digital flash units work

Once you've been a photographer for a while, you start to realize just how much light is required for a proper photo. This is weird because you're so used to seeing things in low light conditions that you assume that a camera can see as well as you do. But your eyes were designed to be much more sensitive to light than cameras. When the camera can't see well enough, it usually decides to pop the flash and try to compensate for the lack of light. On camera flash is a very harsh, unatural light which can also cause the appearance of the dreaded red-eye.

Still, flash has the capacity to provide enough light for stunning photos and there's a lot you can do to improve on it's harsh quality. I recently went to a David Ziser seminar where he demonstrated his flash photography wedding techniques and was duly impressed. David uses a combination of Quantum and Canon external flashes to get outstanding portraits. But before you go out and spend your hard earned cash on flashes, let's take a look at what you need to look for in a good external flash.

On or Off Camera

When purchasing a flash unit, I think you need to consider what it's primary use will be. Lower end cameras usually come with a built in flash...sufficient for non-professional uses, but not very flexible and practically useless for lighting larger volumes. Higher end cameras usually come with a hot shoe attachment where you can plug in an external flash unit.

Even when you're purchasing a hot shoe flash, you have an option to use the flash on-camera or off. My suggestion is that if you want to use the flash attached to the camera, you buy the most expensive flash your manufacturer makes. High end flashes are built to communicate carefully with your camera and give you advanced features like TTL and high speed sync, but only when they are able to communicate directly with your camera.

On the other hand, if you plan to use your flash primarily off camera, you won't be able to use some of those functions and you're better off getting cheaper flashes with a lot more juice. I'm assuming here that you're getting started with off camera flash and you're cheap like me. When you're shooting off camera, your first priority is much of it can you get...everything else is a convenience. Let's talk about power.

Guide Number

A hotshoe flash's power is normally measured a formula called the Guide Number. A low guide number means a weaker flash, a high guide number means a more powerful flash. You want to buy a flash with a high guide number because more power means more light, better exposures and more control.

Technically, the guide number is calculated by multiplying the distance between the flash and the subject by the f-stop required for a correct exposure at that distance. Double the Guide Number means the flash has to shoot at four times the power because of the inverse square law (a subject at twice the distance as another subject from the same light source gets a quarter of the light).

There are other features that are both cool and important to have.

High Speed Sync

Although most cameras can Most flashes can only synchronize at certain speeds with your camera. With my Canon 40D and my E540 EX II, I can shoot at speeds of up to 1/320 of a second. At speed higher than that, the shutter in my camera is unable to open and close quick enough. Here's why:

Depending on which type of camera you have, your shutters generally operate in one of two ways. For lower end cameras, the shutter is built into the aperture controls so that a hole opens and closes for a certain period of time to capture the exposure. The shutter is usually built into the lens. Those cameras have slower maximum shutter speeds than higher end cameras.

For SLRs, or cameras with removable lenses, the shutter control is not on the lens, but is actually controlled by two curtains in front of the chip that captures the photo. The curtains themeselves require a certain amount of time to open and close. Think of them like completely black windows in your house. It would take you a certain maximum amount of time to open up and close the windows. When the windows let light through, the exposure begins.

Like scotty would say, they canna break the laws of physics so it takes time to open them and close them. In order to get them to behave as if they were going faster, the camera does a little trick. The camera has two curtains and it starts closing the second (AKA rear) curtain before the first curtain is fully opened. That makes the camera expose the light

When the flash has high speed sync capabilities, it fires not once, but many times as the curtains travel through the series of positions, exposing the sensor as if it was one continuous bit of light. You have to be careful with high speed sync because it can burn our your flashes if you don't use it sparingly.

Why is this relevant? Because when you are lighting a subject on a very bright day, the appropriate exposure for your subject might be different than the exposure for your background. As a matter of fact, you may require a very high shutter speed in order to get a good exposure of the sky. So high that unless you have high speed sync available to your camera, you won't be able

Rear Curtain Sync

Now that we're in the subject of the flash curtains, some flashes will allow you to control when the flash is fired. For longer exposures, the flash fires much quicker than the shutter speed. As a matter of fact, most flashes fire at very high speeds in the thousandths of seconds range. Most of the time, the flash pops right after the first curtain opens. The camera continues to expose the scene until the rear or second curtain closes the exposure.

For a very long exposure, this freezes the scene at the beginning of the exposure, which may or may not be what you're looking for. In very long exposures, you may want to freeze the action at the END of the long exposure. This is known as rear or second curtain sync. It will allow you to create some special effects with your camera, so it's a good feature to keep in mind. The pic

Front Curtain Sync

The photo above was shot with first curtain sync. I think that the first photo works better for what I was trying to accomplish with the first shot.

Circumference of the Beam

Some flashes let you control how big the cone of light that is produced from the unit. When you fire a flash, the light goes out away from the unit in all directions. This creates a circular cone of light and if a flash allows you to change the size of the cone you can tighten or loosen how much of a frame is lit by the flash. This can come in handy a couple of ways. First, you can emphasize part of an image by tightening the cone, you can also use the edge of the cone to light with a soft gradient. It's a really good feature to have so even cheaper flashes will have this.

Multiple Firing Capabilities

In a dark room, it doesn't really matter how fast your shutter speed is set, the flash can actually freeze the motion of the object at different points. Some flashes will allow you to fire flashes at multiple bursts in the same exposure.


A few things you need to look for when buying a flash in regards to connectivity. Can it connect to an external power source. This is really important for hooking up external batteries to allow the flash to last longer like in a wedding or other long shoot, or alternatively you can connect an AC adapter and use the flash as cheap studio lights. Be careful with this because you can easily burn out your flash if you fire it too quickly.

Another issue is connectivity to the camera. To take better pictures, it's a good idea to take your flash off the hot shoe in your camera completely and shoot with the flash at other positions. This flexibility provides for some extremely creative options. Some flashes allow you to connect to your camera via a cable. Once you take your flash off camera, you'll loose some of the communications features between the flash and your camera like high speed sync and others.

When buying certain flashes you need to make sure they will work with your camera's model specifically. Flash hot shoe connections operate at different voltages and you can damage your equipment if you mix and match the wrong ones.

Remotes & Slaves

You can also fire your flashes using radio or other remote controls. You won't be able to do high speed sync on these, but being able to control multiple lights at the same time will give you some new lighting possibilities. You can also cheaply add a slave trigger to an existing flash to allow it to fire at the same time as other flashes. This is virtually at the same time because sensor in the slaves are light sensitive (and light travels pretty fast) so as soon as one flash fires, as long as it is in the field of view, the slaved flash will fire as well. The shot above was taken by putting an assistant inside the bathroom with a flash remote. I handheld another flash to the bottom left of camera and allowed a slow enough shutter speed to capture some of the very interesting ambient light.


Some flashes allow you to rotate and move the flash up and down so that you can work on getting the light to be more directional. With that, you can bounce the lights onto ceilings, walls and other surfaces and get some shots that can be better lit than straight on flash.

Lighting in two planes

Most people think of a photograph as the finished, two dimensional flat 4x6 that you might see on your screen or as a print. But while lighting the subject, what you're shooting is three dimensional and different parts of the frame are being lit differently. Specifically when you're shooting a subject, the subject may need to be lit differently from the background.

I often shoot the Viva La Música concert every year at SeaWorld in the Spring. The particular stadium I shoot at combined with the time of the year, and the time of the day I shoot at means the sun is usually at the worst possible place for a photograph...right on top and a little behind the subject...photographic heck. If I get a good exposure on my subject, the background is blown out. If I get a pretty blue sky background, my subject is silhouetted...rats. The way to get a good exposure is to throw flash at my subject, but there is a problem. The shutter speed to get the pretty blue skies is outside the range of speeds my flash can normally handle. Turning the flash into high speed sync really helps to solve this problem. I'll be doing some additional articles on this technique in the next few weeks.

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