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Digital Flash Photography

Great picture taking often requires perfect light and often the quality of available light isn't always ideal. In areas like portrait taking we can easily use flash to overcome any light inadequacies with landscape photography the challenge is slightly different.

Light is probably the most important factor in digital photography and using flash can make all the difference in getting the light correct. Most cameras have some sort of built-in flash and without it we wouldn't be able to take some great photographs. There are a variety of popular types of flash and flash modes on digital cameras. I have listed these below along with the situations you would use them for digital flash photography.

digital flash photography - golden retriever

Automatic flash

This is the most common type of flash used in digital flash photography. On all point and shoot cameras with automatic set to on flash the camera detects low light levels and forces the flash to fire.

Flash on or forced flash.

This is usually an option on all digital cameras and it forces the flash on regardless of the available light. It is great for outdoor shots, often referred to as fill-in flash. It is used when the subject is darker, for example you might be taking a portrait shot looking into the sun or in shadowed area.

Flash off

This is usually an option on all digital cameras and it forces the flash off completely - Use this to take long exposure shots at night or where flash photography is not permitted, such as in museums etc.

Slow-sync flash

This is the real gem of digital flash photography. It is usually an option on digital cameras and it forces the flash on. Slow-sync mode activates the flash when using slow shutter speeds and is useful for balancing the light between background and subject, and is great for night time portraits. Often best used with a tripod to achieve the best results.

digital flash photography - baby

Digital Flash photography with red-eye reduction

This is usually an option on all digital cameras. Red-eye can be a problem in portraits taken with flash lighting up the blood vessels in the eye causing them to appear red on the picture. It is most common when subjects are close to the lens. To prevent red eye digital cameras have a red eye setting which causes a single flash shot or a sequence of flashes, or a more prolonged and less distracting light before the main flash is fired. This causes the size of the pupil to shrink prior to the picture being taken.

Freezing Action with flash

The speed of a flash or the duration of a flash is in thousandths of a second and it can be used to freeze the action of a moving object.

Understanding TTL flash with DSLR cameras

Most digital SLR cameras come with a built in flash and a hot shoe for mounting an external flash. The built in flash is a TTL (through the lens)flash and most of the flash units sold by digital camera manufacturers are TTL flashes.

The first thing you need to grasp is that there are two complete metering systems working together when you shoot using a TTL flash. The first metering system is the camera's own metering system used to determine the exposure without flash. The second is the flash metering system which always monitors the centre spot and this is entirely separate from the camera's own metering system. The flash metering system in the body of the camera talks to an external flash via the hot shoe.

Ideally when you shoot flash the camera metering system is set for the correct background exposure and the flash automatically uses it's own metering system to adjust the power level of the flash to light the subject.

To understand what exactly goes on lets go through the sequence of events.

When using TTL flash it is important to remember that the digital camera is effectively using a dual metering system. The exposure you set on the camera is what is used to expose the background which is illuminated by the ambient light. This same ambient light does of course illuminate the subject. How much the subject is illuminated depends on how much ambient light there is. Your TTL flash takes this into account when it meters the preflash pulses and adjusts the flash power to add precisely the correct amount of illumination to the subject.

Indoors, however, the ambient light falling on the subject is usually so weak that the flash becomes the main source of light. In most cases you can ignore the ambient light on the subject and consider the subject to be entirely lit by the flash.

To understand TTL flash and how to get the best out of your camera try this experiment.

Take your digital camera into a moderately lit room and set it to Aperture priority. Focus on a central subject like a cup or something. Adjust the aperture setting to it the lowest f value possible, on many zoom lenses this will be f3.5 or f4. Check the camera speed at this aperture. Ideally we are looking for something like 1/30 to 1/60 of a second. It may be necessary to adjust the iso setting to give you a high enough speed. Now take your picture.

Now set your camera to the Manual mode, switch the flash off and adjust the speed to 1/160 or 1/200 and the aperture to f16. Now take a picture. ideally it should be all dull or black, completely under exposed.

Now put you flash on and take the image again. Ideally the image of the subject should be correctly exposed. The flash metering has done its work correctly. Now reduce your aperture by 1 f stop and take another image. The exposure image of the subject should be exactly the same. Repeat this at a lower f stop, again the exposure image of the subject should be the same. Increase the iso by 200 and take another shot. Again the exposure of the image should be the same.

THE FLASH CONTROLS THE SUBJECT IMAGE EXPOSURE, APERTURE OR ISO HAVE ALMOST NO EFFECT ON THE ILLUMINATION OF A SUBJECT.

If you repeat the experiment with the settings for the first image - you will now see the flash output is severely limited and is effect being used as a fill flash.

Using flash outdoors

When you shoot outdoors and the ambient light is the main contributor to the exposure of the subject this is when you use your your flash to only provide fill to lift shadows on the face of the subject. It is essential to ensure the camera speed is not outside the sync range of the flash or camera and the aperture and ISO have to be adjusted accordingly. It is also advisable to use some negative compensation on the flash to prevent blown highlights particularly on faces.

TTL Flash compensation settings

To avoid blown highlights when using your TTL flash as a fill flash try settings of -0.3 to -1 for indoors and -1 to -2 for outdoors. The brighter the ambient light the more negative compensation you need to lift the shadows on faces.