Introduction to Flash Photography
Basic Principles and Language Of Photography Using Electronic Flash
Jan 13, 2008
Flash photography has evolved over the years from igniting powder through single-use exploding glass bulbs through to the latest electronic marvels. While flashguns have a limited range, they are essential for taking shots where the photographer and has little or no control over timing of an event or the lighting. Typically, this is the situation confronting wedding photographers and photojournalists.
Guide number
This is a measure of the light output power of a flash unit and is used in calculating the exposure settings for lens aperture and camera ISO setting based on camera to subject distance. Normally the manufacturers quote the guide number as meters of illumination at a specific ISO setting. Now, all this is set by modern computer-controlled flash exposure systems that vary the flash output based on the actual lighting conditions. However, it is a useful comparison between flash units from the same manufacture to their relative light output.
In dim light, the pupils of eyes tend to enlarge to let in more light, and a bright direct light from a flash mounted close to the digital camera lens reflects off the red blood vessels at the rear of the eye, giving the famous red-eye effect
Red-Eye Reduction
One way to reduce the effect is automatically firing the flash before taking the picture. The eyes react to the light, reducing the pupil opening. When the main flash burst comes along, the smaller pupil opening reduces the red-eye effect.
Flash Synchronization
Electronic flash is a very short bright burst of light. The key is after the shutter release button is pressed, the flash must wait until the digital camera’s shutter is wide open before it fires, and must fire before the shutter starts to close. Depending on the camera, this can be a quite specific shutter speed or a limited range of the camera’s shutter speeds. This does leads to more specialized flash modes with fast and slow shutter speeds.
P-TTL/ E-TTL
Program Through The Lens, or Electronic TTL according to Canon. This is where the camera and the flash unit talk to each other to decide when to fire the flash and how much power to use to achieve the correct exposure. This method uses the camera's metering system to determine the correct exposure, taking into account the effect of the flash. There is a short pre-flash burst to set the aperture and shutter speed, and to determine the amount of flash power required. The camera also tells the flash what the shutter is doing for some of the advanced sync techniques discussed later.
Daylight Sync/ Fill Flash
Yes, using the flash in daylight. This is to fill in the shadows on people’s faces on dim days or shady locations. Also helps to lift the subject in a portrait out of a dimly lit background.
Slow Speed Sync
This is similar to the previous technique, except in darker light. At a slow shutter speed, the flash will illuminate the near subject while the dark background, which is out of range of the flash, will benefit from the slow exposure to show details using natural light.
Rear Curtain Sync
This is a refinement of slow speed sync, used for moving subjects. A slow shutter speed allows details of the background in the image and capturing any motion of the subject in the low light portion of the image. The flash fires just before the shutter starts closing to properly show the subject in its final position. The overall image gives the impression of the subject moving out of the dimly lit area and into the bright light.
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