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Flash control
Flash control means controlling the flash brightness, a separate topic from the exposure with aperture, exposure time and ISO – and the subject of this page: how cameras find the right flash brightness and how you can regulate it yourself.
Automatic flash exposure
Automatic TTL metering | Digital cameras use modern variants of TTL metering for flash metering. TTL stands for ”through the lens“, a method that became popular with SLR cameras in the 1980s. Its core is:
- During exposure and when the flash is lit, the camera measures how much light is coming through the lens. This is actually a second exposure metering when the flash is on. Before that, the camera can only measure the ambient light.
- As soon as the amount of incoming light is sufficient, the electronics switch off the flash.
Technically, it was a great improvement at the time to be able to measure the actually bounced light this way.
Modern TTL metering | Progress continued and today cameras use variants with their own names such as i-TTL at Nikon, E-TTL II at Canon or ADI at Sony. Their differences to the original TTL are rapid pre-flashes:
- Immediately before the exposure, the camera sends out a a pre-flash and registers how much of its light is coming back into the camera.
- From the exposure metering without and with pre-flash, the camera determines how much light the flash should emit.
- Then actual exposure can start, combined with a second main flash.
In earlier models, the pre-flashes were still perceptible as separate flashes and could reliably cause unflattering narrowed eyes with sensitive people. Nowadays however the two flashes follow each other so quickly that this no longer matters.
Automatic flash exposure in manual mode | Yes, this automatic flash exposure metering also works in manual mode, no matter what aperture and exposure time you set. With an automatically controlled flash, the manual mode also becomes a kind of automatic exposure.
Manual flash control
The flash brightness can also be set manually; it's usual procedure to specify the power in fractions: 1⁄1 for full power and 1⁄2, 1⁄4, 1⁄8 etc. for less. Always halved, just as one always proceeds in steps with a factor of 2 or 1⁄2 when changing aperture, exposure time and ISO. At least with advanced cameras and external flash units.
Which flash output is suitable for a subject can be calculated from the flash intensity and the distance to the subject – but I'll save that here, with digital photography it's easier to try things out with test shots. In addition, modern flash units can offer setting aids such as the display of the distance associated with the flash output. But you will only find this with external flash units, not the small integrated flash units.
Reasons for manual flash control can be, for example:
- The light pulse of the in-camera flash is only meant to trigger a second flash. In this case, the pre-flash of the automatic metering would interfere and the second flash would fire too early. This is how you can trigger inexpensive flash units from other manufacturers that are actually not compatible with your camera. Or even expensive studio flash units.
- You want to set a constant flash brightness for several shots so that the automatic system does not adjust it again, e.g. when the picture is cropped slightly differently.
- You can manually control the brightness of flash units from other manufacturers that are not compatible with the flash system of your camera.
Exposure metering in the flash unit
This is a variant that has advantages for flashes set up independently of the camera and is only found in the expensive flashes intended more for professionals. I mention it only briefly for the sake of completeness:
The flash can meter the light reflected by the subject in order to control its brightness independently of the camera. Then a flash can also be placed far away from the camera and shine where the small TTL metering flash cannot reach.