Normally when turning on the flash and when using an automated exposure mode, the camera selects a shutter speed between 1/f or 1/60 (whichever is higher) and the x-sync speed. This allows the camera to be used handheld without inducing much camera shake. In low light situations, however, this also causes little to no exposure from ambient light. The scene is almost entirely illuminated by the flash, and the result is often a pitch black background.
To increase the amount of exposure by ambient light, some cameras offer a Slow Sync mode. With some it comes disguised as the Night Portrait Subject Program. With this mode, the camera chooses exposure values closer to the ambient exposure values, typically ambient exposure minus 1 stop. Due to the slower shutter speed, there is more exposure by ambient light and less exposure by the flash. The longer exposure time also means that you may have to use a tripod to avoid camera shake. This technique is also known as “dragging the shutter”.
In normal flash mode the flash pulse is emitted right after the first shutter curtain has opened. When there is some ambient light and you have moving objects in the frame, this results in a picture where these objects are illuminated by the flash at the beginning of the exposure and illuminated by ambient light during the rest of the exposure. This looks like the objects have moved backwards.
Some bodies offer a Rear Sync mode where the flash pulse is emitted shortly before the second curtain closes. This means that the flash exposure happens at the end of the exposure. The result is that the picture looks correct with regard to the moving objects.
There are a few restrictions:
- HSS or ADI can not be combined with rear sync
- the shutter speed is limited to 1/60 with most cameras. Only the latest Sony DSLRs allow a higher speed in rear sync mode.
- wireless flash can not be combined with rear sync
One question that you hear often is “how do I turn on fill flash with the Minolta system”? The short answer is “you don't”.
The long answer is that the camera effectively always does fill flash. The camera selects a hand-holdable shutter speed by default, and when there is little ambient light, almost all exposure will come from the flash. When there is a lot of ambient light, a larger portion of total exposure will be from ambient light, and the rest is filled in by the flash. You can change the balance between ambient exposure and flash exposure, e.g. by using Slow Sync when there is little ambient light, or by using manual exposure mode. In M mode you can influence flash exposure with the aperture, and you can influence ambient exposure with the aperture and the shutter speed. The closer the exposure values are to the purely ambient exposure values, the more exposure will come from ambient light and less from the flash. But even in manual exposure mode, the camera intends to fill ambient exposure with flash up to the correct exposure. If you want to influence flash exposure directly, you have to use flash exposure compensation or manual flash control.
In normal flash mode the flash emits a short but strong pulse of light. That pulse is a lot shorter than the exposure time. But since it exposes the entire frame at once, the shutter must be fully open when the flash fires. With focal plane shutters the shutter is only fully open at exposure times slower than the x-sync speed (determined by the shutter construction). At faster speeds the shutter is more like a narrow slit moving across the frame. You can't use normal flash at these shutter speeds because the short pulse would not expose the entire frame.
But sometimes you want to combine a fast shutter speed and flash, eg. when you want to use fill flash at daylight with a fast, wide-open lens. For these situations some bodies and flashes support High Speed Sync (HSS). In this mode the flash output is a series of longer but weaker pulses instead of one short strong one. The flash emits light during the entire shot. This way the entire frame is exposed, even when the shutter is never fully open.
The disadvantage of HSS is that the guide number of the flash unit is reduced considerably, ie. the flash range is reduced a lot.
Using HSS also means using pre-flash metering. TTL-OTF metering can not be combined with HSS, because the flash metering sensors don't see the entire frame when using shutter speeds above the x-sync speed. Parts of the frame are covered by the shutter.
Wireless HSSWireless flash was limited to 1/60 or 1/45 or slower speeds with cameras before the Dynax/Maxxum 7. With the 7 and some later bodies and with the new D flashes it's possible to use higher shutter speeds with wireless flash.
© 2008 Michael Hohner; This page was last changed on 2008-01-05