AF-assist Mode on External Flash
Most digital SLR cameras use what is known as phase detection auto-focusing. There are dedicated sensors on board the camera which tell the lens to move and lock focus. What these tiny sensors actually look for is a contrast in the scene.
When you press the shutter release half-way these sensors are activated. Now, these sensors need some amount of light to be able to operate properly. In extremely low light conditions, they cannot pick up enough contrast to lock on.
This is the reason why when shooting in very low light conditions, your camera lens does that continuous whizzing sound and won’t let you fully depress the shutter button to take the image.
To counter this problem you can use the built-in AF-assist lamp. Most DSLR cameras have this feature set to auto default, so you don’t have to do anything and use it straight out of the box.
This fires a brief pulse of light so that the camera can lock focus on the focal point that you wish to focus on. This flash of light is fired without creating a bias over the exposure and the camera will still take the picture as per the TTL metering.
There is a better method to use, just in case you are using an external flash. When shooting with an external flash, you can set it to fire in AF-assist mode.
The pulse of light is slightly more intense in this case. Mind it, however, that not all flashes have this feature. When buying an external flash always insist on checking the accompanying literature to establish that it has the features that you are likely going to use more often.
One of them is undoubtedly AF-assist. Most of the pricey OEM flashes do have this feature, like e.g., the Nikon SB-900. If you are buying third party flashes make it a point to verify.
To use the external flash in AF-assist mode, without creating a bias on the final exposure, do the following:
- Set your camera in AF mode.
- Attach the external flash to your camera’s hot-shoe.
- Navigate to the menu option and select AF option (SB-900).
- The flash that I am referring to (SB-900) has three options that you can choose from. They are ON, OFF (Flash does not fire), AF Only (Flash only emits the AF beam but does not fire when the image is taken).
- Press OK to save and exit the menu.
Once this is set each time you are in a low light condition, the flash will now only fire the AF-assist beam and not fire the usual flash when you take a picture.