Focussing in low light situations can be tricky. As light becomes scarce eventually even cameras with a high-end focussing system will struggle and finally wave the white flag. But there is a simple measure to counter this problem. It’s in the standard repertoire of your equipment but you might not have thought of it until now.
Most flashes have an autofocus assist-light. Ever wondered what the red plastic window at the front of the flash is for ? It houses sensors and stuff, but also th AF assist light. When the camera signals „Yup, it’s too dark, some help please !“ the AF assist light is activated and projects a pattern onto the subject, thus helping the camera to focus. Then the shutter button is pressed the light is switched off and the flash fires, giving you a perfectly focussed image. So far every photographer should know that this function exists. Now comes the part less photographers might be aware of.
Sometimes using a flash is not an option since it would destroy the atmosphere. What now ? Well, most flashes can activate the AF assist light and suppress the flash. Your flash unit becomes an autofocus assist-unit. The function is called differently with each flash unit. At the Nikon SB-800 one has to set the „FIRE“-option to „NO“. Not so intuitive if you ask me. At the SB-910 there is a dedicated menu for the AD assist light: On, Off, AF-Ill only. Consult your flash manual about this function, it should be described in the AF assist section.
This AF assist light mode enables one to focus even when there is only few light. You can photograph the party (barely) unnoticed. The red focus assist light is activated only for a brief moment.
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