Monday, July 26, 2010

TTL Flash Troubleshooting Checklist (Basic to Intermediate)

Updated: August 18, 2010

Sometimes I take a shot with flash on TTL and the exposure is completely wrong (much too bright or dark), the flash doesn't fire, etc. Here are the settings I double-check to see what may have gone wrong.
  • Flash sync mode. Slow rear sync is automatically selected in program or aperture priority when the sync mode is on rear sync.
  • ISO. If the flash exposure is too high, I check if my ISO is too high. If I'm bouncing the flash and the flash exposure is too low, I check if my ISO is too low.
  • Exposure compensation. With Nikon, exposure compensation does affect flash exposure. If I used exposure compensation with a previous shot, I check if I left it on (With Canon, exposure compensation does not affect flash exposure.)
  • Flash exposure compensation on camera, on flash, on menu. There are three ways to adjust flash exposure compensation in my camera/flash combination (D300 and SB800). First, if the flash is attached to the camera, I can adjust FEC on the flash body (by pressing + or - ). Second, on the camera body, if I hold down the flash button, I can change the FEC using the sub-command dial. Third, I can change the FEC using the flash menu. The three ways to adjust the FEC are *cumulative*.
  • AE-L (auto-exposure lock). If I used AE-L in a previous shot, I check if AE-L has been released.
  • FV lock needed? If the subject is off-center, or there's an object in the foreground, aiming at the subject first then using FV lock before recomposing the picture ensures a more accurate TTL flash exposure.
  • FV Lock - is it active? If I used FV lock in a previous shot, I check if the FV lock has been released.
  • TTL-BL? TTL-BL is a mode where the camera tries to balance the flash exposure with the ambient exposure. The exposure with TTL-BL is often different from that of regular TTL. TTL-BL is the default mode but can be deactivated by pressing the mode button on the on-camera flash, or by switching the metering mode (see below).
  • Metering mode (spot, center, or matrix). Nikon's TTL flash exposure is affected by the metering mode. If the metering mode is spot, TTL-BL is deactivated. If the metering mode is center, TTL can give potentially incorrect flash exposure.
  • Batteries too low? The SB800 unfortunately has no battery meter or even a low-battery indicator. It will do its best to keep firing but when the battery is very low flash exposure sometimes becomes strange before the flash finally quits. If you fire a shot and it takes longer than usual for the flash to recharge for the next shot, that means the batteries are too low.

Additional checklist for wireless mode (CLS AWL):
  • Commander mode activated.
  • Commander mode settings saved. After inputting new settings, press OK, not just menu to go back, otherwise the new settings won't be saved.
  • CLS slave in remote mode (vs. SU-4 mode).
  • CLS slave in the correct group (Group A, Group B or Group C).
  • CLS slave on the same channel as the commander.
  • CLS slave on correct mode (TTL vs. AA vs. Manual). Note: the SB-600 has no AA mode.
  • If flash exposure is still wrong, I try switching to manual control. Once in a very rare while, the flash will still give me the wrong flash exposure notwithstanding the foregoing checklist. On those occasions, I try switching to manual mode.