Question: I'm not always getting good flash exposures with my Nikon D5100 and SB-800 Speedlight. Sometimes the photos are too bright and in other cases they are underexposed. Any suggestions?
Answer from Peter B.: This is a common question from my students who own other brands of cameras. You’re using very sophisticated, high-tech equipment with “intelligent” Matrix metering and i-TTL flash metering, so you should be getting excellent flash exposures in many situations. Of course, that assumes that all camera and flash features are at the appropriate settings. For example, make sure that the SB-800 Speedlight is set for i-TTL mode and that you have not inadvertently set some Flash Exposure Compensation. Also, check to ensure that the metering pattern on the camera is set to Matrix for the greatest reliability (not Center Weighted or Spot).
Whether indoors or out, a flash unit can be an essential accessory. With the right settings, you should get beautiful flash exposures. (ISO 200; Matrix metering; i-TTL flash; -2/3 Flash Exposure Compensation). (c) 2010 Peter K. Burian
Hint: With subjects that are extremely close to the camera -- or very far from your position -- be sure to check the flash range data on the LCD panel on the back of the flash unit. As you change the ISO level on the camera, the data will also change, indicating the effective flash range at the different ISO.
Using bounce flash or a diffuser -- whether the built-in wide flash adapter panel or an add-on light modifier accessory -- will reduce the effective flash range. And if you use the FP High Speed Sync mode, the flash range will be very short. In such situations -- or with a distant subject in low light -- set a higher ISO level (to make the sensor more sensitive to light) or move closer to the subject. When the subject is unusually close to the camera however, set the lowest ISO level and also flip the built-in diffuser down over the flash head.
Full-size flash units with an LCD data panel are particularly useful and convenient. The display provides feedback as to current settings as well as the effective flash range at any ISO level that you set. Courtesy of Nikon
One of the benefits of digital photography is that you can snap a test photo and check the results instantly to see if you need to make any adjustments. The “correct” exposure, especially for fill-flash photos, is often a matter of personal judgment. If you don't like the results you’re getting at default settings, start experimenting with Flash Exposure Compensation. For a more gentle effect outdoors, try setting a -2/3 level on the flash unit (not on the camera). With very light toned or highly reflective subjects however, you may need to set a +1 (or similar) Flash Exposure Compensation level to prevent underexposure.