How can I think logically about getting correct exposure with flash?

Question

I recently got a new off-camera flash (Nikon SB-700), and I'm having trouble thinking about the variables that go into proper exposure.

For example, without flash, I have a little mental decision tree that went something like this:

• If shooting very long exposure then use manual mode with camera on tripod. Choose aperture to suit desired DOF and/or choose shutter speed to suit desired exposure time. Try to use ISO 640 or lower. Do not use exposure compensation (because in manual mode it's pointless).
• Else if shooting fast moving subject, use shutter priority and ISO auto. Tweak exposure compensation to prevent blown highlights or blocked out shadows.
• Else use aperture priority, and choose suitable DOF. Make sure that shutter speed is no slower than 1 / focal length. Compensate for slow shutter speeds by 1) Raising the ISO, or 2) Using a tripod, or 3) Bracing the camera or yourself against something. Tweak exposure compensation to prevent blown highlights or blocked out shadows.

For an amateur like me, the above algorithm covers just about everything I do. I could probably even make a flowchart out of it.

Now that I am trying to learn about flash photography, things are suddenly very, very complex, and I feel lost.

My question is: Is there a similar mental flowchart or algorithm that can I use as a guide for flash?

Answer

For me, the workflow with a single off-camera flash goes something like this:

• Attach gel and/or modifier to flash according to ambient light and desired effect.
• Place the flash where I want it to be - taking into account desired angle, whether I want the flash to be out of frame or behind something, reach, desired apparent size of light source (bigger near subject - gives softer light), desired light falloff (faster near subject).
• Choose aperture and ISO so I would get desired DOF and background exposure level (often underexposed to add impact for the subject) with shutter time no faster than sync speed.
• Turn on/enable the flash.
• Am I in hurry? If yes, set flash to TTL exposure metering and camera to manual or aperture priority mode; if not, use both in manual mode.
• Is the subject moving? If yes, decide if rear curtain sync should be used.

• With manual flash: guesstimate or calculate approximate power level needed, set it.

  Formula for calculating needed power level:

  (distance ⁄ GN)² × (100 ⁄ ISO) × f-stop
  

  The distance should be metered between flash and subject in units used for guide number (could be either meters or feet).

  For example, at 35mm zoom your SB-700 has GN 31.5 on a DX body; with subject 3 meters away from flash, ISO 200 and aperture at f/2.8, you'll need power level

  (3 ⁄ 31.5)² × (100 ⁄ 200) × 2.8 ≈ 1/110 × 1/2 × 2.8 ≈ 1/79
  

  which is quite close to 1/64.

• Take test shots and adjust flash position and power level/compensation to get desired exposure and effect (might skip this step with TTL in hurry).
• Take the shot.

Check more discussion of this question.