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 1

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.

Answer 2

I'm not sure if there is a checklist or algorithm, but here are some givens when working with flash:

• Shutter speed is not the tool you use for stopping motion. Flash duration is. And, flash duration is normally so brief as to be faster than your fastest shutter speed.
• Longer exposures will allow ambient light to be more of a factor in your exposure. Shooting at maximum sync speed will reduce the effect of ambient lighting (or, in some cases render it insignificant).
• You still use aperture to influence depth of field.
• Distance to the subject is now a factor. Your flash's power decrease inversely to the square of the distance to the subject. With the sun, that's not a big deal because a couple of steps this way or that don't move a subject much in relation to distance to the light source. With a flash, which is both closer and less powerful, a step or two can make quite a difference.
• Distance to the background is now a factor. You can use this to your advantage -- If you want the background de-emphasized, just have the subject closer to you and increase the shutter speed to reduce ambient light effect. That will give you a dark background.
• Ideally, you have an exposure you think suits your subject -- say f/5.6 at 1/200 sec. You set your camera on manual, your flash on TTL and it should "figure out" how much light to emit. Don't go overboard on this because shooting a subject at some distance, f/22, 1/200 sec may be more than your flash can handle.

Since you are supplying your own light, you don't really need the camera picking aperture or shutter speed. You can do this (within reason and sync speed). That means, you are better off using manual mode than Av or Tv. Clearly, the exception to this is if you are using the flash to fill a naturally-lit scene; in that case your original decision tree applies.

As an aside, if you are shooting with the flash on-camera, you are using it in about as limited a way as you can. Anything that will take the flash off the camera will improve things immensely.

Answer 3

Using:

GN = distance x f-number

from https://en.wikipedia.org/wiki/Guide_number

and

Full power GN x √(y / x) = Reduced GN

from https://en.wikipedia.org/wiki/Guide_number#Effect_of_power_settings

simplified to:

GN_req = distance x fstop

GN_req = GN_max x √flash output

we can make:

flash output = ((d x fstop) / GN_max)²

this supposes iso 100 so to factor in other iso's:

flash output = ((d x fstop) / GN_max)² x (100 / ISO_req)

To use the example set out elsewhere with

at 35mm zoom the maximum guide number (GN_max) is 31.5

flash to subject distance (d) is 3m

sensor sensitivity (ISO) is 200

lens aperture (fstop) is 2.8

we have the equation:

((3 x 2.8) / 31.5)² x (100 / 200) = 0.0356

to convert this to a fraction use MOD on a calculator or:

1 / 0.0356 = 28.125 or about ^1/₂₈ power.

to convert to ev steps involves natural logs (I have no idea see https://sciencing.com/calculate-log-5144933.html):

log₂ = log(x) / log(2)

or

(log(¹/₂₈)) / (log(2) = -4.8138 ev steps

That said I would just use GN = distance x fstop and the guide number table from my flash's manual to calculate the power.
See also scantips.com from user WayneF on this site, especially WayneF's Guide Number calculator.