I'm trying to understand flash exposure compensation better. I get the relative scale, i.e., -1EV is half as much light as 0EV, but what is 0EV itself? Specifically:

  1. At 0EV, is the flash trying to output enough light to expose the picture properly for the given settings with no ambient light?

  2. What is the E-TTL system metering off of? Suppose, for example, that I have a scene that's correctly exposed with ambient light except for a portion that's completely shadowed. Then if I aim a flash only at the shadowed region and use 0EV, that region should be exposed similarly to the ambient region, right? But if that's the case, how does the system determine which part of the scene to use to calibrate flash output?

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    "At 0EV, is the flash trying to output enough light to expose the picture properly for the given settings with no ambient light?" There is no such thing as no ambient light. If the scene had no light, how is the scene supposed to be measured? There must be light from the scene in order to measure it. – scottbb Dec 19 '18 at 16:08
  • Say I'm in a pitch-black room, exposing solely by flash. I set the camera to my desired shutter/aperture/ISO and flash exposure compensation to 0EV. What's the flash's output? – Asher L. Dec 19 '18 at 16:12
  • Likely maximum power, because in a pitch black room, the metering is essentially -infinity (i.e., below the camera's lowest threshold). So the flash will just add as much light as it can. – scottbb Dec 19 '18 at 16:15
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    But with E-TTL metering, the flash will pre-fire and measure the output. It won't go to maximum power unless necessary. – Asher L. Dec 19 '18 at 16:18
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Yes, compensation of 0 EV is trying to properly expose the picture, meaning a goal of proper exposure level. This is true of both TTL flash exposure and of ambient exposure compensations.

The problem is that if TTL flash is used in bright ambient, the sum of those two proper exposures (the metered flash and ambient) are then 2x too much exposure (of the near subject illuminated by flash). So we necessarily back off, and know to use maybe -2 EV TTL flash compensation when in bright sunlight ... which sum is still a very minor low overexposure, but it can go unnoticed. My site has a calculator of this at https://www.scantips.com/lights/flashbasics4a.html#percent

Then the concept of Balanced flash was invented, when the camera automation tries to properly back off on the flash itself. That menu surely still says 0 EV flash compensation, but the flash (in bright sunlight) will actually be near this -2 EV level, if Balanced flash. Nothing says that, but it's how it works. That reduced flash level is the meaning of Balanced flash, which is often the default today.

However, then (Balanced flash), the flash compensation menu can still modify that somewhat, to be more or less flash. However, sometimes this balanced automation can have its own ideas and goals, and possibly override expected results a bit.

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0EV is the exposure that the camera and or flash meter determines is the correct exposure.

When you turn on the flash your camera will pick a shutter speed that will sync with the camera. If it is not very bright out you will frequently get a shutter speed that is too dark for the ambient light and thus the flash will be what lights your subject. At 0EV your flash will expose the scene correctly.

If the ambient light is brighter your camera and the flash will both expose the correct amount and on modern gear that is happening in real time you will not get over exposure. However you will still frequently feel like the picture looks wrong. That is because the shadows from the ambient light will be over powered and you will get a strange look.

To compensate for that people frequently dial back their flash to -1.3EV, -1.5EV or -2EV. That works well for bright harsh ambient light because it fills the shadows in a little but not so much that they disappear.

In your case of a scene with a dark shadowed area if you set up your flash to light that area only you will get an overall correct exposure. But you might consider - is that what you want? And will there be some strange effects in any transition areas? It could be better to light the shadow area with a little less than 0EV depending on the situation.

PS. By correct I mean as measured by the camera's meter. So, correct is what you want in many situations but you should be aware of what the meter is deciding and possibly compensate a little yourself to get the look you want - if needed.

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A lot depends on which camera you're shooting with and which exposure mode you are using. In order to use flash exposure compensation, one must be using some form of automatic flash power calculation, often called TTL. With an Exposure Compensation value of '0', you're telling the camera to set the flash power based on what it calculates to be "correct" exposure. How the camera calculates "correct" exposure depends on how the camera has been programmed by the manufacturer and what the photographer has selected for various camera settings.

For most cameras, if you are using Aperture Priority (A or Av mode) with TTL flash, the camera is programmed to assume you want to do one of two things:

  • In bright ambient light it assumes you want to use the flash as a 'fill' light.
  • In dimmer ambient light it assumes you want to use 'Slow Shutter Sync' (a/k/a 'dragging the shutter') to expose the background ambient light as well as you can while using the flash to freeze your subject as the subject's primary illumination.

If you're shooting in dim light and don't want to 'drag the shutter', then use Shutter Priority (S or Tv) or Manual (M) exposure modes. Or, if your camera offers it, select a "custom" setting that limits the slowest shutter time when using TTL in Av mode to a set value such as 1/60 or the camera's flash sync speed (usually somewhere between 1/160 to 1/250, with most newer cameras around 1/180 to 1/200).

Any exposure compensation you enter will be applied to the flash power calculated to provide proper exposure under the parameters of the camera's TTL programming.

For more about how flash is calculated using TTL please see:

How do TTL flash metering systems calculate how much power is needed?
What exactly does TTL flash sets its power to?
Why is flash TTL metering independent from ambient light metering?
Does TTL by definition overexpose your subject?

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