Flash Exposure

Before we get into adding flashes/strobes I want to cover flash exposure quickly. It is important to understand that when you add flash you are actually making two exposures simultaneously. One exposure will be of the ambient light, and the other will be of the light from the flash.

There is a "rule of thumb" that says Shutter Speed (SS) affects the ambient exposure and Aperture affects the flash exposure. This is a huge over simplification. It is misleading and at times entirely incorrect.

The rule of thumb makes some assumptions. First, you are using manual flash and not TTL (thru the lens metering). It assumes the selected shutter speed remains within the flash sync speeds. It assumes your ISO is fixed and not changed. And it assumes the flash power (output and distance) is fixed. If any of this is not the case the rule of thumb is wrong.

Exposure is exposure, and Aperture, ISO, and SS always affect it. Here's the catch, for a flash exposure the SS can be anything from the sync speed or slower and the flash will be recorded. So if you change the SS while remaining within the sync speeds the only thing that will be affected is the ambient exposure. If you change the SS (within sync speeds) and also change the aperture to compensate; nothing will be affected.

How much of the flash gets recorded is largely determined by the aperture opening. If you change the aperture size, AND change the SS to keep the ambient exposure the same, then the only thing that will be affected is the flash exposure. If you ONLY change the aperture, both exposures will be affected.

If you change only ISO both exposures will be affected.

If you only change the flash output only the flash exposure will be affected.

IMO, it is best to think of it as two simultaneous exposures using all three variables of the exposure triange. The only difference is that for the flash exposure the third factor is flash power instead of shutter speed. (assuming you stay within the flash sync speeds.) If you are in TTL flash then the flash output is automatically calculated and varied . You have to change it using flash exposure compensation (FEC).

A bit more on TTL fash exposure:

The flash metering is separate and independent of the camera's metering. It doesn't know, or care, where the flash is pointed. The only thing it cares about is that it gets "enough" light returned from the subject; and it will do everything it can to generate that output.

On Nikon's, camera EC also affects the flash exposure (some models let you change that in the menus, i.e. D4). On Canon's, camera EC does not affect flash exposure.

If you read the manual for your flash it probably shows the maximum flash out put as 1:1, flash:ambient. That is misleading. What it means is 1:"correct exposure" based upon the flash metering. The flash metering system will not go above "correct" unless it is told to do so. If you make the camera underexpose the ambient (manual mode or -EC) without changing the flash exposure (by adding the corresponding +FEC for Nikons if -EC was used) then the flash exposure will be above the ambient exposure. You can push the flash as far as it will go and get ratios as high as 3:1 (or 3:0)


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