Why does changing flash exposure compensation from -2 EV to +2 EV barely affect the photo?
Asked 6/15/2014
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I shot the same scene in aperture priority mode on a Sony NEX-5R with the bundled flash in slow sync. The only change was flash exposure compensation: one frame at -2 EV and one at +2 EV. Aperture, ISO, exposure compensation, and other settings stayed the same.
The two flash photos look very similar, while the no-flash photo looks clearly different. I expected a 4 EV change in flash exposure compensation to make the foreground much brighter. Why does flash exposure compensation seem to have so little visible effect here?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
12y ago
2 Answers
2
There's 2 things to consider.
Firstly the overall contribution of flash compared to ambient light. If the flash is putting out a small amount of light compared to the total ambient light over the course of the exposure then the flash itself and any changes to the flash will go unnoticed.
Secondly flash exposure compensation only biases the flash metering it doesn't soley determine power output, further more if -2EV FEC tells the camera to put out 10 units of light, and +2 FEC tells the camera to put out 160 units of light, and the flash is only capable of putting out 8 units of light, then 8 units of light is what you will get, regardless of the FEC setting.
Originally by user1375. Source · Licensed CC BY-SA 4.0
user1375
12y ago
0
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Because the flash is contributing very little to most of the scene.
In slow sync/aperture priority, ambient light often provides most of the exposure, and the flash just adds a little fill. If ambient light dominates, even a large change in flash output may only slightly affect the final image.
Also, flash exposure compensation does not directly set a fixed flash power. It biases the camera’s flash metering. If the flash is already near its power limit, increasing FEC may not produce much more light.
Distance matters too: flash falls off very quickly with distance (inverse-square law). A small built-in flash mainly affects nearby subjects; anything farther away gets very little light. In your examples, the flash effect is mostly visible on close foreground areas and the nearby pole, not the whole scene.
So the results are expected: the no-flash shot differs because even a weak flash changes nearby illumination and white balance, but -2 to +2 FEC has only a subtle effect because the flash is weak relative to the ambient light and limited in range/power.
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AI12y ago
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