The following 4 photos were all taken with a Nikon D300 at 1/250s, f/5.3, ISO-400, in shutter priority mode, with the on-camera pop-up flash. The only difference between them is the exposure bias, which varies from -4EV to +2EV. You can clearly see that the four pictures get brighter, despite having the same exposure settings.
Can you explain how/why this is happening? I have come up with a few possibilities, but I don't know which (if any) is actually correct:
- Is the camera adjusting the output of the flash to compensate for the exposure bias? This seems like the most likely solution to me, based on how the images look, but my understanding is that exposure bias was supposed to control the amount of ambient light in the scene, while leaving the flash alone, whereas flash exposure compensation is supposed to control the power output from the flash. (Flash exposure compensation remained constant for the four images).
- Alternately, is the camera applying some sort of automatic lighting correction in post-processing after the images were shot? (These images were all taken in JPEG mode, not as Raw).
- Is there some other effect at play here that I don't understand?
Edit I've copied-and-pasted the relevant exif data, extracted by exiftool, for the photos. The data is identical in all important fields (I checked using diff), except for the Exposure Compensation and Exposure Difference fields (I don't know what the latter means):
Camera Model Name : NIKON D300
Exposure Time : 1/250
F Number : 5.3
Exposure Program : Shutter speed priority AE
ISO : 400
Flash : On, Return not detected
Flash Type : Built-in,TTL
Flash Exposure Compensation : +2/3
Active D-Lighting : Normal
The Exposure Compensation and Exposure Difference fields for each image, from top to bottom is as follows:
Exposure Compensation : -4
Exposure Difference : -0.3
Exposure Compensation : -2
Exposure Difference : -2.5
Exposure Compensation : 0
Exposure Difference : -4.4
Exposure Compensation : +2
Exposure Difference : -6.6