When I use flash for indoor pictures, will a manual setting of shutter speed 1/60s and aperture f/4 give the same exposure as shutter speed 1/240s and aperture f/2, assuming the focal length and ISO are the same?
The two different shutter speed and aperture combinations should give you equivalent ambient/background exposure but for subject exposure, the answer can vary depending on the exact shooting situation. Let’s make it simple with a few assumptions, but this should cover most practical shooting situation. The assumptions are:
- The ambient light is sufficiently dim so that the light from flash dominate the exposure of the subject
- The shutter speed is within the camera’s sync speed limit
If the above assumptions are valid, shutter speed does not affect the flash exposure of the subject because the flash duration is typically much shorter than the shutter speed (time for which the shutter is open).
Weather or not the change in aperture will affect the subject exposure depends on weather you are using manual flash power output. If you let the camera automatically adjust the flash output (TTL), and you are not trying to use matrix metering (Nikon term), or whatever metering mode that causes the camera to try to balance the subject and background exposure levels, the camera should be able to adjust the flash output level to obtain a proper exposure of the subject within the camera and flash’s ability to do so. Therefore the answer is yes-the subject flash exposure is about the same, If your flash output is manually controlled, the second exposure of 1/240s and f/2 would yield two stops more exposure because f/2 is two stops over f/4. The shutter speed simply does not matter.
In general, when shooting flash indoors a good practice is to use a manual exposure (aperture, shutter speed, and ISO combination) that produces an appropriate ambient/background exposure, then adjust the flash power level to obtain a proper main subject exposure by using flash output level compensation (not exposure compensation). The manual exposure control for the ambient/background gives you the most control. For example, if the background is too distracting or has drastically different color temperatures, you could underexpose the background to eliminate the problems or reduce the distractions.
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Keywords: Digital photography, Exposure, FAQ, Flash photography, Frequently Asked Questions, Indoor Photography, Q&A, Questions and Answers, Sync Speed