If you're choosing the 24p setting simply by choosing a scene file setting it's not just the frame rate that's changing- it's the gamma, iris, detail, etc. Check out all of the controls in your camera. 24p doesn't affect the overall brightness of a shot, if anything switching from just 60i to 24p should become brighter because the camera is giving slightly more exposure time to each frame at 24fps than at 29.97...
Noah
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