First, by shooting in M mode, you are locking the exposure. You're setting both aperture and shutter speed, which effectively locks the exposure (if you're not changing ISO).
The design of (nearly all) DSLRs requires that an image be taken in order for histogram data to be available. When the viewfinder is in use, the mirror id in the "down" position, which blocks light from the sensor. Thus, no LCD preview, and no pre-shot histogram is possible.
ETA: Here is a tutorial on understanding the histogram.
Message edited by author 2007-06-29 07:59:48. |
