I read some info about the 51 point AF system for the new Nikon D300.
The dpreview showed a graphical presentation on the locations of those
51 points within the targeted digital frame (approximately covers 2/3
of the square, centered in the middle of the frame). Could someone
provide a little more detail about this 51 point system?. I may be a
little confused about this concept against the exposure metering in
some old digital cameras.... averaging or spot metering. I guess that
this is not about the light exposures, but instead it is about the
distance measurements from the object(s) to the sensor plate. If the
later is correct, then how do they manage to get a razor sharp picture
if you have to average the distances over 51 spot locations? Or, did
the camera actually take average distances from those 51 locations? I
guess you cannot have 51 different focused distances to the sensor and
each measured distance will be registered into a single 2D plate, can
you? Or, I may miss something completely and more fundamental here? I
think that taking average distances over 51 points will not definitely
work to produce a razor sharp image on the main focus/target area,
unless each of those 51 points have identical distances (or close to
identical distances) to the sensor.
Thanks for discussion and comments.