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Nikon 51 point AF system in D300
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. |
#2
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Nikon 51 point AF system in D300
wrote in message ... 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. I can't help with the D300. However, my D70 has several points, but only one is in use at any given time. The only connection between AE and AF is that some cameras use the distance to the focus point as a guide to set the exposure. Jim |
#3
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Nikon 51 point AF system in D300
wrote in message ... 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. More likely that the uP reads the various distances, decides which are the near ones (and thus likely the subject(s)) and adjusts the focus point for those, possibly adjusting exposure by aperature at the same time to ensure sufficient depth of field to cover them all. -- Woody harrogate three at ntlworld dot com |
#4
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Nikon 51 point AF system in D300
in article
, at wrote on 1/1/08 00:50: Or, I may miss something completely and more fundamental here? The sensor points are not magic 'rangefinders' that output distances. They are phase detection sensors. Some are 'cross type'- i.e they detect in two planes at the same time. See the following pages on Nikon's site: http://preview.tinyurl.com/2mnrns http://preview.tinyurl.com/36dj2z There are many different focus modes on the D300: € Single point AF € Dynamic Area AF [9 points, 21 points, 51 points, 51 points (3D-tracking)] € Automatic-area AF I think in dynamic area mode, you can set whether it biases the choice to closer subjects, or not. It will also automatically start focus tracking (predicting where a subject will be, and focussing there before the subject reaches that point). Combine all these options with AF-S and AF-C modes, and you're looking at quite a few combinations of settings. In Live View mode, it will use contrast detection AF on any desired point in the frame (i.e. change the focus, see if this increases contrast, if not, go the other way, etc.). All other AF modes won't work, as the focus sensors are in the bottom of the mirror box, and they get their image from a secondary mirror that reflects light from the middle of the frame (that has passed through the semi-silvered primary mirror). As the mirror is up during live view, this can't work. I think there is an Olympus live view method where the live view comes from a second colour CCD in the prism housing, and so the mirror is down. HTH, Ben |
#5
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Nikon 51 point AF system in D300
In article , Ben Micklem
wrote: In Live View mode, it will use contrast detection AF on any desired point in the frame (i.e. change the focus, see if this increases contrast, if not, go the other way, etc.). All other AF modes won't work, as the focus sensors are in the bottom of the mirror box, and they get their image from a secondary mirror that reflects light from the middle of the frame (that has passed through the semi-silvered primary mirror). As the mirror is up during live view, this can't work. I think there is an Olympus live view method where the live view comes from a second colour CCD in the prism housing, and so the mirror is down. the nikon d3/d300 live view has two modes. in handheld mode, the mirror will drop and autofocus will use the usual 51 point focus sensors, then the mirror goes back up to resume live view. in tripod mode, contrast detection is used off the sensor itself and the mirror does not drop to focus, with the focus point placed anywhere in the frame. |
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