Hi Nikon-a-holics,
A couple of other questions have been asked regarding my explanation of the F5's
colour matrix meter. One by Thomas, and three by Bryce, one of which is pretty
much the same as Thomas's. So here goes. :-)
The two easy ones from Bryce first.
Camera orientation.
About the absence of mercury switches for orientation detection. I gathered
this information from the "Magic Lantern Guide" on the F5 published by "Silver
Pixel Press", authors - B. Moose Peterson and Rutger Huer (sorry about the name
spelling, don't have the book with me). At one point in there it is stated that
the RGB meter is used for this operation and not a mechanical method as in the
F4. Not willing to pull my F5 apart, I'll accept their explanation. If another
contributor can definitely confirm the presence of mercury switches (or similar
devices) I'm happy to accept their assurance, either way, as long as it works
I'm satisfied.
Memory usage.
As for the memory requirements, 256x256x256 is actually 24 bit colour not 16
bit. However, the camera is not trying to store 16+ million bits of
information. Each sensor in the meter, 1005 elements, reads the light falling
on it and presents, at 256 levels, 8 bits of information (256 in binary is 1111
1111 or 8 bits) so this is close to 8 kbytes for the colour information.
I don't specifically know that the meter uses 256 levels for each sensor. I
chose that number as it is used in most PC paint programs to represent colour.
I was attempting to describe the principle of operation not the technical
specifications. Should it use only 64 levels of colour on each sensor, the
memory requirement drops to only 2k. Even 64 levels per sensor would be
adequate to determine the PREDOMINANT colour of the scene (perhaps even 32
levels - 1k memory).
Now for the 30,000 pictures. The way I understand the literature that Nikon
have distributed about their matrix meters is that 30,000 actual pictures were
used to develop the database in the camera, however, each of those 30,000
pictures had a single exposure so we are only storing 30,000 exposure indexes.
This would not be a great memory burden.
This question was asked by both Thomas and Bryce.
Now for the more difficult question, how does the meter know that the value
46,46,46 (as an example) is a grey card in full sunlight or a white card in the
shade (46,46,46 is approx 18%).
By itself the RGB sensors can't determine this without a baseline for the
determination. But there are more pieces of information gathered by the meter
than just the colour values. The brightness and contrast of the scene are also
obtained. Your grey card in the sun is going to be less bright than a white
card in the sun. In the shade the white card will still be brighter. When this
is combined with the colour information the program in the camera comes up with
the correct exposure.
Take an example of data from the meter 10,25,50,14,22 being
red,green,blue,brightness,contrast, although distance is also used I believe
that this is primarily for flash work. Now imagine you have a huge box with
other boxes inside, take out the #10 red box. Open this and take out the #25
green box. Open this for the #50 blue box, open this for the #14 brightness box
and then the #22 contrast box. Inside this is a piece of paper with the
exposure index on it. Convert this to speed and aperture and away you go.
I don't feel that I have the gift of writing and explanation for such a
complicated process as this, for myself I'm convinced it works. I have personal
experience that it does. If someone else can make a clearer explanation please
come in and we'll all be richer for the discussion.
The best to all and keep the great shots coming.
Eric
