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To guess your subject type and determine what really is white in sunlight the Matrix needs
to know the absolute level of light outside the camera. Remember that the light inside the
camera will differ from the light level outside the camera depending on the speed (f/stop) of
your lens.
Therefor the Matrix needs to read the true f/stop of the lens. The FA camera read that with
a special new lug on the back of AI and newer lenses. AF cameras read this electronically.
Other cameras have not needed this, since they did not try to guess what sort of subject
you were photographing and therefore were happy only knowing how much light made it
through your lens to the film.
For instance, the Matrix knows how bright daylight is, so it knows if it sees something that
is bright enough to be bright sand in full sun it knows to add exposure to make it look light
and not just gray.
If the camera can't tell the actual maximum aperture of the lens then it can't determine
absolute light levels and cannot do Matrix metering.
Absolute maximum aperture coupling
Manual AI lenses have a special internal mechanical coupling lug on the back of the lens
that tells the F4 and FA what the exact maximum aperture is, like f/4 or f/2.8. All AF lenses
have these same mechanical lugs for the FA and F4 (thank you, Nikon), and also have
electronic contacts for the AF cameras.
Heck, the camera also wants to know the light falloff of the lens, and I think that's also
coded into the depth of the mechanical lug. The camera uses this to get the right readings
for the meter segments on the sides of the image. This is entirely different than the lug on
the external aperture ring that tells the camera relationship between the aperture you set
on the lens and the maximum aperture. I don't think any cameras were ever designed to
read the falloff mechanically, just as AI lenses also have a mechanical lug to couple the
focal length of the lenses to cameras that were never built.
All AF cameras read the f/number via electronic contacts. Except for the F4, no AF camera
has a feeler to read the mechanical lug from the back of the manual lenses, and therefore
all AF cameras (except for the F4) will revert to center weighted when you put a manual
focus lens or teleconverter on them. This is a defect in the design of AF cameras probably
designed to make you have to buy new AF lenses.
I believe that the AF lenses also tell the Matrix about the falloff of illumination so that it can
more accurately measure the corners of the image.
Use with teleconverters
The only way to get real Matrix metering on an AF camera is to use a TC-14E or TC-20E
(or the new "II" versions). These only work with the exotic AF-I and AF-S telephoto lenses.
There is no other way to get real matrix metering with other TCs on AF cameras other than
the F4.
© 2007 KenRockwell.com 63 converted by Sándor Nagy