There have been some mails concerning matrix metering (AF-bodies) and MF
Pentax lenses and why this does not work properly.
Maybe this helps clarify the issue:
Matrix metering needs specific information on the properties of a certain
lens at different f-stops for the following reason:
All lenses show some vignetting at the largest apperature. In normal and
telephoto lenses (50mm focal length and up) vignetting is usually less than
1/2 an f-stop and negligible but in wide angle lenses it can be considerably
more (2/3 of an f-stop, difference center to edge, on my A 2.8/28 and even
1.5 f-stop on my Tokina MF 3.5/17 lens). Fortunately a considerable amount
of vignetting disappears when a lens is stopped down!
Now light measurement in all Pentax AF cameras is done prior to exposure
and at the largest diaphragm. This leads the camera electronics with a
dilema: light distributing across an image is abviously different at the
f-stop at which it measures light (for example 2.8) and the f-stop at which
the image is actually recorded on film (for example 5.6; 8 or 11). This
problem is solved by transferring specific information on the lens'
vignetting properties from lens to camera.
Only 'F' and 'FA' lenses are able to transfer this specific information on
vignetting to the camera. 'A' lenses only transfer the maximal aperature (as
PZ1 owners have noticed the diaphragm of an attached A-lens is correctly
displayed in the viewfinder).
What practical consequences does this have?
The Pentax PZ1 and PZ1n bodies do not prevent the user from using matrix
metering with MF lenses (K, M or A). If the lens does not show much
vignetting (lenses from 50mm upwards to telephoto lenses) there is no
problem and matrix metering is correct. But if Pentax PZ1/PZ1n users use
wide angle lenses, which show much more vignetting when fully open, the
matrix metering is confused: It sees a light falloff from center to border
and compensates for this (plans longer exposure). But since vignetting is
much less stopped down this compensation (appropiate at f2.8) is of course
much too much at f8 or f11. Thus the final image is overexposed. This
problem has been encountered by many PZ owners I know, especially when using
slide film (it does not matter that much for prints, since negative film is
quite tolerant to overexposure). PZ owners using MF (A/K/M) wide angle
lenses should thus make sure that metering is switched to centerweighted or
preferably to spot measurement when using wide angle lenses (with my Tokina
17mm lens only spot mesurement gives the right exposure).
Since this offset with MF wide angle lenses and PZ bodies has lead to
complaints to Pentax that the metering system is off, the MZ series do not
allow for matrix metering with A (or earlier M/K) lenses any more and
automatically switch to center weighted measurements (at least that is what
someone claimed in a recent posting. I do not own a Pentax MZ). If it is
true, it is a pity, since matrix metering with A/M/K TELEPHOTO lenses (e.g.
over 50mm focal length) is quite correct with PZ bodies!
Hope this helps,
Knut
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