Originally posted by SMC-A I previously did not know that not showing the chosen aperture and not having an exposure lock was "a pentax thing" on so many models.
As for that, maximum + offset -> your chosen aperture.
And it has to know, how else to calculate the exposure time with open aperture metering?
At least thats the only logic I can come up with.
The Fuji cameras actually had a more elegant solution to the problem, offering the same functions.
Between the lens bayonet and a single contact pin is a resistor - corresponding to the maximum aperture of the lens. The chosen aperture is transmitted via a mechanical feeler. Full time aperture and speed are displayed.
I am planning to buy another Super-A body and investigate the possibilities of manipulating the "Frame 1" lock switch and the viewfinder illumination. Maybe I can turn this into a how-to thread
The K mount transmits the relative aperture the lens is set to by a lever at about 3 o'clock on the mount (looking into the camera), this controls a variable resistor connected to the exposure meter. The 'crippled' K mount (DSLRs and some 'MZ' film bodies) don't have this lever and can't open-aperture meter (at less than full aperture) unless the lens has electronic contacts. The lenses with aperture rings have moving levers to transmit aperture changes to the camera, the FA-J & DA lenses have static pins to indicate minimum aperture (the aperture that bodies not in program mode use).
The exposure meter measures the 'full aperture' reading then uses the aperture simulator variable resistor (above) to determine the offset down from full aperture (it doesn't need to know the actual aperture value - just how many stops down from the full aperture reading). The electronics in the body are not able to turn this into an absolute aperture value for display in the finder. When the lens is set to A, the aperture data is passed digitally through the contacts and as the camera controls the aperture, it knows what it is using.
My MZ-M is my only camera that displays the aperture in the finder (when using FA lenses) with the aperture ring not set to 'A'. This camera also has an illuminated LCD in the finder which may suit your needs better. It offers the same exposure modes, has a meter lock and I prefer the shutter speed dial to the buttons.
I don't know why you want to change the 1/1000th speed before the film is fully loaded? The speed reverts back to the previously set speed after the film advances to frame one and you won't gain anything. If you fiddle with anything and don't know what you're doing, you may find it will only ever fire at 1/1000th.
EDIT - It's not really a 'Pentax' thing, it is the way the original K mount works and had to be the same for the KA and later lenses to maintain backward compatibility.