OK, so without an aperture simulator the camera doesn't know what the working aperture is unless it's set to "A", when the camera tells the lens what to do, rather than the lens telling the camera what it's done. This exchange is made electrically through the A contacts on the face of the camera and lens mounts.
BUT- There is NO electric stop-down mechanism in A, F, FA or even FAJ lenses. No motors or solenoids exist in the lenses to close the diaphragm. This is done by a MECHANICAL LINKAGE from the camera body, which is shared with the DOFp function. The lenses' default state when off the camera is stopped down. When mounted, their stopdown lever is brought to bear against the camera's corresponding stopdown lever, which has enough spring loading to open the diaphragm out to full. When DOFp is activated or the shutter tripped the camera's stopdown lever is backed off which lets the lens return to its set aperture. Ever since the "A" setting appeared, these levers have had a linear relationship between their degree of movement and the diaphragm's f-setting so that the camera could control aperture selection by only moving the lever enough for a certain aperture instead of all the way every time. Pre-A lenses were only made to be stopped down to their limit of travel as defined by the f-number selected on the aperture ring, which is why they cannot function properly as camera controlled lenses. Otherwise you'd only need to select the minimum aperture and fool the camera by mount-masking, but if you did that the mid-apertures would be non-linear and unpredictable from lens to lens. The flaw in Pentax's logic, in disabling the diaphragm operation of pre-A lenses, is it presumes that aperture control needs to be centralised in the camera body. But it obviously doesn't. If you use a manual aperture lens or an M42 lens with a K adapter you get to use the lenses at any aperture (although I haven't absorbed enough information to know if stopped-down metering is supported in these cases). It's only the meter which needs to know the current aperture for the purpose of full-aperture metering. But there's more to cameras than metering, some of us are content to have a meterless camera (cases in point: in a studio doing pack-shots, at a school or at a mall doing bulk portraits, or anytime that an external meter is appropriate). So why for Godsake can't we have meterless manual at any ring-selected aperture? Is it so hard for designers to believe that not every feature in a camera needs to be used all the time, or that some features might NEVER be used by some owners? Obviously the stopdown mechanism is present in the *ist and *istD, because it needs to be (otherwise even "A" able lenses couldn't stop down). Apparently the mechanism is disabled when it doesn't recognise a lens set to A, or an FAJ lens. Manual diaphragm and screwmount lenses get around this because they are already stopped down and there is nothing the *ist can do to change that fact. In protecting us from ourselves Pentax is presuming that we are all idiots who don't know our apertures from our a***holes. Why should they care if some owners don't RTFM and ruin a few photos? The camera wouldn't be harmed and maybe the careless photographer would learn to RTFM in future. And the photographers who want to use pre-A lenses would be happy. GIVE US METERLESS MANUAL WITH FULL APERTURE FUNCTION ON PRE-A LENSES PLEASE, PENTAX. It's only a software action that disables aperture function, so fix it! Pleeeeeeeeeeeeaaaaaaassssssse :) regards, Anthony Farr
