> > head). Full aperture meaning the lowest setting, like 2 or 5.6 something? Wide > open? If the camera can't read the aperture, how can it know which f stop is > full open on the lens?
"Full aperture" is wide open (using the full area of the lens, not just a part). The camera *doesn't* know what f stop it is, but it doesn't need to; it knows how much light is coming into the camera, so it knows what shutter speed to pick to get the right exposure. It could be an f5.6 lens in bright sunlight, or an f1.4 lens in a shady grove - makes no difference to the camera. That's because when the lens is mounted on the camera the mechanism of the lens engages wth the mechanism of the camera to hold the lens "wide open" so that the image on the viewfinder screen is as bright as possible. Before that (introduced on the Spotmatic "F" in the mid-70s) it was quite hard when using smaller apertures (those with larger f numbers, just to be as confusing as possible :-) With the exception of a very small number of specialty lenses, all K-mount lenses work in this full-aperture metering mode, rather than the alternative (stop-down metering, where the light is measured at the taking aperture). The camera doesn't need to know what the actual aperture selected is, but it does need to know the difference between the metering aperture and the taking aperture. This information is communicated to the camera body by a mechanical linkage consisting of a lever on the lens (which moves as you turn the aperture ring) and a lever on the camera which in turn is moved by the lever on the lens. It's this lever that is absent from the *ist-D body (the perjoratively-nicknamed 'crippled' K-mount), making it impossible for the camera body to read the position of the aperture ring on the lens. > Is there any logical way to compensate for the above? > Something one can do to guesstimate an accurate meter reading? Sure. Use the "Sunny 16" rule: on a bright, sunny day, with the lens set to f16, the shutter speed should be one over the speed of your film. (i.e. if you are using Kodak Gold 200, either 1/180 or 1/250). If it's not sunny, but instead cloudy, or shady, or overcast, or twilight, ... use progressively larger apertures. There used to be an insert in the box with every roll of film that gave a table of appropriate values; that no longer seems to be the case, but an abbreviated table (handling some of the more problematic cases) is printed on the inner surface of the box for at least some currently-available films. The camera I learned on - my father's old Agfa fold-out - had settable aperture and shutter speeds, but no built in light meter. While he did have a hand-held meter, he rarely used it, and he taught me to rely on my own exposure estimates. It's easy to guess to within a stop in daylight conditions (and usually to within half a stop), which is as accurately as you could set the exposure on the camera!
