thanks for your excellent summary on DOF preview. However, all you write is in accordance with my idea: A camera capable of DOF preview that has the information that the aperture ring is not in "A" position could in principle stop down the lens by fully releasing the aperture coupler. The lens then would stop down to the selected aperture and the camera could measure the amount of light regardless of what the selected f-number is. If this would only happen while the release button of the camera were pressed half-way down, you would get shutter speed and DOF-information right then, and I wouldn't mind working with such a camera.
Arnold
Bojidar Dimitrov schrieb:
Hi Arnold,
I am not able to follow exactly:
Let me explain my idea a little more: If a camera (not necessarily
the *ist) has no aperture simulator, it can still properly measure
the amount of light provided that it does not meter at open
aperture. As a camera like the *ist offers e l e c t r o n i c
depth-of-field preview, such camera might automatically close the
aperture with every lens that is not in "A" position. In this case
the camera would not need to know the selected f-number (like it
does not know with the K28/f3.5 shift lens).
Still, here is a small explanation of how DOF preview works. All lenses with aperture rings have a spring that tries to close down the aperture as much as possible. Moving the aperture coupling of the lens open the aperture. Thus, regardless of which aperture is selected via the aperture ring, the aperture can only move between fully open and the selected aperture. When you mount a lens on a body, as you turn the lens to lock it onto the body, the DOF lever of the body catches the lens aperture coupler and opens up the aperture.
Now, when you select a numeric aperture (anything other than "A") and
press the DOF button, all that happens is that the aperture coupling is
FULLY RELEASED, and the lens closes ONLY DOWN TO THE SELECTED aperture. This "ONLY" is a function of the lens, not the body. This works
correctly with K and M lenses (with their non-linear aperture lever) and
with A, FA and FA lenses (with their linear lever).
Z/PZ cameras do the same also in P and Tv modes, and then the aperture closes down to the smallest lens aperture. This is of course wrong. So MZ/ZX bodies have electronic DOF preview where the lens aperture is released ONLY AS MUCH AS NECESSARY in order to set the f-stop selected by the body. This requires calculating the exact amount that the lever needs to be allowed to move, and thus does not owrk with K and M lenses.
The DOF lever is the same one that controls the aperture during exposure, so it must be present on all bodies, even the *ist. And it needs to possess the functionality of selecting any chosen aperture, also on the *ist.
Once again, the trouble with the crippled bodies is that they lack another lever. All bodies meter before the exposure, through the lens, using the aperture opening that the lens currently uses. Cameras that have the lever in question know how much the lens will close down during the exposure, and increase the shutter times accordingly. Lenses with manual diaphragms have such a lever, and that lever is at a constant position, thzus saying "the aperture will not close down during exposure". Cameras that do not have this lever do not know how much the aperture will close down, so they assume 0. Thus the overexposure with the MZ-50.
Now, the FAJ lenses have only one aperture setting, "A", so they do not need such an indicator of how much the aperture will close during exposure. This is something that the body controls via the other (the DOF) lever. All I'm saying is that if the FAJ lenses do not have this functionality, and if they are a perfect match for the *ist, the *ist may not have the mechanical coupling to read the position of this non-existing lever.
Hope this all makes at least a bit sense... Boz
