Manual variable aperture zoom on a digital Pentax

Started 3 months ago | Questions thread
GossCTP Veteran Member • Posts: 5,657
What I've observed

alex_virt wrote:

Can anybody explain to me how a manual A-series zoom with variable max aperture behaves on a digital Pentax camera?

For example, I have an F3.5-4.5 zoom.

Do I understand correctly that the camera doesn't know the actual aperture and treats the lens as a constant F3.5 zoom and underexposes at the long end?

If I set the aperture in camera to F4.5, will it remain F4.5 through the zoom range and become wide open at the long end? Or will it remain stopped down to the actual aperture of F5-something at the long end?

Here's what I have observed. The lens communicates focal length so at some point it changes the wide open metering from f/3.5 to f/4.5. The lens itself though moves in a smooth continuum from one end to the other. You can observe this with DOF preview. Zoom in small increments with the lens set to wide open - there is no point where the blades ever stop down in the slightest. Therefore at the point where the lens jumps from 3.5 to 4.5 there will be an exposure error because the real aperture is at f/4 but at that point that the meter jumps 2/3 of a stop as the real aperture shifts very little.

I think the problem may be transposed even when stopped down, because the camera is stopping down with a fixed stop distances. In other words, if set to f/8, the camera stops down the lever by 2 and 1/3 stops. At the 3.5 to 4.5 threshold point it will change and stop down by 1 and 2/3 stops to get the same f8 at the long end. I haven't tested much on this second part as I ran out of time and interest.

If set to a small aperture, e.g. F8, will the actual aperture remain F8 through the zoom range?

I'm confused...

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