Manual variable aperture zoom on a digital Pentax

Started 2 months ago | Questions thread
OP alex_virt Senior Member • Posts: 2,398
Re: Manual variable aperture zoom on a digital Pentax

MightyMike wrote:

alex_virt wrote:

MightyMike wrote:

alex_virt wrote:

alex_virt wrote:

MightyMike wrote:

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?

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

I'm confused...

Its an A-series lens, the camera therefore knows the aperture, the camera also therefore knows when the aperture is not F3.5 anymore when you zoom. So set the camera to F3.5 at the wide end, zoom the lens and watch the aperture change on the back of the camera (there is an off chance you may have to tap the shutter button to see the change) That said, when set to F4.5 its F4.5 regardless of what zoom position you're at, ditto for F8.0.

On my KP it doesn't change and remains F3.5 at both ends.

I've just experimented with my film camera Z-1P. The F3.5 didn't change either.

However, the shutter speed decreases by 1 stop at the long end on both cameras.

You've jogged my memory, I seem to recall the A35-70mm F3.5-4.5 or was it the A28-80mm F3.5-4.5 or both having the same effect. It must be something to do with the simplicity of the aperture identification system and variable aperture zooms of the time.

That said, despite it reporting the wrong aperture wide open at the long end when setting it to an appropriate aperture F4.5 or higher it will maintain that aperture as you zoom. and despite the aperture not being correctly reported it seems the metering sill works. The difference between F3.5 and F4.5 is 2/3rds of a stop but I wouldn't worry if the camera adjusts the shutter speed by 1 stop, it could be how you have the camera setup or variability in the test or close enough to not make a significant difference.

If I set the aperture to F8, the shutter speed still drops by a stop at the long end. It means that the actual aperture at the long end is about F11.

Maybe it's because the KP and Z-1P were made to work with newer electronic lenses that communicate the actual aperture to the camera. I wonder how these lenses behave on older manual cameras ca. 1980s...

Ok, well its not behaving the way I expected it to but my expectations are likely wrong and your lens/camera combo are working the way they probably were meant to. Seems like a strange omission on Pentax's part though.

Conclusion: the actual aperture doesn't stay constant, but the metering remains correct, and the camera compensates for the aperture change by adjusting the shutter speed that reduces gradually from the wide end to the long end.

Probably when production of new zoom lenses with electronic contacts began, Pentax started to cut corners and reduced maximum apertures from 3.5-4.5 to 3.5-5.6

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