Manual variable aperture zoom on a digital Pentax

Started 3 months ago | Questions thread
James O'Neill Veteran Member • Posts: 5,751
Re: What I've observed

GossCTP 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?

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.

There is a difference between A-Series lenses which have 5 pins which send just aperture info, and FA / DA / DFA lenses which send focal length and current min/max aperture at that focal length. You can see this in (e.g.) exiftool. for example with my smc PENTAX-DA 18-250mm F3.5-6.3 ED AL [IF] I see this.

Lens Focal Length : 100.0 mm
Nominal Max Aperture : 5.7
Nominal Min Aperture : 38
Max Aperture : 5.5

Lens Focal Length : 250.0 mm
Nominal Max Aperture : 6.7
Nominal Min Aperture : 45
Max Aperture : 6.2

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.

Yes! You have wide open which might be any value between f/3.5 and f4.5 and then 1/3 stop down, 2/3 stop, etc until you reach the smallest aperture.

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.

It doesn't really jump. What has actually happened is the meter will see a will think the lens is at f/3.5 and the light out side is say EV20, when it's really f/4.5 and the light is nearer to EV21. It might think it is stopping down to f/8 but but it's really going to f/10 but that's OK because the light is brighter than the camera thinks it is.

 James O'Neill's gear list:James O'Neill's gear list
Pentax K-5 IIs Pentax K-1 Pentax smc FA 50mm F1.4 Pentax smc DA 18-250mm F3.5-6.3 Pentax smc FA 43mm F1.9 Limited +3 more
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