Why do modern Pentax DSLRs require stop-down metering with older K-mount lenses?
Asked 3/25/2015
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My Pentax ME Super can meter with an older manual-aperture lens at any aperture-ring setting, but when I mount the same type of legacy Pentax lens on a newer Pentax DSLR, I have to use stop-down metering to get a correct reading. Why did Pentax change this behavior? Is it simply because of open-aperture metering, or do modern bodies lack the mechanical coupling needed to know the selected aperture on older lenses?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
11y ago
2 Answers
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Legacy lenses tend to have a mechanical and or electrical coupling that tells the camera where the aperture ring is set without actually stopping down, so the meter in cameras built for this can take a reading at full aperture, do a quick calculation based on the selected f number. When you press the shutter release, the mechanical linkage causes the diaphragm to spring shut to the selected aperture during exposure, then spring back to wide open.
You can fit that legacy lens to a current DSLR, but the electrical linkages are different and the mechanical ones are simply not there. Your DSLR has no way to tell what f number you've selected, and no way to operate the diaphragm either, in just the same way as it can't autofocus with that lens.
Gist of it is, to get a meter reading through the lens, it has to be stopped down so the meter uses the actual amount of light that will hit the sensor.
tl;dr: legacy lenses will mount on a modern body and will focus to infinity, but forget automation of the diaphragm, focusing, etc. They're either not present or not compatible.
Originally by user38565. Source · Licensed CC BY-SA 4.0
user38565
11y ago
0
Generated from our catalog & community — verify before relying on it.
Older Pentax film bodies could meter manual-aperture lenses wide open because the lens and body had mechanical coupling that told the camera which f-stop was selected. The camera could then meter at full aperture and compensate for the chosen aperture value, while keeping the viewfinder bright for focusing.
Modern Pentax DSLRs were designed around electronic lens communication, and many no longer include the mechanical aperture-ring coupler used by older lenses. With a legacy lens, the DSLR often cannot tell what aperture you selected on the ring, and may not be able to control the diaphragm in the same way either. So to meter correctly, it must physically stop the lens down and measure the actual light coming through.
Open-aperture operation is also useful for focusing: a lens wide open gives a brighter viewfinder and helps autofocus systems work faster and more accurately.
So the main reason is not that stop-down metering is impossible to keep, but that the older mechanical linkage was removed in favor of simpler, cheaper, and more electronic lens-body communication.
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