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On my Pentax ME Super with a legacy lens (Pentax SMC 50mm 1.7) my camera meters at all aperture ring settings. Yet, if I transfer this lens to a modern Pentax DSLR, I need to stop down the lens to meter properly. By pressing the stop-down button, the camera closes the aperture blades to the selected aperture, and then reads the reduced light amount.

I find this behaviour annoying and wish that modern Pentax cameras can meter a legacy lens without having to do the stop down procedure. I have read elsewhere that this is because modern cameras do open-aperture metering. However, can't we keep the metering behaviour of old cameras with respect to legacy lenses?

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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.

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    \$\begingroup\$ I'll add that in the Pentax system, the mechanism needed for old lenses to meter wide open is called an "aperture simulator," and Pentax/Ricoh have never put one in any model of DSLR. \$\endgroup\$
    – coneslayer
    Mar 25, 2015 at 15:08
  • \$\begingroup\$ Thanks, but why can't cameras have both? Is it just a cost savings thing, or does open-aperture metering interfere with the "aperture simulator"? \$\endgroup\$
    – Alex
    Mar 25, 2015 at 23:05
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    \$\begingroup\$ Weight, cost, complication/reliability, and relatively small demand. Measurement systems, materials have all improved since those days and most people have only a fleeting interest in the old technology. Of your Pentax me were to cost $100 more to buy and weigh 25% more, would it still be as attractive a proposition? \$\endgroup\$
    – Bruce Bracken
    Mar 26, 2015 at 4:56
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Open aperture metering also allows open aperture focusing, which is perhaps even more significant. The wider a lens is opened, the faster and more accurate a phase detection auto focus system can be. Note that open aperture metering became popular well before digital cameras hit the mainstream market. It became popular when automated focus systems started to proliferate.

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  • \$\begingroup\$ But with legacy lenses I am using open aperture manual focussing anyway, as with manual focussing it is best for the aperture to be wide open as well? \$\endgroup\$
    – Alex
    Mar 26, 2015 at 23:58
  • \$\begingroup\$ With the aperture at its widest, the depth of field is at its narrowest. That also means more blur for the same amount of missed focus (compared to a narrower aperture), so the blur is easier to see in the viewfinder. This is true until around f/2.5 or so on most DSLRs. At that point the focusing screen effectively acts as a second aperture for the viewfinder. Please see photo.stackexchange.com/questions/21152/… \$\endgroup\$
    – Michael C
    Mar 27, 2015 at 0:45
  • \$\begingroup\$ yes, I completely agree with you, but the point I am trying to make is that I focus with my legacy lenses wide open as well, so I am not sure why open aperture metering gives an advantage in this area. \$\endgroup\$
    – Alex
    Mar 27, 2015 at 1:26
  • \$\begingroup\$ It is not so much that open aperture metering gives an advantage as it is that providing for both systems, when probably less than 1% of potential buyers even would use the legacy system, is a distinct competitive disadvantage. \$\endgroup\$
    – Michael C
    Mar 28, 2015 at 7:46
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Modern Pentax SLRs don't have stop-down aperture coupler on the body mount - mechanical linkage that links to lens aperture ring.

Why? Because modern cameras communicate with the lens using electronic or digital signal protocol, thus rendering mechanical protocol unneeded. Because mechanical components have higher costs (more finely machined, moving parts) and lesser reliability (again, finely machined, moving parts), Pentax decided to cut that coupler out of their cameras somewhere around 1997. with introduction of MZ/ZX-50 camera, and haven't reintroduced aperture coupler ever since, not even with full-frame K-1 (it even went to worse with MZ/ZX-30 and MZ/ZX-60 cameras). Colloquially, K-mount without aperture coupler is called "crippled" K-mount.

Unfortunately, there's no way to use K- and M-series lenses on modern bodies with full open-aperture metering. Actually, there is historically one modern, film camera introduced after K-mount got crippled, that still have un-crippled mount - Pentax MZ-S. So, it is possible to have both, there are no technical reasons not to have both, except cost cutting.

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  • \$\begingroup\$ They could at least have added a mechanical feature that allows you to just switch your lens to full-time working aperture metering (as is these days considered perfectly acceptable with adapted lenses on DSLMs).... \$\endgroup\$
    – rackandboneman
    Jan 15, 2019 at 16:55
  • \$\begingroup\$ Agreed! That would be something like Auto-Manual switch on M42 lenses. But, Pentax does not have a modern mirrorless camera system with an EVF, so working-aperture lens on traditional optical viewfinder would be of little help. \$\endgroup\$
    – elkarrde
    Jan 16, 2019 at 21:42

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