Originally posted by rawr Forgive my ignorance, but when I mount a Pentax (or any other) manual lens on my NEX-F3 via a simple dumb PK-NEX adapter, as I adjust the aperture, the camera just continuously meters the scene according to the light the lens lets in. I then simply manually focus and shoot.
Would a stop-down coupler enable Pentax lenses without the A setting to behave the same way on a Pentax DSLR (ie no need to use the green button to meter the scene with K or M series lenses, just adjust aperture on the lens, focus and then shoot)?
Yes -- and moreover, unlike the adapter scenario you described, the aperture would remain wide open until you take the picture. So you would still get the brightest possible viewfinder, for easier focusing.
The coupler tells the camera what setting the aperture ring is at, so the camera can use that plus the amount of light coming in (at wide open) to deduce the correct exposure.
So you get both full-time metering as with adapted lenses (like M42)
and wide-open focusing and composing as with K/M lenses.