[mikemike]

What does the lens discuss with the camera? I know that the lens seems to have a role in certain decisions like focus, metering, and distortion correction (am I missing anything here). I am confused as to why it has a say in something like focus or metering. How does a lens get any kind of information about something like that since it is designed to let light pass through instead of intercepting the light.

[scratchpaddy]

I think it's a passive communication. The lens tells the camera what it is and what it can do (focal length and aperture especially), and the camera works within those parameters. The focal length determines how the camera handles shake reduction. The aperture contacts on the lens tell the camera what it'll get when it presses the aperture lever a certain amount, letting it calculate the necessary shutter speed when stopped down even though it meters wide-open.

I'm interested to know about the focusing part, though. Like you, I don't know how that's communicated.

[audiobomber]

I'm reasonably sure my FA 35mm communicates better with my K-01 than my FA 28mm does. The FA 28 gives me crazy high shutter speeds in P mode (1/90s, 1/125s). Minimum it wants to go is 1/45s. The FA 35 is often at 1/30s.

[Adam]

For AF communication I'm willing to bet that it's just speed and direction, which is just how screwdrive AF works.

Adam
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[audiobomber]

What does the lens discuss with the camera? I know that the lens seems to have a role in certain decisions like focus, metering, and distortion correction (am I missing anything here).

In addition to what scratchpaddy said, there's CA correction and MTF data for modern lenses.

[Lowell Goudge]

The lenses also communicate maximum and minimum aperture for exposure control and metering correction/compensation. This data is also used for matrix metering and P-TTL flash

It also gives lens model, and focal length for shake reduction, control of the zoom head on flash, and with zooms control of the MTF functions and tracking the selected exposure curve as a function of focal length

[mikemike]

Interesting, if it is just passive, why can't the data for lenses be passed in outside of that lens communication such as a file stored on your SD card and when you put a manual lens on instead of inputing focal length you can choose from a list of lenses that you have profiles for in that card?