Originally posted by potatostar I was told I would need to use an external light meter since those lenses can't communicate with the camera.
You can use K and M lenses with no adapters or restrictions in camera M mode. Since camera does not know aperture range of the lens using K and M lenses in P, Av and Tv modes is not possible and that is the only restriction.
Here is how to do it, M lenses in M mode. Two ways:
1) Holding DOF will activate meter (aperture symbol on on/off switch): while holding DOF preview rotate aperture ring or move camera around and see how reading changes (lower right corner in the viewfinder). Since lens is stopped down (that is, aperture blades closed to their preset value) image will get darker.
2) Using AE-L button sets correct shutter speed for set aperture: point at the target, hit AE-L and camera will stop down, meter, and adjust Tv. It all takes just a moment! (I use this method 99% of time.) Point at various subjects and keep hitting AE-L to see how it works.
Note: for consistency change "AE-L bttn on M expsr" in custom menu to "Tv Shift". That way hitting AE-L in M modes will work the same way for all lenses (from oldest K to modern DA): setting Tv based on Av.