Originally posted by Wayno ...By the way, I don't really understand why metering should be accurate when manually stopping down an M42, but not accurate with the K mount lenses using the green button. Perhaps the lens isn't stopped down long enough to get an accurate reading? Thanks in advance for any advice.
There is a general problem with meter accuracy for all non-A lenses on the K10D and K20D and to a lesser extent on the KxxxD (mirror prism) bodies. Strangely, the meter behavior is somewhat
lens-specific, is aperture dependent, and is worse (by my observation) for lenses with a maximum aperture wider than about f/4 regardless of mount. My Jupiter-9 85/2 is by far the worst offender with about two stops underexposure wide open.
The general pattern is:
- Underexposure wide open
- Essentially correct exposure at f/3.5-f/8
- Mild overexposure at f/11 and smaller assuming adequate light for shooting at those apertures
A short note regarding stop-down metering at narrower apertures in moderate or low light. The published sensitivity for the meter on the K10D is 1 EV (ISO 100). My experience has been that sensitivity is somewhat less with linearity falling off at about 2 EV (ISO 100). Now you might think that 1 or 2 EV is pretty dark and it is when metering wide open. However, when metering stopped down in moderately low light (say f/2 @ 1/30s, ISO 100), at f/11 using stop-down metering, the amount of light actually striking the sensor is only EV 1 (ISO 100) and is at the limit of sensitivity for the meter.
The result is that the meter gives the same reading for all apertures below a certain point at a given light intensity resulting in severe underexposure. This loss of meter linearity in low light and narrow apertures is true for all stop-down metering cameras:
- Many M42 film cameras
- All K-mount cameras (film and digital) when mounted to M42 lenses
- All crippled KA mount cameras (film and digital) when mounted to non-A lenses
Steve
(Much more than a simple answer I know...I guess I am in a verbose mood...)