Originally posted by beholder3 I can't even remotely see how the presented deviations of 0-1 EV are of any significance to anyone outside theory.
Good point! The posted graphs were used as illustration because they are the ones I have for "A" lenses where "expected" was the actual metered value for that lens. They were part of a larger test comparing the stock screen with the Katz Eye product. I did not want to get bogged down with too much detail in the post, but with real world subjects the situation with the K10D was much more alarming with some lenses. The worst example might be my LZOS MC Jupiter-9 85/2. Wide open with a real world subject, the stop-down reading would yield 2.5 - 3.0 stop underexposure with readings at f/5.6 being much closer to expected. As noted in another comment, a quick test yesterday with an XR Rikenon 50/2 was grossly overexposed at f/5.6 for a green-button reading of my well-lit breakfast nook (a common test subject).*
Unfortunately, most reports are unavoidably anecdotal and in the real world, many lenses manage just fine. Here are a few general notes:
- Current generation bodies are much better than those from a decade ago, at least for M-mode metering
- Complaints/questions about stop-down metering continue even with today's cameras. The general form is, "My Sony meters my Taks just fine, but metering on my K-S2 is all over the place".
- Faster lenses tend to be more problematic
- Moderate maximum aperture lenses tend toward more predictable metering. I don't even chimp with my Tamron 70-150/3.5 on the K-3
- Most lenses (the above Rikenon excepted) are fairly predictable at moderate stop-down apertures
- It is difficult to separate the mechanical accuracy/precision of the aperture mechanism from meter problems
- Stop-down metering at narrower apertures is complicated by lack of linearity at the limits of meter sensitivity
It is that last point that I forgot to include in the original post. The manuals for stop-down cameras such as the Spotmatic typically include a table indicating a so-called meter coupling range, with combinations of ASA(ISO)/shutter settings outside the approved values on the table being outside the range of meter sensitivity and/or linearity. Moving forward to the present, the RGB meter in my K-3 may be sensitive to -3 EV (100) for
stop-down readings with a f/1.4 lens wide open, but not linear in the same light with the same lens stopped down to f/2. Under those conditions, the camera will set too high a shutter speed and underexpose.
It is a complex subject.
Steve
* Yes, the diaphragm is good on the Rikenon. Aperture performance (based on histogram centering) is linear. Why stop-down metering is flaky is beyond me.