I have three Pentax digital meters. They read identically. One of them I keep in brand
new condition to check the other two. I also once had a Minolta spotmeter which read identically to all the Pentax ones. But over time or hard use meters need period recalibrating. In my case, this occurs when something is only 1/3 stop off at the
extreme end. But this has happened only three times in the last thirty years.
My Zone VI modified Pentax Spotmeter V (analog) is in close agreement with my Sekonic 580 spotmeter. I've since picked up a Pentax digital spotmeter, and it's also within about 1/3rd stop of the analog version.
Folks,
My Pentax Zone VI meter in question went to Richard Ritter for a checkup. Here's the results of that:
My meter is on for the low and mid-range, off 2/3 stop on the high end compared to a new Pentax meter he was using for reference.
The Pentax Zone VI meter should be off about a stop from the Sekonic. The Sekonic is calibrated to 13%, the (some) Pentax to 18%. Richard also stated that he'd seen Pentax meters calibrated to both 13% and 18% depending on when they were manufactured. This seems to be consistent with replies to my OP indicating that other folks Pentax spot meters both agreed with or were a stop over compared to other meters.
When comparing my Zone VI Pentax in the field to my friend's non-Zone VI, we were off about a stop metering the shadow values on the ceiling of the steeple above the bell in the attached jpeg. Richard stated that the Zone VI meter would not see the same amount of IR as the non-Zone VI would and he would expect them to differ.
There have been numerous threads about the "K factor" and whether meters should be calibrated to 13% or 18%, and I don't think there is a "right" answer for B&W photography any more than "do I need a center filter". But what was more important to me is that my meter is (or was) apparently not linear, and that does need to be fixed.
Cheers, Steve
Steve, that's a much bigger difference than I would expect. Sekonic use K = 12.5, and Pentax use K = 14, only about 1/6 step. I don't know the nominal Zone VI calibration, but my Pentax V and my Zone VI Pentax Digital give very similar readings except in very reddish light. Of course, the nominal calibration doesn't tell the whole story; meters can differ considerably in spectral response, and can agree metering one target but differ considerably on others. An unmodified Pentax has a very broad response; I assume the Zone VI modification narrows this, but I've never had my modified meter tested. Minolta (and presumably now Kenko) meters had a response something like the 1931 CIE observer, though they weren't as carefully controlled as Minolta's precision luminance meters. I don't know the response of Sekonics, and have never owned one, so I don't even have a rough idea.
Meters aren't really calibrated to a reflectance; rather, they're aimed at a source of known luminance and adjusted to give a certain reading, with adjustments usually made at three or four different luminances. It's possible to envision reflectance if a reflected-light meter is compared with an incident-light meter. Two approaches are possible: comparison with the incident meter's flat sensor, approximating what would obtain from metering a perfectly diffusing gray card perpendicular to the line of sight, and comparison with the incident meter's hemispherical sensor, approximating what might obtain from metering a three-dimensional scene. In the former comparison, most incident meters use C = 250, and the equivalent reflectance works out to 15.7% for Sekonic and 18% for Pentax. Comparison with a hemispherical sensor is a bit tougher because calibration constants vary a bit among manufacturers; calculated reflectances can range from about 11.5% to about 13.7%. The concept of "reflectance" with a hemispherical incident receptor is of course a bit different from the formal one.
It should be born in mind that calibration constants for reflected-light meters derive from broad-angle averaging meters measuring entire outdoor scenes, so they weren't really intended to measure gray cards. If a concept of reflectance is required, it's similar to that with a hemispherical incident-light meter: it's the light reflected by a typical outdoor scene, including objects of different reflectances and orientations, as well as the effects of some shadows. It can be shown that this "reflectance" can range anywhere from about 11% to 14%; it is not, however, a calibration standard.
I explain this in more detail in Exposure Metering (PDF), and look at the effects of meter calibration. It should be obvious that, as Steve said, there is no "right" calibration. It would, of course, be nice if all of your meters gave the same reading, whatever it might be. But, for reasons given above, it probably ain't gonna happen.
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