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45PME vs spotmeter


New Member
I have a 503CW with 45PME and a 132C digital back. For most images I've found using the PME in averaging mode works just fine, since the Imacon back has almost endless shadow detail. For extremely bright skies, I've taken to shooting a second image two stops down, to merge later.

Here's my question: before I took to just stopping down, I tried using my Pentax spotmeter as I would when shooting chromes, taking a reading off the brightest point and making that a zone VII. But I immediately discovered a difference between the two meters that I've since confirmed by measuring the output of a light box (diffused with a couple extra layers of white plexi): Whether in spot or averaging mode, the PME will read as EV12 what the spotmeter reads as EV 11. The one-stop difference seems the be consistent up and down the curve.

If anything, I'd think that the protective layers on the digital back would require more exposure, not less ... but in any case, the PME doesn't know what kind of back is on the Hassy, digital or film.

I have not had the two professionally calibrated, but I've been using the spotmeter for years with entirely predictable results. Can someone think of a structural reason, other than miscalibration, why these two would give different readings of the same light source?

jim schaefer