Calibrating a DSLR for spot metering

I've been using my D70 for spot metering with decent results, today I thought I'd try to calibrate it to 18% Middle Grey.

I set up an 18% Card in three different areas with three different lighting conditions. One under cloud cover, and two separate shaded areas next to my house.

I used my Minolta Flash Meter III to make sure the 18% Card was evenly lit, and to verify the exposure. I then metered with the D70s on spot, at 135mm, close enough to have the spot meter only the card. After I checked to make sure the exposure hadn't changed with the Minolta III again.

The results I got were less than consistent:

http://www.5onitnyc.com/JPEGS/MinoltaIIId70Chart01.jpg

I tried the experiment a ton of times and kept getting the same results. In the Cloud-Covered Sun the exposure compensation was -0.3, in the shade it was -0.7, regardles of what white balance I set, the lens focal length, or the camera ISO.

Anyone have any thoughts? I understand I'm going off faith that the Minolta Meter is reading an accurate 18%. Even so, what would cause the differences in shade to sunlight, the increase of the color temperature?

-Omar

I'll add that I understand what's important would be the consistency of using the same meter, with the same film, but I'd still like to establish consistent readings with multiple meters.

The minolta is mostly an incident light meter, but can do a 40 degree reflected use with the proper adaptor on it. The Nikon isn't being used as an incident light meter, but as a spot meter. So you have three different sorts of metering you can do with two meters. The reflected meter on the minolta is probably more like a CW metering on the nikon.

I'd say you are getting pretty close results despite the differences.

If you are focusing close with the nikon you might be losing some light due to the focus setting, sort of like bellows factor on a LF. Setting the lens focus to infinity might produce different results, and a filter on the lens might account for something too. I'd just turn the exposure comp +0.5 on the nikon if you want to have something comparable based on your consistent results.

I have been working on a procedure for calibrating a DSLR for use as a photometer, to read in candelas per meter squared. There are several corrections to be done, light falloff, linearity, spectral response and calibration factor, the corrections done in Photoshop. For your use, only the spectral response and a calibration factor are needed. The calibration factor is, for your data, "subtract .5 stop". The spectral response is the problem. The Minolta LS-110 I use is supposed to match the human eye response. I need to make a large correction for the DSLR, increasing the green a lot, and who knows what your Minolta needs. But neither matches the film, each film is different. If you use 1/2 stop for the calibration factor, your error is 1/5 th stop, good enough. Better than that requires a LOT of effort.

Short answer, the nikon is not set up to meter 18% grey as middle gray. And likely the Minolta isn't either.

More details here:
http://dpanswers.com/content/tech_kfactor.php

As long as there is consistancy and you know the offset, you should be fine.