> how should I angle the gray card in sunlight?(posted 8395 days ago)Good question. The reason the angle is important is that the "standard" is about 13%, not 18%.
Kodak used to state that the grey card should be held vertically, taking care not let it shade itself or hold it at an angle that produces glare, and hold it at a 45-degree angle to the light source. The reason was that this would produce around 13% reflectance, even though the card was 18% reflectance.
This vanished from the grey card instruction sheet a decade or so ago.
After a recent debate about 13% vs 18%, Bob Shell contacted someone at Kodak regarding the change in instructions. It turned out that when Kodak revised the packaging for the grey card, someone _forgot_ to include the instructions about how to hold the card at the proper angle!
I believe that's back in the instruction sheet, but since I haven't bought or looked at it I don't know.
Anyway...the _correct_ method is to hold the card vertically and turn it to about a 45-degree angle. Which is very difficult to do consistently.
The difference in how you hold the card results in about a 1/3 stop change; that much error isn't desirable but otoh isn't huge either.
I avoid the problem entirely by simply using 18%. After all, if my meters get set 1/3 stop higher or lower than someone else's it makes no difference at all. Actually to eliminate the vagaries of using a grey card a Wallace ExpoDisc (www.expodisc.com) works great.
Over the years I've found that my EIs resulting from calibrating with the 18% ExpoDisc have usually agreed with others not using exotic developers within 1/3 stop, sort of a reality check.