The problem with gray cards is that it's difficult to use them correctly.
They're supposed to be pointed so a line perpendicular to the card bisects the angle
formed by the lines from the subject to the light source and to the camera lens.
It's not always easy to determine exactly what that orientation should be.
The reading can vary significantly depending on card orientation.
The meter reading is taken along the line collinear with the lens.
- Leigh
If you believe you can, or you believe you can't... you're right.
I think when you perform Zone System calibration, you often "find" a rated speed that is half box speed. But I don't think the plan was originally to use half the rated speed. Half-speed makes an effective shortcut because black and white negative film gives satisfying results when slightly overexposed.Originally Posted by michael slade
There's nothing in the zone system that says the "real" film speed is 1/2 the box speed. The film speed is determined by testing. It might end up being 1/2 the box speed or it may not.
Brian Ellis
Before you criticize someone, walk a mile in their shoes. That way when you do criticize them you'll be
a mile away and you'll have their shoes.
Exactly. I have found through testing several different types of films that for me the rule has been that I use 1/2 the box speed. It has also proven to be helpful when shooting color negative film too. Trust me, I didn't come by this idea blindly.
I’ve calibrated and tested my equipment being used like I use them.
Ends up that box speed is best for me.
One-half box speed would have worked fine too in most of my b/w shots.
Actually one of the classic ways to use an incident meter is called duplexing. It's described in Dunn & Wakefield's Exposure Manual.
Duplexing uses readings from different directions and averages them to find exposure settings.
You can't depend on your eyes when your imagination is out of focus. ~ Mark Twain
I've been using an incident meter for years, I've always given negative film an extra 2/3 stop and rated slide film at box speed. Never liked black and white at box speed.
Found this article about grey cards "Meters don't see 18% Grey"
http://www.bythom.com/graycards.htm
"I don't recall ever reading Phil making mention of any 'existing standards' as being incorrect, but would appreciate any references."
See Beyond the Zone System (pp. 112-113 of the 3rd Edition).
He provides 2 possible historical reasons why the 18% standard became the standard.
"The origin of the 18% reflectance gray card standard is unclear, but it appears likely that it was intended to be used with artificial lighting in the studio and that the original instructions suggested increasing the indicated exposure by 1/2 stop when using the card outdoors. Alternately, it may have originated in the graphic arts, where the luminance range of typical copy subjects is approximately 1.5 in log terms. For subjects of this range, the gray card is in fact, a middle gray because its 18% reflectance is approximately equal to a log value of 0.75.
The normal range of an ordinary photographic subject, however, is generally considered to be 7 stops, or 2.1 in log terms. The middle gray of this range is 1.05, which translates to a reflectance of about 9%. If the geometric center of the normal range is considered to be middle gray (which can be debated) the standard gray card is a full stop too light in value."
Could you please summarize that thread ? I'll have to read it several times to discover the "takeaway" of what Stephen Benskin has explained. It's going over my head at the moment.
If that were true, wouldn't all current digital cameras with meters corresponding to middle grey also then be off by a stop? And who cares where the middle of the scale is mathematically, doesn't middle grey correspond simply to what we see as "middle grey?"
On the one hand we propose that engineers have already solved the problem, because cameras already work correctly - which is a tautology, isn't it ?
On the other hand, we're suggesting that engineering doesn't matter in the end. But doesn't that negate the value of the first proposition ?
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