Reflective Meter Mystery

[Max Power]

A couple of days ago I was messing around with the TTL meter on my Minolta X-700 and my hand held meter in order to conduct a film test sequence. The sequence called for the use of a piece of black card stock instead of an 18% grey card because a reflective meter would read the black card as 18%.
Wanting to see if it was really true, I set my black card in an evenly lit spot in the shade, and then filled the viewfinder, set it on infinity and took the reading. I also took a reading with my hand-held meter. I next took away the black card and placed a Kodak grey card in precisely the same spot and at the same angle. I came up with one full stop difference in the readings between the grey card and the black card, and with both my TTL and hand-held meters.

Does someone have and idea as to why this might have happened? Shouldn't the TTL and hand-held meters have been 'fooled' into reading the black card at the same value a grey card?

I'm confused :confused:

Thanks,
Kent

 

[glbeas]

Your readings won't be the same because of the differing reflectivity, but if you had followed through by exposing and developing some film you would have seen the resulting negs would have been very similar in density to each other.

 

[dancqu]

I came up with one full stop difference in the
readings between the grey card and the black card, and with both
my TTL and hand-held meters.

Shouldn't the TTL and hand-held meters have been 'fooled' into
reading the black card at the same value a grey card?

That is an intensity reading. Then there is the meter's recommended
exposure which is correct for the gray but not the black. If anything
the meter is never fooled. The reading it gives is always for that
gray. You'll need to stop down two or three stops for that
black reading to produce a black density.

Worry about that one stop difference. Perhaps the black material
reflects considerable blue while your gray card absorbs considerable;
or the other way around. I'd think two or three stops, maybe four,
minimum twixt black and 18% gray. Dan

 

[Kevin Caulfield]

Isn't there the possibility that the Minolta sees the dark subject, and then assumes that it is an average 18% grey (as do many camera metering systems), so then gives an exposure compensation?

 

[chuck94022]

Shouldn't the TTL and hand-held meters have been 'fooled' into reading the black card at the same value a grey card?

I'm confused :confused:

Thanks,
Kent

Yes, Kent, the problem is your confusion, not your camera's confusion.

Reflective light meters are very, very simple beasts. They do not know what they are looking at. There is no computer. What they *all* do (with the exception of very modern meters, see below) is measure the luminance of the area of meter coverage (some "weight" one part of the coverage area more than another, but don't worry about this for this discussion).

The meter then tells you what exposure you need to set to make that luminance record as 18% gray on film. It doesn't matter if you point the camera or meter at a black card, gray card, or fresh sunlit snow, the result is the same. The exposure calculated is the exposure needed to record that amount of light as 18% gray on film. (The resulting *action* is the same, but the reported *exposure* recommendations will be very different in each case.)

In camera meters *usually* work because most average scenes that the average snapshot shooter points a camera at, typically averages out to 18% gray over all the values in the scene.

But if you happen to go out in the snow, for example, you'll find that if you trust your meter (and then let KMart process the result) your snow will render as 18% gray, not the brilliant white you expected.

Regarding the one stop difference you found between the black and the gray card, that could be for a variety of reasons. The lighting may not have been exactly the same. The angle of the meter to the target may have been different. The black card may not have been truly black. But with careful measurement you should have found that when metering the black card, both meters should have indicated an exposure several stops more open (or slower shutter speed depending on how the meter works) than the gray.

The only differences have happened recently with the very newest cameras, which add processing to the analyzed image. New Nikons actually compare the image being metered to a database of similar images, and recommend exposure settings similar to those images. Amazing and something I will forever avoid. Perhaps someday new cameras will detect where you are standing with GPS, where you are pointing with an electronic compass, realize you are in Yosemite National Park, and just record one of Ansel Adam's images. (That's what you were going for anyway, right?

-chuck

Edited to add the following:

ps: While the meter's operation is very simple, this simple but precisely predictable behavior of the meter is critical to calculating proper exposures for serious photographers. Anyone using the Zone System or similar systems depends on the meter calculating what exposure will make a particular spot in the scene as 18% gray. They can then adjust their exposure to "place" that spot on the value they choose. Typically using the Zone System, a photographer will measure a shadow area that they want to represent as their minimum area of detail in the image, then reduce the meter's exposure value by 2, 2.5, or perhaps 3 stops to "place" that shadow area into "Zone II or III", which is significantly darker than middle gray.

 

[Max Power]

Chuck,
Thanks very much for that explanation; it clears up the question for me.

Cheers,
Kent

 

[Claire Senft]

IR response

It could very well be that both meters are being influenced by Infra Red light. Set the cards up and the a photo of each one under the conditions mentioned. If there is a difference, which I imagine there will be, there is a very good possibilty that both meters are being influence by IR light. This is a very common anomaly.

 

[dancqu]

... there is a very good possibilty that both meters are being
influence by IR light. This is a very common anomaly.

He was working in the shade. His near same meter readings for
the black and gray cards I think more likely a character of the
material being metered and a preponderance of blue.

Perhaps Max will update us with other blacks and grays; fabric,
paint, other papers, etc. Dan