If 18% Grey Is A Midtone ....

[Scott Fleming]

Using my grey card:

Then why is Oak tree bark in semi-shade 4 stops darker and white puffy clouds on a bright partly cloudy day only 2 stops brighter?

Just for added info .... light green grass (not the dark bleu green burmuda stuff but that bright spring green) in full but hazy sun reads a full stop brighter than my grey card.

[Jim Rice]

So.....I took my Kodak grey scale (thanks, Jim G.) out to an oak tree in the yard. It turns out that oak bark is considerably darker than 18% (1.30~1.60 as opposed to .70). I'll take it out again on the next partly cloudy day.

[John Layton]

Oh, this can be tricky. Using a grey card under non-controlled lighting conditions can be a very inexact science. I recall a workshop I led, years ago, at the beginning of which I gave each participant a grey card. Readings were all over the place - with lots of variables, including types of meters, overall light intensity, even some specularity exhibited by the grey cards themselves. The only sure bet about testing for true relative brightness is to have all of your subject matter at the same distance away, and exhibiting a texture close to that of the grey card, with areas of consitent tonality large enough to be adequately and accurately read without any bias creeping in. Then there are meter cells themselves, which, even if calibrated "properly" might be truly accurate at only one light intensity.

So go back and look more closely at your tree bark - to see if there aren't a number of "microdensities" introduced by the surface texture itself, which might throw off an intuitive guess about its relative brightness. And you mentioned a "white cloud" but on a "partly cloudy" day. Clouds are full of variable and continually changing densities, and the extent to which they can be isolated in the sky can make them difficult to assess in terms of their actual relative brightness.

You also need to give more info. What type of meter are you using? How is it calibrated - and have you had it calibrated recently? It might be best to begin with these questions, then you might want to run some log exposure/density tests for different tonalities, repeating these for maybe three overall light levels to test for liniarity. Might seem like a pain in the rear, but when its done you'll have a better idea of whats up, and you might even decide to throw away your grey card!

[Mike Davis]

There are many internet resources describing the proper use of the gray card.

http://www.cameraguild.com/technology/gray_card.htm
http://home.nc.rr.com/tspadaro/The_Grey_Card.html

I don't use a grey card. I use a spot meter and objects in the scene (unless I'm using strobes in which case I use an incident flash meter).

It doesn't matter that the oak bark is darker than the grey card. Nor does it matter that the clouds are brighter. What does matter is the difference between the shadow and the highlight. If its too great, detail will be lost and printing will be more difficult. If its too small, the photo will look too flat. But if its right, the photo will print more easily and the result may be more pleasing.

With the spot meter I determine the difference in values between the shadows and the high lights. I then set my shadow, select my speed (is there wind, is the subject moving, etc), and determine my f stop. Based on the luminance difference between the shadow and the highlights (which I try to note in my notebook), I determine development. N, N-1, N-2 etc.

Obviously I'm simplifying the Zone system, but I like simplicity and this method works just fine for me. When I'm actually measuring things, I look at the Exposure Values on the meter and make my adjustments. It's not rocket science and its not magic.

[Scott Fleming]

Thanks all.

I usually depend on green grass or folliage to find my midtones and it works most of the time. I have the Minolta Spotmeter F. I usually measure the entire scene to see how many total stops I'm dealing with and which end I want to alter. Almost always the high with NDGs. If I meter the shadows then the highlights and average the two I'm almost always within a half stop of just metering bright green grass.

I never use the grey card. It just seems too unreliable. I can watch the meter change as I angle the card in the sun. I just had it out today experimenting and it seemed far from the middle of things tonality wise when using typical tones found in the landscape. The treebark was 20 yards away if that helps and I used the whitest part of a cloud with the sun on it that I could.

[Jim Rice]

I'm an incident meter man myself. Once acclimated to it I would suggest that for the work I do (color trannies) it simply works.

[Alan Davenport]

In an ideal photographic world, and with an accurate meter, an incident reading should probably agree very closely with a gray card reflective reading.

I shoot mostly transparencies, and my spot metering is limited to 7½° (Gossen LunaPro F.) When I use the "spot" attachement, I find that I can scan around the various parts of the subject and use the LunaPro F's scale (the one on the needle, which is calibrated in +/- stops) to set an exposure. I just hold the meter "on" and watch the needle as I scan around. Best case is when I can put the darkest and brightest parts of the scene inside the marks for +/- 2 stops; when the brightness range exceeds that of the film it is very easy to see it and adjust accordingly.

I hope that description made sense to someone reading it. It works for me; I seldom get a bad exposure this way.

[Alec Jones]

"I usually depend on green grass or folliage to find my midtones and it works most of the time."

The problem with that is that green grass reflects a lot of IR. That's why it's white in IR photographs. Not a good choice unless you filter it out from the meter and the film.

[Scott Fleming]

alec,

Well this is new info. Never heard this anywhere before. So how many stops does IR throw off a spotmeter? What method do you recommend? Metering the entire scene and averaging?

[Alec Jones]

Fred Picker discovered that in developing his Zone VI Modified Pentax Spotmeter. The meter and the film see things differently than the eye - and they don't agree with each other either!

It sounds like you're starting right in determining the range of values, but if you make your exposure determination from the high values, and open up enough to arrive at a zone v value exposure, you'll be there.