D200: strange "mid-tone" metering of painted surface?

[kenneth_logan]

I'm trying to understand the metering of my new D200 as I anticipate

an important photo shoot overseas.

 

I shot a fairly-uniform off-white painted wall, exposure

compensation 0. The shot registered at an average of approximately

180 on a 0 to 255 scale. (The in-camera histogram shows it as

approximately that, as does Nikon Capture 4.4.) I would call that

approximately 1 stop overexposed, because I think that this should

yield a result around 127 on the scale. I do have that right, don't

I?

 

Now, varied-light scenes seem to do pretty well with the matrix

metering system, though I'm still checking that out.

 

So--why would this fairly-uniform surface meter so strangely?

[kuryan_thomas]

If you were using matrix metering, that's probably why. Matrix metering is trying to

"understand" what the scene actually is and set exposure accordingly. Maybe it thought the

wall was open sky? Or a grassy field?

 

Try with the spot meter or the center-weighted meter.

[tom_luongo1]

I just tried a similar test with the D70. The histogram was a moderately narrow spike near dead center of the frame. (i.e. around 127 on the scale)

 

Perhaps the camera software is doing something tricky to improve the SNR ratio. Yeah, I know that's lame idea. I get my D200 next week and could let you know then. Anyone else out there with a D200?

[andrewbennett]

For off white, 180 sounds about right. Was your wall as dark as the attachment? That is neutral middle grey which would measure 128.

 

Andrew

[tom_luongo1]

Andrew, the meter doesn't know whether it is pointing at a white wall or a dark wall or something in between. It assumes that the average reflectance is the same as an 18% grey card and sets the exposure accordingly.

 

Tests with my D70, under the same lighting condition and differently colored objects (more or less uniform color - I deliberately defocussed to make that) always yielded a histogram more or less peaked around the middle.

 

I'm assuming that Kenneth had his camera set to an automatic exposure mode. Metering a grey card and using that exposure to shoot an off-white wall would indeed yield something like a 180 average exposure.

 

Another possibility is using the Shutter priority mode with a speed slower than the aperture can compensate or Aperture priority with an aperture wider than the shutter can compensate to get the correct exposure.

[walterh]

Kenneth - what did the histogram (for each color) look like? Did you loose highlights or details in the shades? This would be more important to me than absolute values of one object in the image. There is no law saying that off white painted walls shall have 128,128,128 values. In my opinion a good metering system will try to keep as much of the histogram of the scene within the limits of the image histogram. In other words if all values of the scene were within the dynamic range of the scene who cares about the absolute values? ( I assume RAW images even though you did not mention this). At least the absolute value would be JUST ONE aspect out of many for an optimized exposure.

[EricM]

"It assumes that the average reflectance is the same as an 18% grey card and sets the exposure accordingly."

 

meter's aren't calibrated to 18%, haven't been for years. most are around 13% and if i remeber right, the ansi standard is 12.3%

[EricM]

and there's some useful info here too,

 

http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=00G9Ih&tag=

[tom_luongo1]

Eric, you are right, I meant to write something like "meters are calibrated to a 'theoretical' 18% grey card."

[andy a.]

I guess I'm most interested in what will happen if you shoot the wall again in center-weighted. I'm betting the hump will be on the 50% line in the histogram then, and matrix is just trying to be smart here. I can imagine a scene with nearly all blue sky and a tiny bit of foreground where this would make sense.

[twmeyer]

<i>matrix is just trying to be smart </i>... very funny, and most likely correct, too. I don't really trust The Matrix :^)... t

[curt wiler]

When shooting this kind of test, matrix metering will just add to the confusion. Use center-weighted or spot. And then remember the meter will try to make the surface 13% grey, not 18%, unless you dial in a correction.

[kenneth_logan]

I'll try center-weighted and spot and see what happens and report back in! I think that "middle gray" (whether 18% or 13%) should equal approximately 127.

[kenneth_logan]

I tested the same D200 with the same lens (180 f/2.8 D AF) with spot, center-weighted, and matrix metering (presumably 3D color matrix with this AF D lens), this time with the slight variation of basically of adding about 4" of pencil (maybe 2% to 3% of frame) at frame left to focus on, off-white wall as basic background. Metering was about the same: exposure compensation of 0 resulted in an average value of approximately 180, rather than approximately 127 as I would expect for a mid-toned rendering. It was aperture priority with a setting of f/5.6 or so (2 stops down from wide open).

 

I also tested same combination at 0, -.3, -.7, -1.0 exposure compensation, lawn grass in sun, matrix metering. The most-balanced exposure, according to the histogram, was -.7 or even -1.0.

 

Well, maybe this D200 just has a meter calibration situation leading it to seek to overexpose. If that's consistent, it's no great problem to exposure compensate. I really wonder, though if it will be consistent.

 

Any further insights or corroborating experiences with this model's type of metering?