Middle gray not at the center of histogram


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LBL2009

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I wanted to find out where my camera puts middle grey on the histogram.

I don't have a 18% grey card, but I understand that camera will take the average of the metering and give us middle grey. If we shoot a single color subject, whether it is white or grey or black or any color, it will always come out with the tone of middle grey. And the histogram will show a peak at the center.

Well, not for my camera.

I filled the frame with the blue sky, again the cloud and then the off-white wall. I shot in M mode with spot metering. In all the shots, the peak is about one stop to the left. I have to increase the exposure by one stop to bring the peak to the center.

Have I missed out anything in my experiment or Nikon designed my D40 to underexpose by one stop? :dunno:
 

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osocan

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Just tried the experiment: found that the histogram does shift to the left but it was due to uneven illumination from on the wall. However when I shot OOF into a bright screen, the histogram was centred. I also heard that certain lenses consistently overexpose, eg the Voigtlander 58mm f/1.4. Maybe you can try with another lens?
 

Dream Merchant

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Nikon does not calibrate to 18% grey, but around 12%.
 

osocan

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Nikon does not calibrate to 18% grey, but around 12%.
Didnt know about that.

Would that affect the histogram display? or the whole histogram is shifted to reflect the calibration?

Thanks
 

LBL2009

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Just tried the experiment: found that the histogram does shift to the left but it was due to uneven illumination from on the wall. However when I shot OOF into a bright screen, the histogram was centred. I also heard that certain lenses consistently overexpose, eg the Voigtlander 58mm f/1.4. Maybe you can try with another lens?
I did it with my kit lens, will try another kit lens. I suppose it is the sensor and calibration which decide where to put the light on the histogram. Lens shouldn't play a part unless the lens has built-in data to instruct the camera to make an adjustment?

Nikon does not calibrate to 18% grey, but around 12%.
So, we should expect the peak slightly to the left for Nikon. The one stop to the left is too much in my camera. Have to try more to see whether the sun direction has anything to do with it.
 

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Dream Merchant

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The way a meter is calibrated would naturally affect the entire behavior and response of the metering, and that would in turn be reflected in the histograms.
 

Dream Merchant

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So, we should expect the peak slightly to the left for Nikon. The one stop to the left is too much in my camera. Have to try more to see whether the sun direction has anything to do with it.
Apparently. Ever notice how many Nikon users have their EC constantly set to +0.7?

And yes, even slight variations in metering angle can result in different readings. If you really want to check, meter something of known value, evenly lit, with the camera perpendicular to the test target.
 

LBL2009

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Apparently. Ever notice how many Nikon users have their EC constantly set to +0.7?

And yes, even slight variations in metering angle can result in different readings. If you really want to check, meter something of known value, evenly lit, with the camera perpendicular to the test target.
+0.7. Close to one stop.

It is 3.35pm in the afternoon. Sunny day, blue sky but with big clouds. Will find a fine day and shoot the blue sky with the sun behind, in-front, to the left and right and see what happens. :bsmilie:
 

Dream Merchant

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Have FUN, and DON'T WORRY! There's absolutely nothing wrong with your meter!



Here's why:

It seems that the rest of the world has literally adopted an automatic assumption that 18% REFLECTED grey is THE middle grey standard.

Camera meters are calibrated to ILLUMINANCE, instead of REFLECTANCE, if I understood correctly, and that's where all the confusion begins.

If you're into the physics of it ... you have to go back to metering basics (reflected/incident).

...I wanted to understand certain things about this.

In digging into the "American National Standard for General-Purpose Photographic Exposure Meters (Photoelectric Type)" ANSI PH3.49-1971 I have come across a very plausible explanation for this.

As we know, an illuminance of one footcandle falling on a perfectly reflective surface produces a surface luminance of one footlambert. Since the reflectance is diffuse, the footcandle and footlambert are related by a factor of pi.

So knowing that and that incident light is measured in footcandles while surface luminance is measured in footlamberts we can examine the equations in the ANSI standard:

2^EV = A^2/T = BS/K = IS/C

EV equals exposure value T = effective exposure time in seconds A = actual f-number of lens diaphragm S = ANSI speed of film (using the ASA series, not the old DIN degrees) B = field luminance in footlamberts K = exposure constant (reflected light) I = incident light in footcandles (illuminance) C = exposure constant (incident light)

Now, the ANSI standard gives the value of C as 30 +/- 5. If you use a value of 30, and run through the calculations based on an ISO 100 EV value, the illuminance comes close to matching a stand-alone Minolta illuminance meter (different receptor geometries). Also, if you work backwards and forwards with these numbers, I also confirmed the Sunny-16 rule, although as one might expect in SoCal in the middle of summer that it was closer to Sunny 16.5.

Interestingly, using the value of 30 for C rather than the implied value of 27 in the Minolta book provides closer correlation to the Minolta illuminance meter that we have at work.

Then after some more math in the standard, the value of K is derived as 3.64.

After all is said and done, all we need to do is look at the ratio of K/C which is 3.64/30 or 12.3%. That is where the 12-13% comes from. It is NOT 18%. Using the 18% gray card as a metering reference will cause approximately 1/2 stop underexposure as the reflected light meter is assuming 12%.

Hope this helps!
Here's more to 'clear up the mind' over the weekend'!

Ref:

http://www.richardhess.com/photo/18no.htm
http://www.bythom.com/graycards.htm


If that's all too confusing, just think of the issues regarding discussion about color temperature.

The warmer the color temperature, the cooler the color of the light. Saying the the deep orange color cast of say a tungsten bulb is 'very warm', and that the deep blue of twilight looks 'very cool' is a direct contradiction to the physics. LOL!
 

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LBL2009

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#10
Thanks.

It is more practical to find a working reference (the usual common subject) and know how our camera processes it.

You are right about high temperature is cooler and low temperature is warmer. To learn to use a camera, we have to think in opposite direction because the camera does thing in reverse.

White balance is only one example. :bsmilie:
 

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