Metering, some myths explored

Wilt wrote in post #18106404
I simply put a white piece of paper under uniform illumination and shot it in Av mode at ISO 100. Then I shot an 18% gray card under the same uniform illumination and shot it in Av mode at ISO 100. Both stored as JPG.

I took both the above JPGs, and read them into LR and also into Paintshop Pro. I used the respective white balance eyedroppers to neutralize the image (since it was not shot in daylight). Then I used each eyedropper to display numeric values under the sample point.

• LR sampler measures on a scale of 0% -100% in tenths of percent, a separate value for R, G, B; 'middle' is 50%
  
• PSP sampler measures on a scale of 0 - 255, a separate value for R, G, B; 'middle' is 127 or 128
  

That is the procedure I used. No adjustments except White Balance.

Something is wrong with your procedure. RGB of 137 translates into 25.5% brightness or -2 stops, which doesn't match up to any previous results.

Would you mind sharing these jpegs?

Frugivore, just PM your email address, and I will send both to you.

Wilt wrote in post #18106477
Frugivore, just PM your email address, and I will send both to you.

I used the color picker tool and found the white paper to be uniform throughout, but the gray card had a slightly magenta tint in two corners. I would expect the spikes in both histograms to be in the same location, but they're not. Maybe your light was not uniform for the gray card? Did you use spot metering instead of evaluative?

And this was done with the 40D? I thought I had done the same test with my 40D years ago, but I can't locate the images now. I am wondering if the metering middle point was changed by Canon over the years. Did Canon find that more people were shooting outdoors and they lowered the middle point to account for the wider dynamic range? That is my guess.

In regard to my tests, it just occurred to me that because I used flash to illuminate my white paper instead of ambient, that NEVEC reduced the exposure. I must try this again during the day with only ambient to see if there's a difference.

Frugivore,

My camera was on Spot metering, but with Evaluative I get EXACTLY the same reading.
And with my Minolta Autometer Vf incident meter, I get EXACTLY the same reading, too.
(My Minolta 1 degree spotmeter could add +0.33EV to the above two readings, make the histogram spikes even further to the right.)

.

Larry Johnson wrote in post #18104117
I won't pretend to understand everything in this thread, but I have noted that there seems to be a lack of threads discussing metering.

I can't understand any of it either.......it's like there's math involved or something. Definitely not what I would call "easy reading".

But that's ok, because I don't really rely on my camera's metering system anyway. I just take a shot of the scene, look at it on the LCD playback display, and then make whatever exposure adjustments are necessary. That takes the camera's metering system out of the loop altogether, which is just how I like it.

If I relied on the camera's metering system, then there would be times when I would get stuck with an exposure based on what the camera thinks. I want my exposures to be based on what I want, not on what the camera thinks is right.

.

Larry Johnson wrote in post #18104117
I won't pretend to understand everything in this thread, but I have noted that there seems to be a lack of threads discussing metering.

Tom Reichner wrote in post #18110163
I can't understand any of it either.......it's like there's math involved or something. Definitely not what I would call "easy reading".

Guys, it is NOT complex, nor involves ANY math. Let me try to help you with a simple summary of the first 3 posts...

POST 1:

• A. Tonal Scale from A to Z is shown, with the extremes (black, white) and 24 other intermediate steps. The Tone M is 'middle' between black and white, and is also '18% gray. IOW, 18% is the 'middle of the tonal scale, between black and white'
  
• B. 18% gray is NOT the 'target tonality used for metering properly' as many believe...or else the peak for 18% gray would be exactly in the middle of the histogram, rather than just to the left of center as shown!
  
• C. 18% gray IS indeed the 'target tonality used for metering properly', because the peak for 18% gray is precisely centered on the histogram shown.
  

• A. The above claim is disproven, with the target printed on glossy paper...the peaks for black and for white are NOT centered laterally on the histogram
  
• B. When metering is performed with an incident meter, with the target printed on glossy paper...the peaks for black and for white ARE centered laterally on the histogram
  
• C. The above claim is disproven, with the target printed on matte paper...the peaks for black and for white are NOT centered laterally on the histogram
  
• D. When metering is performed with an incident meter, with the target printed on matte paper...the peaks for black and for white ARE centered laterally on the histogram (just as it was centered with the target printed on glossy paper
  

Points A and C disprove the claim. It demonstrates that incident meters work properly, regardless of the target black and white being on glossy vs. matte paper. This also disproves any claims that 'incident meters were designed for film'...they work properly for digital cameras, too!
Points B and C also disprove statements that digital cameras have their meters set so that 'white does not clip' and fall off the histogram.

POST 3:

• An 18% tonality peak IS centered on the histogram, and is mid way between the peaks for 'black' and for 'white', corroborating point 1A....when properly exposed.
  

• Hand (back), disproven as useless
  
• Hand (palm), shown to be 'in the ballpark' substitutute know to be 'about +1EV (or a bit more) brighter than 18%
  ...but don't forget the real target for best accuracy is 12%, not 18%!
  
• Sky, disproven as useless
  
• Green grass/trees, not discussed in this thread but they are NOT a good surrogate target... useless
  
• Value of using sugar as a photo subject which helps in the retention of 'white with detail (not 'detailless white') when 'shooting to the right', proven