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Checking for exposure accuracy

Discussion in 'General Technical Discussion' started by hillrg, Jul 4, 2005.

  1. I am looking for help, trying to determine whether I have an exposure problem with my D2X. I took the photo of a kodak grey card and another calibration card from photobox.

    Subscribe to see EXIF info for this image (if available)

    I also have the companion TIF file for the photobox calibration image. The areas the arrows point to are supposed to be 128, 128, 128 RGB.

    I did a WB preset and I spot metered on the grey card, shooting in JPG fine. In the sun, metering off the grey card matched the incident reading from my sekonic light meter. In the shade the sekonic was metering one stop slower than the D2X spot meter. I had to add +.67 EV to get an RGB of 114, 117, 124. At +1 EV the RGB was 133, 133, 141.

    The blue value was consistently high even with the WB preset, both in the shade and sun. I have also read that the nikon metering is calibrated to a 12/13% grey, not the 18% kodak - is this true?

    Any suggestions? How do you test your cameras?
  2. Commodorefirst

    Commodorefirst Admin/Moderator Administrator

    May 1, 2005
    Do you have to fill the viewfinder with the grey card on your D2x when doing a preset adjustment? I know you don' thave to when shooting raw, you can just meter on the smaller portion on your card, but i thought that whne doing a measure in JPG you needed to fill the viewfinder.

    At least that is how it works on my D70


  3. Yes, I filled the viewfinder. I also tried setting the Kelvin to 5900, my estimate of the lighting, which turned out to be almost the same as the preset.
  4. Okay, I've figured it out. The kodak card is 18% gray, which corresponds to an RGB of [128,128,128]. Nikon metering is calibrated to a 12% gray, which is approximately 0.5 EV, or 1/2 stop darker than the gray card, or an RGB of about [102,102,102].

    Therefore, you have to open up by 0.5 EV from the reading you get on a gray card.

    The net result is that my D2X is still under-exposing by about 1/3 stop. For now I have adjusted custom setting b7 (fine tune exposure) by +2/6.

    I confirmed this by taking a frame filling photo of the gray card, which resulted in an in-camera histogram with the spike right in the middle. I think the histogram is slightly asymmetrical due to uneven lighting from a halogen lamp I used.

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    Hopefully this will offset my natural conservative exposures.

    I would be interested in hearing how other nikon bodies expose, if anyone else is interested in trying this.
  5. Commodorefirst

    Commodorefirst Admin/Moderator Administrator

    May 1, 2005
    So how did you find out that nikon meters at 12%? this is the first I have heard about this.

    Interesting, I wonder if different cameras are different?

    thanks for the info.



    Ok my historgram seemed more balanced than yours when I shot a whibal card full frame and checked out the histogram on the cpu. D70 (hmmm...)
  6. MontyDog


    Jan 30, 2005
    #1064 - You have an error in your SQL syntax;
  7. biggstr6


    Apr 26, 2005
    For what its worth , my D2x underexposes alittle compared to what I was used to. I checked it with a Kodak grey card and it was a little underexposed too.

    Im considering adjusting b7 so I can continue shooting the way im used to instead of adjusting the way I shoot. Not sure yet.
  8. Iliah


    Jan 29, 2005
    Dear Rory,
    No. 50% gray corresponds to 117 in gamma = 2.2 space; and above that, resulting image is not gamma = 2.2.

    Please shoot the gray card varying the aperture, and lets see if your aperture is properly calibrated. Of course, light should be consistent, but tripod, setting camera to shutter priority and bracketing to full stop, shooting a series of 5 frames should be close enough.
  9. Hi Iliah - thanks for the help - much appreciated. The following histograms were generated shooting with the 105/2.8 micro after doing a WB preset in a dark room with only a bright halogen light source which remained consistant throughout the photo series. I metered the gray card at f/8 and then shot a series from f/11 - f/5.6 in manual exposure mode. Of course, I reset b7 to zero.

    I generated the histograms in NC 4.3.0. BTW, I ran the series twice, and both times I got the same histogram jump between f/6.3 and f/5.6.

    Paul - yes I read Thom's article. I got the RGB 102 value from Ron Reznick's ebook.

    Subscribe to see EXIF info for this image (if available)

    So, what does this mean?
  10. Iliah


    Jan 29, 2005
    Dear Rory,

    Can you just post converted pieces of gray card images, 100x100 crops done in Nikon Capture, JPEG Excellent quality, sharpening set to off? Better in Adobe RGB, if possible, as sRGB is not a constant gamma space. Or you can watch info palette in NC at 33% zoom level and post the numbers (RGB+average)

    Jump of brightness between aperture values can be for many reasons -

    - weired custom curve (shoot with "normal" for this test);

    - weired Capture default gamma correction curve (tonal curves in RAW converters do not conform to simple gamma correction, instead LUTs or complicated formulas are in use);

    - problems with lens and/or camera - less likely.

    To be sure, NEFs (compressed HSC are OK) are needed.
  11. I am afraid I picked up every little detail on the card. Maybe I should back off or defocus to smooth things out.

    Subscribe to see EXIF info for this image (if available)
  12. Iliah - I followed your advice. I shot the gray card from a distance after getting a spot meter exposure. I shot a NEF with the tone set to normal. The average swatch was 116 - exactly the same as Paul!

    My previous exposures were all using the auto tone curve - that must have been what was screwing me up.

    Thanks very much for the help!
  13. Iliah


    Jan 29, 2005
    Dear Rory,

    Are there any jumps in brightness between neighboring aparture settings? Have you used eV compensation to get the value of 116?
  14. Iliah

    I have no EV compensation to get 116. Also, after setting the tone to normal (from auto), there were no jumps between neighbouring apertures.
  15. Iliah


    Jan 29, 2005
    Dear Rory,

    So, all looks good now :) 
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