IR still bites on D2X

Discussion in 'General Technical Discussion' started by Iliah, Aug 26, 2005.

  1. Iliah

    Iliah

    Jan 29, 2005
    nowhere
    This photo is taken at the altitude of about 2000'. Relativly sunny day, 2 hours before sunset. The shirt is near black in reality.
    [​IMG]
     
  2. TOF guy

    TOF guy

    208
    Mar 11, 2005
    The shirt is blue-ish rather than magenta-ish as I'd expect with the D70. This would mean that the color filter in front of the CMOS is such that the IR response in the D2X comes mostly from the blue pixels. But the others - and it particular the red filters - which one would expect to be the most transparent to IR because of the wavelength proximity - cut IR well in the D2X.

    Iliah, have you done a shot of a IR remote ? What does it look like ?

    Thierry
     
  3. I just use the eye dropper in post processing to drive it to black if I feel it is important for it to be so. Of course as you know, the more light that hits the shirt the lighter it will become (if the EV remains constant). I would say that the shirt leans a little toward navy blue in your image.
     
  4. cwilt

    cwilt

    Apr 24, 2005
    Denver, CO
    Any way you can do a test with and without a B&W filter to show the difference?

    I have one on order but don't have it yet.
     
  5. Iliah

    Iliah

    Jan 29, 2005
    nowhere
    Dear Gordon,

    Of course it is not a problem to post-process the shirt to its original colour. It is a "solid piece" and "solid tone", any selection is easy. But - there are at least two other problems, associated with IR: more noise in darker colours, and that other tones are also affected with IR.

    If face is not overexposed, shirt also should not be IMHO. I agree it is closer to navy blue on this image.
     
  6. Hi Iliah, your exposure on this image is great and I was not implying otherwise. The face; however, is not in direct sunlight and the shoulder of the shirt is. My only point was that you can drive a black object (shirt) to near white with enough light. I guess I was wondering why you are making the point about IR being a culprit in the color of the shirt as a novice like me certainly could not come to that conclusion. Based on past experience I would guess that your comment was right on but you couldn't prove it by me. :smile:
     
    Last edited by a moderator: Aug 27, 2005
  7. Iliah

    Iliah

    Jan 29, 2005
    nowhere
    Dear Gordon,

    I agree with your comments re black turning white under good light :)

    When I'll get home, I'll take colorimeric measurements of the shirt :) As you know, L value may vary with the amount of light, but hue ("a" and "b" values) should stay.
     
  8. heiko

    heiko

    May 15, 2005
    Israel
    Hi Iliah,

    What happens if you WB for a black shirt?

    I just happened to be in the alps recently and luckily I did a WB preset with my grey card. I was at an altitude of 2800m (nearly 10,000 feet), sunny weather and snow in the background. When post-processing some pictures I tried to set the WB to sunny - that was way off and the whole picture took a blueish color (saw the same problem on film many years ago). Anyway, with correct WB the pictures look just fine, except perhaps for some little noise in the dark areas. The camera was D70 with 18-70 DX with UV filter and Sigma 24mm and Nikkor 70-300 w/o filters. I have just started processing the pics, so no conclusions re noise. Can't say anything about D2X, except that I wouldn't mind getting it as a present.

    Your picture looks perfectly exposed and I wouldn't know that the blue shirt is black if you hadn't said so. By the way, I WB the picture in PS so to get a black/grey shirt. Only then I noticed that the skin color around the nose is slightly blueish, which disappears when you WB on the shirt.

    Please correct me if I'm totally wrong here, but could this simply be a WB issue?
     
  9. Iliah

    Iliah

    Jan 29, 2005
    nowhere
    Dear Heiko, hues of hair, jeans, skin amd bag are OK; bag is slightly less yellow. The shirt is actually very dark grey with just a slight blue tint. When part of the face is in shadow, it is often to have shift to cyan.
     
  10. heiko

    heiko

    May 15, 2005
    Israel
    Hi Iliah,

    Thanks for your response. I should have known better and trusted your wits :redface: . Anyway, this seems to be my way of learning a thing or two :smile: .

    If I understand you correctly, the black/dark grey shirt became blueish due to IR noise affecting the dark parts of the scene - a problem of the filter. Does this affect other parts of the picture as well (meaning not the dark/black parts) and cause a shift in color? Can it be avoided by using a lens filter?
     
  11. Iliah

    Iliah

    Jan 29, 2005
    nowhere
    Dear Heiko,

    B+W and Heliopan both have reliable "hot mirror" filters that cut that IR contamination off. Those filters are expensive and usually on backorder, so I use them only when shooting assignments, not for myself. IR afects lot of tones, including skin tones and green foliage/grass. IR also adds to noise in red and blue channels, the kind of noise/blotches mostly visible in shadows.
     
  12. heiko

    heiko

    May 15, 2005
    Israel
    Thanks, Iliah!
     
  13. My experience shooting clothing tells me that it is not IR
    contamination, but it is the type of dyes used in the shirt
    that are throwing the colour off.
    Cheers,
    Robin
     
  14. JeffKohn

    JeffKohn

    Apr 21, 2005
    Houston, TX
    But if it were metamerism wouldn't it also be visible to the human eye? If the shirt looks black in person but blue in the image, then it seems I don't understand how that could be caused by metamerism.
     
  15. Jeff,
    I can't really explain it except to say that some materials
    and their dyes don't reproduce well. Usually if you are going
    to see this it happens to greens, and no you cannot know
    ahead of time whether a material will shoot well or not
    by just looking at it.

    In my experience

    Cheers,
    Robin
     
  16. MontyDog

    MontyDog

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

    Iliah

    Jan 29, 2005
    nowhere
    Dear Paul,

    I would say D2X is much less prone to IR then D2H. Still sometimes I see poor greens, troubled shadows, and the positive effect of hot mirror filter on exposures with flash.
     
  18. cwilt

    cwilt

    Apr 24, 2005
    Denver, CO
    What about UV? Is the D2x as sensitive as the D70 to UV? From looking at EXIF data of IR shots I'm guessing the D2x is 2-3 stops less sensitive to IR than the D70 is.


    Still waiting on my #486 filter. Testing at 6000 feet could be interesting.
     
  19. Iliah

    Iliah

    Jan 29, 2005
    nowhere
    Dear Charles,

    Exposure of pure IR shots may be not so good indicator, as contamination on "normal" shots occurs due to near-IR.

    UV, as Bjorn explained, plays important role in D2X exposures. He suggested a rule of thumb to use UV filter where one can get a sunburn.
     
  20. cwilt

    cwilt

    Apr 24, 2005
    Denver, CO
    That is anytime the sun is up. :eek:
     
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