Using the "blinkies" to judge clipping

Discussion in 'Digital Photography' started by kallisti, Jan 22, 2010.

  1. kallisti macrumors 65816

    kallisti

    Joined:
    Apr 22, 2003
    #1
    I assume many know this, but wanted to post about it "just in case."

    With many (most?) modern digital cameras, it is possible to review the image after it is taken and either look at histograms or highlight warnings (the "blinkies") to help judge whether you achieved a good exposure on the shot.

    If using histograms, you generally look to make sure that you aren't too far to the right. If using "blinkies," you make sure that nothing important to the shot is blinking on your LCD.

    The caveat to both of these methods is that the "generic" setup (the white histogram or the RGB highlight warning) on many cameras only displays information from the green color channel. It ignores data from the red and blue channels. This is true for most point-and-shoots and is the default setting on Nikon bodies (can't comment on Canon). This is counter-intuative--one would think that the composite channel (or the RGB channel for cameras capable of displaying histograms for all three colors) would display a summation of clipping information obtained from all 3 color channels. This isn't the case. Instead only the green channel is displayed for reasons that escape me.

    This is actually a fairly major design flaw, the importance of which depends on the colors present in your shots. For general shooting it often works. But you can get seriously screwed if the predominant color in your composition is red or blue, or if your composition has important highlights that are either red or blue. Relying on the RGB histogram or the presence of "blinkies" can create serious exposure problems in these situations. I have personally found this to be an issue when shooting red or blue flowers. The RGB (white) histogram looks fine and there are no blinkies on the LCD. I get back to my computer and find out that the red channel is completely blown out and the images are worthless.

    This is easy to test out (assuming you have a body capable of displaying either histograms for each color and/or highlight clipping for each color ("blinkies")). Overexpose a shot of a red subject. Examine your "white" histogram (what should be the composite of the red, green, blue channels). Compare it to the histograms for each individual color. Or look at the "blinkies" for the RGB channel and then switch your LCD to display the R channel. Lot's more will be blinking now. The same experiment can be done with blue subjects.

    Just wanted people to be aware of this. There is nothing worse than being on vacation and taking a picture of something where red or blue are important colors, the RGB histogram indicates a proper exposure (or you don't see any "blinkies" suggesting clipping) only to find out when you view it on your computer that you lost critical detail because the red or blue channel was actually overexposed and blown out, resulting in unusable images.
     
  2. Edge100 macrumors 68000

    Joined:
    May 14, 2002
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    #2
    Not sure about other cameras, but my 1DmkII allows the user to review the image with an RGB histogram or a luminance histogram. Not sure which channel is used for the luminance histogram, but by evaluating the 3 colour histogram, I can quickly judge overexposure.

    Also, given the noise performance of modern sensors, and given that most new DSLRs have 14-bit sensors, the need to "use the bits", or to expose to the right as it's called, is less and less important. 14-bit sensors (16,384 levels) allow you to underexpose by two stops and still have the same S/N as a 12-bit sensor (4,096 levels), all other things being equal.

    The moral of the story: back off a bit from overexposure. Adding exposure in PP is ok in most circumstances (unless you're at ISO3200 and are 2 stops underexposed, for instance). Noise I can deal with; clipped highlights I can't.
     
  3. Phrasikleia macrumors 601

    Phrasikleia

    Joined:
    Feb 24, 2008
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    #3
    The other major caveat is that both the histogram and the clipping warnings ("blinkies" as you call them) are relating info from your JPEG preview, not from the raw data. So it may tell you you've clipped something when in actuality you haven't. This misinformation can cause you to make bad exposure decisions, usually resulting in underexposure and noisy shadows.

    The only way to get an accurate histogram or "blinkies" is to use UniWB. No cameras yet support representation of raw data on their own.
     
  4. John.B macrumors 601

    John.B

    Joined:
    Jan 15, 2008
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    #4
    On my Canons I can choose to look at a composite histogram, or the three individual channels on one screen. I can evaluate that on the same screen that shows the "blinkies". Not sure how Nikon handles that, I won't know until the D900 ships.

    I've only recently become aware of the (somewhat controversial) ETTR phenomena, where moderate overexposure is completely recoverable in post (assuming a raw workflow, of course) and allows you to maximize dynamic range in a way that the on-camera histograms may not accurately reflect.

    It never ceases to amaze me how much I don't know about photography and the digital arts. :)
     
  5. Edge100 macrumors 68000

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    #5
    Could it be that RAW capture has an advantage over JPEG capture? We should start a thread about that, or something...;)
     
  6. Phrasikleia macrumors 601

    Phrasikleia

    Joined:
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    #6
    I'm a big proponent of ETTR. That's my MO these days, but I only do it in combination with UniWB.

    LOL. :p Yeah, completely forgot to mention this particular benefit in that thread.
     

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