How can you tell if an image is blown out?

Discussion in 'Beyond the Basics' started by zedin, Jul 24, 2006.

  1. zedin

    zedin TPF Supporters Supporting Member

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    So I have been trying to get better and have things right in camera and not have to do any RAW tweaking. The problem is that from what I have read the d70s' histogram is only single channel (green). So while it might say everything is fine and not blown out in reality red and blue could be completely blown out. Since the d70s does not have all the channels in the histogram how can you tell if the image is blown? I have tried looking at the actual picture but really how much can you tell on the little LCD? Is there a general rule you can follow to preserve the colors? Like assume any one color won't be more then 1 stop more then another color nad just underexpose everything 1 stop and fix it with a level/curve adjustment in PS?
     
  2. Arch

    Arch Damn You! Staff Member Supporting Member

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    I'v not heard about the D70s having that issue...... and i think you can still be sure that if the far right hand side of the histogram has some very high peaks you've more than likely blown an area.
    I slightly underexpose for most things anyway..... a blown area can ruin an image..... but you can always bring to life a slightly underexposed one.
     
  3. dsp921

    dsp921 TPF Noob!

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    When viewing the image, hit the right multi-selector button a few times, I forget extactly how many, but after the info screens and the histogram there is a highlight screen, the blown highlights will flash.
     
  4. ksmattfish

    ksmattfish Now 100% DC - not as cool as I once was, but still

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    No general rule. Just make sure you aren't too close to the right edge of the histogram, and when you buy your next DSLR it will probably have a full RGB histogram.

    In the future RAW software may be able to extract more info from files that seem blown out today.
     
  5. markc

    markc TPF Noob!

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    Something to remember is that the histogram is showing you the typical 2.2 gamma adjustment applied to the 1.0 gamma RAW data. Just because the histogram or flashing pixels (or highlighted pixels in the case of Canon) show a blown area, that doesn't mean the core data is truly blown, just the results of that adjustment. Applying a different gamma curve at the time of converstion by playing with the exposure and shadow sliders you can rescue some of that data.

    If the image is so bright that it's blown in the actual RAW data, there's no way to rescue what's really there, now or in the future. Software might be able to guess at it, but the distinction between pixels has been already lost.
     
  6. ksmattfish

    ksmattfish Now 100% DC - not as cool as I once was, but still

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    If you go to, or past, the level of 100% white you are correct; the info is gone. But a 12 bit (they call it 16 bit, but it's really 12) raw file has 2048 potential steps in the stop at the highlight end. That means 2047 are not true white. Currently available software is not written to be able to make 2048 levels of tone distinction in a single stop, and current monitors probably couldn't show it anyway; you may get solid white even if you really have managed to expose it a few dozen levels short of the right side of the histogram, but not actually gone into that last step of 100% white (or over). Future technology will allow us more precise control over these borderline tones. The information exists in the raw file, but our current software can't make such fine distinctions.
     
  7. markc

    markc TPF Noob!

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    That's been my usage of the word "blown". It's an area that's gone to pure white, not just a bright area.

    Agreed. The whole range a 12-bit file can cover is 4096 steps per color. I wouldn't consider the top half the highlights, though. It all depends on how the image is exposed. (--edit-- Since the top stop is 2048 levels, it does make sense that it is considered the "highlights". I'm not sure why I wrote that. You were talking about a linear RAW file, not a TIFF.)

    At first I was going to say that I didn't follow what you meant, but then I read this: http://luminous-landscape.com/tutorials/expose-right.shtml

    Photoshop can do this, we just need to learn how to master it. It's the conversion software that I think is a bit limited.

    The software doesn't care what it looks like to us. The file is just a bunch of numbers. The camera will have a certain number of stops that are covered by those 4096 steps, but Photoshop isn't limited by that. As long as there is a distinction between two pixels (like 4096,4096,4096 vs. 4096,4096,4095), I can can change their relationship between them. And the difference between 0,0,0 and 10,10,10 could be 1/1000 of a stop or 1000 stops. The image won't look much like what we see with our eye, though. I guess most digital cameras capture about 5-6 stops effective levels.

    From what I've read, RAW conversion software is basically just applying a logarithmic curve adjustment to the linear 1.0 gamma data so that the image is now at gamma 2.2, plus or minus any adjustments. You could do this manually if you wanted.

    I completely agree with you that there can be more data in the RAW file than we see when we run the conversion, but that's only if the conversion blows out an area.

    One of the misunderstandings with shooting RAW is the idea of underexposing. Since RAW is linear, each stop has half the info that the one above it has.

    Stop 0: 2048 levels
    Stop -1: 1024 levels
    Stop -2: 512 levels
    Stop -3: 256 levels
    Stop -4: 128 levels
    Stop -5: 64 levels
    Stop -6: 32 levels
    Stop -7: 16 levels
    Stop -8: 8 levels
    Stop -9: 4 levels
    Stop -10: 2 levels

    So if you underexpose so that you don't use that top stop, you instantly lose half of the info that your file can hold. The shadows don't hold much distinction. Stops -4 through -10 hold less info (254 levels total) than stop -3 does. By overexposing as much as possible without blowing, you get as much gradient detail as possible. This can really open up the shadow detail. Instead of having it all blocked up in, say, 128 levels, you could have it spread across 510 levels.
     
  8. Luke

    Luke TPF Noob!

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    there's a simple way to do it with out a histogram:
    Have a basic understanding of light, and what you're shooting.
    Sounds kooky, i know, but most photographers actually like to understand light.
    Sorry for dissing, and i know this will prompt a reaction from many.
    But really, did weegee worry bout ****ing histograms?
     
  9. hobbes28

    hobbes28 Incredible Supporting Member

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    I understand your frustration with people not knowing it all but that is the reason a lot of people are here... to learn. ;)
     
  10. Luke

    Luke TPF Noob!

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    First of all, what I said was rude, sorry guys.
    But, I'm not frustrated with people not knowing, I just find it so naive to ask 'how do i know if my highlights aer blown out?' when the answer is 'look at the friggin picture'.
    In short ill say this, learn to understand what your actually photographing, and how your camera reacts, and you dont need histograms, you don't even need a meter, all you need is the little aperture and shutter changer. If you look at pros, they aint worrying bout histograms, they know, they just know that theyre spot on, or if they screw up, they know.
    And soon you'll see, i insult you guys because i LOVE you guys. ;)
     
  11. ksmattfish

    ksmattfish Now 100% DC - not as cool as I once was, but still

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    Nope, but then again, he was known for being in the right place at the right time, not his darkroom work. His off-the-clock work (and there was a lot of it) is virtually unknown.

    Plenty of other film photographers worried, and still worry about the toe and the shoulder are of the characteristic curve of the particular film they are using. If you understand the histogram then you understand it's just a way of expressing luminosity, in other words light. The histogram is just another tool, really an extremely simple one, and some of us use it to our advantage. Yeah, I could wait until I got home and made a print to see that I've exposed it too far to the right, but that defeats the purpose of digital for me. I don't know what camera you are using, but the LCD on my 20D is fairly unreliable for assessing subtle differences in tone. On the other hand, the histogram does an excellent job, and it takes me about a second to interpret it.

    It's no different than when I'm using the zone system with film. I assess the abilities and limitation of the materials and equipment I'm working with, and apply my knowledge to make the best print I can.

    Do you have to do things the same way I do? No, you don't. There are many paths to get to the end result, and you have to travel the one you like.
     
  12. ksmattfish

    ksmattfish Now 100% DC - not as cool as I once was, but still

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    If I had a quarter for every naive question that gets asked here I'd be a rich man after just a few days. I think that's sort of the point of many internet photo-forums.

    Here's the most naive question of all "Which is better? Canon or Nikon?" But it gets asked everyday.
     

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