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I tried reading this thread about histograms, but couldn't understand it well enough to answer this question.

I took some shots yesterday where the main peak of my Histogram was actually off the chart. The histogram had one large peak and most everything else was very low. What exactly does this mean, and if it's something bad (I expect that it is) how can I avoid this in the future?

Here is the Histogram in question :

and the image can be found here. (Update: I altered the photo, so the link is taking into account Jrista's excellent suggestions. This histogram no longer corresponds to this photo.)

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1  
a warning for arachnophobes: you may not want to look at the original image (; –  drfrogsplat Sep 1 '10 at 7:14
    
@drfrogsplat Lol, I hadn't considered that issue, heh. –  BBischof Sep 2 '10 at 15:28

4 Answers 4

up vote 10 down vote accepted

The height of each individual, vertical line in a histogram indicates how much of that particular tone your image has, or the tones intensity. As tones in a histogram progress from left (black and shades) to right (brights and highlights), where a particular vertical line lies indicates how intense the tones represented by that line are.

If one particular tone is extremely intense, and the rest are very low, then the image is primarily comprised of that (or those) particular tones. Generally, this happens in the highlights when they get blown out. It it happens somewhere else, then the histogram is simply telling you that your image is almost entirely composed of that particular range of tones.

It should be noted that the histogram generally does not show enough vertical lines to represent the entire tonal range of an image. Every individual vertical line in a histogram usually represents a small range of tones.

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5  
Good explanation. I'm thinking it's probably worth pointing out explicitly to the OP that a peak in the histogram can't actually be off the top of the chart. Off to the right for blown highlights, or off to the left for clipped shadows, sure, but a peak in the middle cannot be "off the chart". Just further reassurance that it doesn't sound like s/he has lost any data from these particular images. –  Conor Boyd Aug 29 '10 at 22:54
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@BBishof: Technically speaking, a histogram stretches infinitely vertically, so as Conor stated, you really can't be "off the chart" from a line height perspective. You can be "blown", which means your highlights cover tones beyond the right edge of the histogram, or "blocked", which means your shades and blacks are shifted beyond the left edge of the histogram. Depending on how many pixels are in an image, any particular part of the histogram may reach a hight beyond what is normally represented, but that doesn't mean anything specifically. In your histogram, you have a high-key image. –  jrista Aug 30 '10 at 0:47
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Based on your shot, it looks quite under exposed. The fact that the bulk of your tones are bunched up in one fairly narrow spike is also an indication of why your image appears to have such low contrast. I would expect that there be a spike around the highlights area, as about 3/4 of the scene is "white", but I would also expect that other tones would be more exposed. –  jrista Aug 30 '10 at 17:11
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If you encounter a similar scenario, I would try to find a way to light the scene better. If you have to grab a nearby lamp and rip off the shade to illuminate the scene, I would (I actually do that myself, particularly when trying to photograph insects indoors.) Best to avoid flash with insects (unless you have the right kind or a diffuser.) You could just mess with exposure, however the ultimate end result is most likely to be that you brighten everything, but end up with the same histogram...and same low contrast...only more to the right. Light is your best friend. –  jrista Aug 31 '10 at 3:42
1  
White balance can be adjusted, so the yellow light (probably incandescent) can be corrected. Digital cameras can do some correction...the "auto" white balance setting should take care of a lot of that. If the camera is insufficient at correcting color casts, post-processing software like Lightroom can usually correct it. The two controls, color temperature and tint, generally cover the whole spectrum. Just use the "neutral balance selector" tool, and click on an area that was a neutral white or gray in reality. Lightroom should then automatically remove any orange cast for you. –  jrista Aug 31 '10 at 4:40

You might read this histogram like this: histogram w/ description

There's nothing wrong with you image. (At most, this histogram says there are "unused" bright tones, so you could maybe expose the photo a bit more, to "move" the histogram a bit to the right -- e.g. use more you your dynamic range to get more of the spider's dark area. But moving to the right might get you overexpose if you add too much light. So it's tricky. And this photo is fine anyway.)

What might give you better idea about tones present is switching the histogram to logarithmic scale. That way you'll be able to see more of the differences between various darker tones.

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Thank you for this detail analysis, of my particular picture. –  BBischof Aug 30 '10 at 13:59

If the histogram shows everything bunched up at one end, then have either a very dark or very light image.

If the histogram is bunched at the right this means you have a very light image - most of the pixels have recorded lots of light, and lots of pixels will have the maximum value. This is often referred to as blown highlights.

While there are some occasions where having a very light image is a deliberate choice for artistic reasons, mostly it means that you have overexposed the image. Unless you are in a fully manual mode, you should check you haven't bumped the Exposure Compensation control off the middle, and if you have an image that is hard to meter accurately you should use the Exposure Compensation control to make a darker image. (Provided you notice when reviewing immediately rather than later). If you are in manual mode you could reduce the shutter speed or use a smaller aperture (higher f-stop number).

And vice versa. If the histogram is bunched at the left this means you have a very dark image - most of the pixels have recorded very little light, and lots of pixels will have recorded zero. The fix is the opposite.

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I am sorry my question was unclear before about the location of the fall-out. I have edited the question in an attempt to make it more clear. –  BBischof Aug 29 '10 at 23:08

It simply means there are lots of pixels in your photograph at that brightness level.

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