Histograms???
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Old 06-15-2007   #1
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Default Histograms???

Hey People,

I have been messing around with my histogram on my camera. It took awile to get a hang of it...but now that I know how to use it....my photos have seem to improve.

My first goal was to learn how to move the data on the graph around a bit. When I first started, I noticed that I had a tendency to underexpose a bit. I think I got that part under control.

However, I have noticed, I tend to peak my midtones off the chart....

I could't find a picture of what I mean, but if you look at my attachment and imagine the "peak of the mountain" going off the chart...that's what I am doing now.....

Is that bad?

Pauly

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Old 06-15-2007   #2
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Default Re: Histograms???

Well, whether it's bad or not is really a matter of question. How does the image look?

The Y axis shows the number of pixels and the X axis shows their brightness... the lightest are on the right., darkest on the left. When you put all the pixel information together on a graph, you get the histogram showing what tonal values are present in the image.

Think of all the possible brightness and darkness tones possible in an image and then recognize that your camera's sensor, no matter how good, can't capture all of them. When the histogram goes "off the scale" at the top (meaning extreme right), it means that there are color or data characteristics that you have been unable to capture. The lighter and darker, or problematic characteristics of the image can be seen in whether the histogram is bunched up on the left or right, depending on what you are trying to achieve, such as a white on white shot... naturally... such a shot will have it's histogram looking a tad different.

(This could turn into a discussion on the dynamic range of a sensor)

After you "tweak" an image, you probably note that the histogram shifts and there are little gaps. This is missing information because you've "compressed" your values between color gradations and the software doesn't have the color or pixel information to fill in. So it is indicative of data lost through manipulation of the image. Such gaps in data are not normal when you simply capture an image.

This is one of the problems with repeated manipulation of jpg images. Such is why manipulation of any image, in photoshop, should be done in layers so as not to destroy the original. You can always go back to the original in case you get a little over-enthusiastic with manipulation of an image. Also, always save manipulated images to another file name. Never over write an original.
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Old 06-15-2007   #3
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Default Re: Histograms???

I kind of get it....

So then, if I got to much in the middle, that means that I have too much of that pixle(s).

My histogram is indicating that (again...for a fictional "good photo") my shadows are ok and my highlight seem to be ok too...but my midtones are "blown out". Dose that sound right?
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Old 06-15-2007   #4
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Doesn't mean they are blown out... off the left or right means the sensor didn't record them

I wanted to clarify this point......

The image could actually look fine.
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Old 06-15-2007   #5
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Default Re: Histograms???

Well, your midtones are not "blown-out" as you say. There is just more "color" there than the camera can grab.

I wouldn't worry about it actually. If the histogram has "a peak" on the left or right that is worse than a off the chart peak in the middle.
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Old 06-15-2007   #6
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Default Re: Histograms???

I have read through this post and am a little confused on histograms...now. It was my understanding that the historgram was not a static graph. The top line is determined by the height of the pixels with the greatest number of occurrence. For example, if a midtone had 130 pixels, and that was the highest pixel count for that image, then the top of the histogram would read 130 pixels. If the highest pixel count was 580 then the top of the histogram would be at 580.

Theoretically, if you take a picture of a black, white and gray card, you would have three single spikes, all reaching the top of the histogram. None of them would be blownout, they would just have an equal number of pixels, given the area of each section of the card is the same. A photographer at our school showed me the histogram of his black/white/gray card, and it was exactly as described.

Correct me if I am wrong, but I have heard this explanation from a couple of sources. Tonal quality is determined not so much by the height of the pixels, but by the overall position of the "hump" for lack of a better term.
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Old 06-15-2007   #7
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Default Re: Histograms???

Maybe I should clarify....

A histogram is made up of lines. Each of these represents the number of pixels the image contains for each of the 256 values that comprise the tonal range. In color, the basic histogram is a composite of red, green and blue histograms. So, yes Kelly, that is correct.

Maybe I went a little deeper than I should have in the explanation above as I started getting into explaining how the histogram boils into curves and how the curve can compress gradations of color variation in an image via manipulation.
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Old 06-15-2007   #8
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Default Re: Histograms???

I was just concerned about Pauly trying to get his histograms to read where there wouldn't be a tone touching the top line, and it is my understanding, that regardless of what you shoot, at least one column of pixels will abe touching the top line. I think your explanation was very good information for anybody using photoshop to understand. I don't know if people less familar with PS are aware that manipulating images over and over begins to degrade them. I was just teaching my daughter today the value of layers.

Now if only those radio triggers would get here so I can get out and test that Metz flash.
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Old 06-15-2007   #9
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Default Re: Histograms???

Quote:
Originally Posted by Nikonfreak View Post
Doesn't mean they are blown out, it means that they were outside the range of your sensor to record them.

The image could actually look fine.
Not true. If the middle is maxed to the top, this has nothing to do with blowing/blocking shadows or highlights, nor does it have anything to do with being outside a sensor's dynamic range. This just means you have lots and lots of pixels at that tonal range.

Having a mountain shaped histogram is not bad, not good either. It just is, and I would NOT go for a particular shape. What you should be worrying more about is the far left and right of the histogram. Not saying the midtones should be ignored, but really it's the extremes.

As to the height of the histo, I would NOT ever worry about that, or at least it should come only secondary to the right/left extremes.
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Old 06-15-2007   #10
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Default Re: Histograms???

So if they're not on the histogram, you believe the sensor recorded them? Note, I didn't say anything was blown. I merely stated that they aren't recorded by the sensor.

Interesting.


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