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 Author Topic: Another terrific article on Dynamic Range  (Read 1392 times)
Guest
 « on: July 16, 2003, 08:51:31 AM » Reply

Peter,

Left to right represents dark to light. Up and down represents quantity. To keep the scale within reason most displays will simply clip the top off a segment that is predominant in a scene.

The clipped top on the shot of the moon is the vast amount of really dark areas. The moon itself is the little ripplies on the right side of the histogram.

In summary...

Left is dark. Right is bright. Up is a lot. Down is a little.

It's easy.

Michael
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Guest
 « Reply #1 on: July 17, 2003, 07:44:02 AM » Reply

Dave,

Isn't that what I said?

Thanks for the elaboration. Sometimes I try and simplify too much.

Michael
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Peter Gregg
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Posts: 62

 « Reply #2 on: July 15, 2003, 10:16:15 PM » Reply

I have to say again that a room for "featured Articles" is needed here.

You newest article Michael is fabulous. The one with T. Knoll and dynamic range.

I will embarrass myself, but until I read your article on reading Histograms, I thought the histogram worked like a sound meter. If you hit the top, you are distorting the sound.

Now, I come to realize it is in fact from left to right. And according to your article, the right side of the histogram is preferred.

One question nags in my mind. in your picture of the moon on the Understanding Histograms article or lesson, when the histogram hits the top very hard like the moon shot does, I still cringe thinking I am blowing stuff out of the picture. Then people talk about the red, green, and blue part of the histogram hitting the top.

What exactly happens when the histogram readings hit the top (ceiling) of the histogram? Is it blown? Do people who show a red, green, or blue line hitting the top of the histogram and stating the "reds have been blown" really not saying the truth?

A blown histogram is one going off the right side correct? Is there any other way to say a histogram is blown? I ask this because it seems there are people out there who think like I did and when you hit the top of it you are blowing out the picture. This is wrong thinking correct?

Hope I made myself clear.

Regards,
Pete
Peter Gregg
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Dave Millier
Guest
 « Reply #3 on: July 17, 2003, 07:03:55 AM » Reply

Peter

Just to back up what Michael said, the histogram is in effect a frequency chart that shows the relative proportions of pixels of different brightnesses in the image.

You can think of it as a lot of little buckets placed left to right across the screen. The leftmost bucket is for all the pixels that are of brightness 1 and the rightmost bucket is for all the pixels that are of brightness 255.

So when the histogram analyses the image it simply examines each pixel at a time and drops it in the relevant bucket. So at the end of the process you will have (for example) 6 million pixels all placed in the buckets sorted left to right by brightness.

The height of the column at any point is simply a graphical representation of how many pixels are in the bucket under the curve at that point. If you shot a picture of a black card for example (exposed correctly), all the pixels would be in the leftmost bucket as a single thin line with nothing in the other buckets.

Because the histogram is squeezed onto a small LCD panel there isn't much room to show the true height of the curve so I guess there is a bit of artistic licence used in th exact proportions of the graph, but this doesn't matter because the height is only used to indicate the relative proportions of pixel brightnesses - the exact count is really irrelevant.

Cheers

Dave Millier
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