Histogram Workings?

[sprockett]

Hi guys, i know the basics of "achieving a nice bell shaped curve/nothing shd leap off the left/right edges"

But i have a few qns..

1. What does the X axis stand for?
2. What does the Y axis stand for?
3. I know there are 4 graphs (RGB and INTENSITY) whats intensity for?

4. Assuming I have stuff going past the right side,or left side, i shd adjust my exposure until the graph is smack in the middle. WHAT if SPECIFIC colors are over exposed only? how do i bring everything to the centre?

Thanks!!

[robertwgross]

You've made some overgeneralizations, and it is a little hard to understand some of your text ("shd"?).

First of all, in all likelihood, you will never get a perfectly bell-shaped histogram. That is nice in theory, but with a real world image, it won't happen. In general, a well exposed image will have histogram lines extending from the left to the right, and maybe evenly from the left to the right, but maybe not so evenly. A well exposed image should be roughly even between the left half and the right half, but again, that is in theory. If you have all of the histogram lines stacked up next to the left side only, then that probably means an underexposed image. If you have all of the histogram lines stacked up next to the right side only, then that probably means an overexposed image.

The histogram lines on the far left mean that there is a lot of dark. The lines in the middle mean there is a lot of medium-tone, and the ones on the right mean lots of brights. So, the bottom axis is the spread of light intensities from dark on the left to bright on the right. The side axis is the relative percentage of the image that has those intensities.

In some cases, there will be a histogram for red, one for blue, one for green, and then there will be another histogram for the combined intensities. In some programs, they don't split it up and you see only the single combined histogram.

Why would you assume that you have light intensities going past the left or the right? If you do, they aren't going to register on the sensor, so ignore it. If you have light intensities that appear to flatten out against the top line, then that may be a problem. It typically means that you have lost detail into the intense dark or the intense bright.

If you have normal green and normal blue, but overexposed red, then you can ignore that or filter it to reduce the red. It depends on what you want for the finished image.

---Bob Gross---

[hommedars]

Take a look at this article:

http://www.luminous-landscape.com/tutorials/understanding-series/understanding-histograms.shtml

Joe

[rdenney]

sprockett wrote:
Hi guys, i know the basics of "achieving a nice bell shaped curve/nothing shd leap off the left/right edges"

But i have a few qns..

1. What does the X axis stand for?
2. What does the Y axis stand for?
3. I know there are 4 graphs (RGB and INTENSITY) whats intensity for?

4. Assuming I have stuff going past the right side,or left side, i shd adjust my exposure until the graph is smack in the middle. WHAT if SPECIFIC colors are over exposed only? how do i bring everything to the centre?

Thanks!!

The histogram is a statistical representation of the color, taken either as luminance--intensity--(which is probably the average of RGB), red, blue, or green. The X axis is the value of the color in question on a 0-255 scale. 0 is zero brightness, which makes black, and 255 is 100% brightness, which makes white. The Y axis is the number of pixels in the image that have that value.

A bell-shaped histogram means you have an image with lots of middle tones. A foggy scene will produce a bell-shaped histogram. Histograms of scenes with sky in them often have two peaks, one for the foreground, and another for the sky.

If you have pixels that are off the ends of the luminance histogram, that means that they are either 0, 0, 0 for black or 255, 255, 255 for white. These colors must be printed as full black or paper white, or they will make the print look muddy. What causes it is a scene with more contrast than the sensor can handle.

If the histogram trails to nothing at one end, it means that the picture ran out of pixels at that end. For example, if it falls to nothing a qarter of the way from the right edge, that means the image has no true highlights. If it falls away at the left edge, it has no true blacks. If it falls away on both ends, it is a flat image, and if you want to spread those values out to make a full-range print, you'll do that with Levels in Photoshop. If it goes to zero on the right end but is clipped on the left end, then the image is underexposed, and if it falls away on the left but clips on the right, it's overexposed.

Rick "who fervently wishes PS had a histogram display in the Curves dialog" Denney