A histogram can tell you whether or not your image has been properly exposed or scanned, whether the lighting is harsh or flat, and what adjustments will work best. It will not only improve your skills on the computer, but as a photographer and scanner as well. Each pixel in an image has a color which has been produced by some combination of the primary colors red, green, and blue (RGB). Each of these colors can have a brightness value ranging from 0 to 255 for a digital image with a bit depth of 8-bits. A RGB histogram results when the computer scans through each of these RGB brightness values and counts how many are at each level from 0 through 255. Other types of histograms exist, although all will have the same basic layout as the histogram example shown below. There is no one "ideal histogram" which all images should try to mimic; histograms should merely be representative of the tonal range in the image.

Access the histogram by going to image/histogram the dialogue box on the left will open. This graph is called a histogram. The width of the graph represents all 256 possible brightness values, starting with black (shadows) on the left and progressing to white (highlights) on the right. The height of each vertical line in the graph demonstrates the number of pixels in that brightness value. Therefore the histogram shows you the distribution of the light and dark in the image. The histogram is a valuable tool, permitting you to corroborate the colors you see on screen. In the levels dialogue box the graph is the same but the sliders allow you to adjust the image.You can look at individual channels in the histogram.

 

Levels

Levels is a tool in image editing programs (such as Photoshop) which can move and stretch the brightness levels of an image histogram. It has the power to adjust brightness, contrast, and tonal range by specifying the location of complete black, complete white, and midtones in a histogram. Since every photo's histogram is unique, there is no single way to adjust the levels for all your photos. A proper understanding of how to adjust the levels of an image histogram will help you better represent tones in the final image.

Moving the midtones slider compresses or stretches the tones to the left or right of the slider, depending on which direction it is moved. Movement to the left stretches the histogram to the its right and compresses the histogram to its left (thereby brightening the image by stretching out the shadows and compressing the highlights), whereas movement to the right performs the opposite. Therefore, the midtone slider's main use is to brighten or darken the midtones within an image.
When else should one use the midtone slider? Consider the following scenario: your image should contain full black and white, and even though the histogram extends to full black, it does not extend to white. If you move the white point slider so that it reaches the edge of the histogram, you end up making the image much brighter and overexposed. Using the midtone slider in conjunction with the white point slider can help you maintain the brightness in the rest of your image, while still stretching the highlights to white:

This histogram tells you that this was a good scan or file. Notice how the graph tails off at before the ends.This tells you that all information is there.

This histogram tells you that this was a bad scan or file. Notice how the graph ends abruptly before the end. Also notice in the height it is cut off. This tells you that there is missing information (missing pixels).

This histogram tells you that this image cannot take any more!!, it is known as "hammered". The vertical lines show it has previously been adjusted. They also show that there is is a lot of missing infomation as shown by the gaps (white lines) in the graph.

16 bit mode versus 8 bit mode. As we have said before 16 bit files are a lot better to work with, so if you can always scan in 16 bit mode. The pictures below will explain why, but remember when you have finished to convert back your file to 8 bit mode.This is because most image editing programmes cannot handle 16 bit files. Photoshop lets you do some things in 16 bit but not all. Also most page layout software ie In-Design QuarkXpress etc do not handle 16 bit files.

Tip: You can also use the arrow keys on the keyboard to fine tune the levels as this will move the sliders one level at a time. Leave the eyedropper over the part of the image you need to adjust and you can see the density you have in the info palette.

OTHER USES FOR THE LEVELS TOOL
The levels tool can be used with CMYK and grayscale in addition to the RGB histograms shown above, including luminance and color histograms. Performing levels on a luminance histogram can be useful to increase contrast without also influencing color saturation, whereas levels on a color histogram can change the color balance for images which suffer from unrealistic color casts (such as those with an incorrect white balance).
Levels can also be used to decrease the contrast in an image by modifying the "Output Levels" instead of the "Input Levels." This can be a useful step before performing techniques such as local contrast enhancement since it avoids clipping (because this technique may darken or brighten the darkest or brightest regions, respectively), or when your image contains too much contrast.