Photoshop Tutorials - Luminosity Range
Tone of the digital image is the luminosity of the pixels that it consists of. Image pixels have different luminosity. The part of the overall luminosity range that is used for the image is called the tone range. The wider is the tone range, the deeper are the colors and the better image detailed are developed. The tone and shadows correction resolves the problems in this area. Theoretically, the complete tone range should be used for the image. If you will break the tone range in three parts, then the darkest part would be the shadows, the lightest would be the highlights and between them would be the midtones.
In Histogram window the shadows are located to the left and the highlights are located to the right. The tone range of the image is the diagram that shows how the pixels are distributed based on the scale of their luminosity.
The more pixels are located in the certain tone interval the more hues the interval contains.
When the image looks dim you can go to Window > Histogram to check the luminosity range of the picture.
Histogram is the diagram where the pixel luminosity ranges from 0 to 255 along the X axis, while the Y axis values display the amount of pixels of each luminosity level.
Open any dim image to check its Histogram. Most likely, it misses one or both darkest and lightest tones. It looks dim, because the pixel luminosity levels do not differ much. You need to stretch the color range to make the image look live and bright. The correction of such image will be limited to cutting off the most bright and dark tones that contain only few pixels. In this case, the existing tones will spread equally along the tone range and as the result the image contrast will increase and the image will now have attractive shadows and flashes. Note that the amount of the tones will become less then 256 (basic value) as the part of the range will be removed and the rest stretched. If the image tones get too stretched the image may start looking as a poster.
To change the tone range you can use Auto Levels option in the Image menu under Adjustments. The result will look way better then the dim image we have started with. How does the command know what we need? It is simple: Auto Levels command makes the brightest areas of the image white and makes the darkest image areas black. The tone range is stretched between the white and black colors that were generated. Now the Histogram will look different. It has the same shape but the image now contains solid white and black areas instead of light- and dark- grey colors. The contrast increased and the colors started looking more natural.
Photoshop automatic tools for tone range correction are Auto levels, Auto Contrast, Equalize that are located in Adjustments submenu in Image menu.
Auto Contrast command adjusts the contrast and luminosity automatically. We will speak about the Equalize command later.
Unfortunately Auto Levels command is not all-powerful. Place solid white and solid black dots on the image before correction and Auto Layers will be unable to edit it, because black and white areas already would exist in the image. In this situation you cannot stretch the tone range because solid white and black areas already exist. Therefore, the image should be changed in other, more complex way. This can be done using Levels command.