Photoshop and other image editing programs contain a tool known as levels that can take an image histogram and move or stretch its brightness levels. It has the ability to change brightness, tonal range and contrast of total black, total white and any mid-tones in a photographic image. These tips will give you a better handle on levels and enable you to efficiently use it to its full advantage, which will give you better photographs as a result.
Three Main Components of Levels
- Black point slider
Change input levels of histogram, mapped to output levels with default setting being black (0)
- White point slider
Change input levels of histogram at opposite end of color spectrum, mapped to output levels with default setting being white (255)
- Mid-tone slider
Change input levels, redefining locus of middle gray (128)
The above components of levels are examples of settings on a RGB spectrum, but the tool can just as easily be used to enhance an individual color’s channel by changing levels options in the “Channel” box located at the top of the screen.
Black and White Point Levels
Questions to ponder before making any adjustments:
Are there any areas in the photograph that should be completely black or completely white?
Does the histogram reflect the conclusion reached for the question above?
The majority of images are best displayed utilizing the entire range of the histogram, extending from completely black (0) to completely white (255).
Photographs that do not fill the entire spectrum tend to look washed out and lack artistic impact.
Adjusting the sliders is the best way to extend the histogram to fill the tonal range completely.
Do not habitually push black and white sliders to edges of the histogram without considering the objects and content in the image.
Foggy, hazy or soft-lit photographs rarely—usually never— have completely black or completely white areas.
Adjusting levels in images such as the one described above can ruin the mood of the photograph and make lighting seem harsher than it was in reality.
Also, adjusting sliders to either end of the histogram can clip desired shadows and highlights.
Sliders conveniently show where clipping will occur in an image, which allows photographer to decide whether or not it is detrimental to the intended mood of the photograph.
A fully black or white image while dragging a point slider across means no clipping occurred.
Counts on the histogram will appear in regions that have been clipped by movement of point slider.
Remember: Sliders showing clipping on an image does not mean entire area has become white. It means that a least one color in the spectrum, either red, green or blue has reached the maximum (255) and is white.
Mid-tone Point Level
The main use of the mid-tone slider is to make image midtones brighter or darker, depending on the subject and mood of the photograph.
Slide left stretches histogram to the right, compressing histogram to left, which brightens photograph by stretching shadows and compressing highlights.
Slide right stretches histogram to the left, compressing histogram to right, which darkens photograph by compressing shadows and stretching highlights.
Are there any other times to use the mid-tone slider?
When an image needs to have both full black and full white, moving the mid-tone slider to its right and the white point slider in tandem will stretch highlights to white while maintaining the integrity of the image by allowing the brighter areas to stay true and not appear overexposed.
The same idea works to darken shadows and maintain mid-tones by moving the slider to the left.
Mid-tone slider is always at 1.00 even if the black and white sliders have been moved so it acts as a measure relative to the both of the other sliders.
Mid-tone “input level” number represents a gamma adjustment.
Values greater than one mean more levels are to the slider’s right.
Values less than one mean more levels are to the slider’s left.
Using the Dropper Levels
Dropper tools on far left and right can be used to set black and white by clicking on the necessary areas within the image.
Not as precise as sliders because it does not show where clipping may occur.
Black and white dropper tools are better suited for computer-generated graphics instead of photographs.
Middle dropper tool does not function like mid-tone slider. Instead it sets the point in the image that is colorless, using that as a reference point as well.
Levels: Other Uses
Increases contrast without influencing color saturation in luminance histograms.
Changes color balance in color histograms whose images have unrealistic colorcasts.
Decreases contrast in a photo by modifying “output levels” as opposed to “input levels” which avoids clipping.
Take These Precautions
Stretching image histogram may cause posterization so minimize use of levels tool.
Levels use on luminance histograms can clip a color channel, but might allow for darker black points and brighter white points.
Level use on an individual color channel might affect color balance, so only use levels when intentional color shifts are needed.
I hope this has been of some value… You can learn more here on photoshop.