Photoshop Levels
Summary

Summary

Table 1 below is a summary of what we can achieve with the Levels adjustment and how to do it.  In addition, the table tells us the equivalent Curves adjustment. 

Table 1.  Levels Adjustment Summary Comparison

WHAT	HOW...	...OR	Equivalent Curves Adjustment	WHY
Lighten highlights and overall image	Move Input Levels white point slider to the left	Change Input Level 255 to a lower number.	Move the 255 point anchor horizontally to the left.	Remaps image highlights closer to 255 and more pixels become lighter than mid tone because gamma slider automatically readjusts
Darken shadows and overall image	Move Input Levels black point slider to the right.	Change Input Level 0 to a higher number.	Move the 0 point anchor horizontally to the right.	Remaps image shadows closer to 0 and more pixels become darker than mid tone because gamma slider automatically readjusts
Darken the overall image. Bring out detail in the highlights but possibly lose detail in shadow areas	Move Input Levels gamma slider to the right	Change Input Level 1.00 to a number less than 1.	Move the 127 point horizontally to the right.	Remaps lighter than mid tone to darker than mid tone
Lighten the overall image. Bring out detail in shadow areas but possibly lose detail in highlights.	Move Input Level gamma slider to the left.	Change Input Level 1.00 to a number greater than 1.	Move the 127 point horizontally to the left.	Remaps darker than mid tone to lighter than mid tone
Lighten highlights and darken shadows, thus increasing overall contrast.	Move both the Input Levels black and white point sliders toward the center till they are just outside the ends of the histogram.	Increase Input Level 0 and decrease Input Level 255.  The clipping preview method, described on the Input Levels Clipping Preview page, is highly recommended when making this adjustment.	Move the 255 anchor point horizontally to the left and the 0 anchor point horizontally to the right.	Stretches the image's tonal range to the full 0 - 255 digital tonal range.
Lightens shadows, reduces contrast for an overall lighter, but flat appearance.	Move Output Levels black point slider to the right.	Change Output Level 0 to a higher number.	Move the 0 anchor point straight up.	Removes black and some dark tones from the digital tonal range and forces the image tonal range to fit.
Darkens highlights, reduces contrast for an overall darker, but flat appearance.	Move Output Levels white point slider to the left.	Change Output Level 255 to a lower number.	Move the 255 anchor point straight down.	Removes white and some light tones from the digital tonal range and forces the image tonal range to fit.
Darken highlights and lighten shadows, thus decreasing overall contrast.	Move both the Output Levels black and white points sliders toward the center.	Change Output Level 0 to a higher number and 255 to a lower number.	Raise the 0 anchor vertically and lower the 255 anchor vertically.	Removes solid whites and solid blacks.
Activate Clipping Preview	Press the Alt (Option) key while moving either the Input Levels black or white point sliders.	n/a	n/a	Turns the document window into a Threshold-like display.  All pixels are temporarily displayed as either solid black or solid white.  As the black or white point Input Levels sliders are moved, pixels that will be remapped to tone 0 or 255 will appear.
Set image white point.	Use the white eyedropper and click on the lightest, not specular, part of the image.	n/a	Same	Remaps that point to white as defined in Photoshop's Color Picker.
Set image black point.	Use the black eyedropper  and click on the darkest part of the image.	n/a	Same	Remaps that point to black as defined in Photoshop's Color Picker.
Neutralize color in an area	Click the area with the gray eyedropper .	n/a	Same	Equalizes the individual color channels while trying to maintain original tone.
Reverse an image	Move the white Output Levels slider to the far left and move the black Output Levels slider to the far right.	n/a	For Input value 0, set Output value to 255.  For Input value 255, set Output value to 0.	Tones and colors are reversed
Create color sampler target in the image	Up to four color sampler targets can be added to an image.  To add a target while the Levels dialog box is open, turn Caps Lock on, mouse to the desired location in the image, and Shift - click.  Caps Lock changes the cursor from an eyedropper cursor to a crosshair , which gives a little more precision in creating a target.  Turning on Caps Lock is not mandatory.	n/a	Same	Color Sampler targets do not affect the image.  They place information in the Info panel that can be used to monitor changes to the image.
Delete a color sampler target	To delete a target while the Levels dialog box is open, mouse to the target, hold down Alt + Shift (Option + Shift) until the cursor turns into a pair of scissors and then click to delete.	n/a	Same
Move a color sampler target	To move a target while the Levels dialog box is open, mouse to the target, hold down the Shift key until it becomes a black arrowhead, then drag the target to the new location.	n/a	Same