Editing Photos - Editing Levels with Sliders

Editing an image's levels can be an easy way to improve image quality.  Images can be darkened, lightened or have their contrast enhanced by editing its levels.  The use of sliders is a common method offered by image editing software when it comes to editing levels.  This is a fairly basic method for editing levels and well within the skills of most people.

How to Use Sliders to Edit an Image

The method used to edit an image's levels depends on the software involved.  (Look for the Levels or Histogram tool.)  I use GIMP to edit my images.  GIMP is open source software that offers fairly advanced features for editing images.  It's free to download and use.  In GIMP, the Levels tool is listed under the Colors tab on the top toolbar.

Opening the Levels tool brings up the image's histogram and the editing options.

The first thing to do is examine the histogram.  The histogram provides useful feedback when it comes to editing the image.  In this case, most of the pixels are on the left three-quarters of the scale.  There is a large gap between the slight peak corresponding to white and the next lightest color.  There is also a small gap at the left.  While this image is slightly dark it does not contain any true black.

GIMP uses two sliders when editing levels.  The top slider is located directly below the histogram.  Using this slider will alter the values of the images.  Using the lower slider will alter the range of values used in the image.

Moving left side of top slider:

 

Moving this slider to the right will darken the picture..  The program will reassign any pixel with a value  below the setting to a value of 0.  (Any details contained in those pixels changes to black will be lost.)  Those pixels above the setting chosen will have their values recalculated to reflect the new range.  The darkening effect is more pronounced for lower value pixels with white pixels being unaffected.

If the histograms contains a gap between the darkest value and true black, the left value can be moved right without causing the image to lose any details.   (This will enhance contrast.)

Moving right side of top slider:

Moving the right side of the slider to the left has the opposite effect.  This lightens the overall value.  The effect is more pronounced on the lighter pixels than on the darker pixels.  Any pixel with a value above the setting chosen will be rendered as white.  This can cause loss of detail if too many pixels are effected.

With this image, moving the right setting to 190 has little impact on detail.  Most of the pixels have values below that point.  Changing the right setting in this case lightens the overall image and add contrast to the brickwork.

Moving the middle setting of top slider:

Moving the middle setting on the top slider alters the mid-range point.  That point is given a value half-way between pure white (255) and pure black (0).

Any pixel located to the right of the pointer will have a value above the midway value.  Any to the left will have a value below the midway value.  This means moving the pointer to the left will increase the overall lightness and moving it to the right will darken the image.  (This seems counter-intuitive at first, but the effect is always visible before actually making any change.)

Changing the middle setting will impact contrast.  Moving the pointer left increases the overall lightness of the image.  This will also decrease the contrast in the lighter half of the image while increasing it in the darker half.  The opposite occurs when moving the pointer to the right.

Why does this happen?  The pointer starts at the half-way mark, which is a value of 128.  If you move it to what is currently a value of 78, the pixels that currently stretch from 78 to 255 (a 177 range) now have to fit between 128 and 255 (a 127 range).  This decreases the distance between each value available on the light half of the image by over 25%.  The software has to decrease the contrast between those pixels in order to fit them within that range.

The pixels on the other side of the pointer originally only filled a 78-point range.  They now have to fill a range over 128 points.  This requires increasing the value distance between those pixels.  (Also known as increasing contrast.)  The opposite occurs when moving the setting the other direction.

The numbers used on the middle-pointer in GIMP correspond to the Gamma correction tool offered by other image editing software.  This is why the value scale starts at 1.00 in this program.


Using the bottom slider:

The bottom slider will decrease the range of values used in the image. 

Moving the left pointer to the right the sets the darkest value to a point other than black.  This lightens the image and decreases contrast.  (The pixels now have to fit in a smaller range of values).  Moving the right pointer has the opposite effect.

Moving both sliders will decrease the value range, decreasing contrast.  The effect on overall value will depend on how much each pointer is moved.

There are times when using the lower slider can be useful.  For example, it can be used to prevent clipping when using the contrast tool to increasing contrast.

Here's the same image after setting the left side of the Output slider to 30 and then using the contrast tool to increase contrast.

Using the Level tool's output feature before enhancing contrast prevents any details being lost in the darker portions of the image.  Looking at the histogram, you can see that the values have been stretched to the right with greater distance between the peaks.  (There are other ways to increase contrast without losing details in light or dark areas, this is just to illustrate one of the uses for Output slider.)


Channels:

Each channel (red, green, blue) can be edited separately.  This will alter the image color.  In channel mode, any effect that would increase brightness will increase the strength of that color.  Effects that decrease brightness in value mode will instead decrease color strength.

This allows the Level tool to be used to address the effect of shooting under artificial lighting conditions or correcting color shift that occurs with older film or slides.

Editing channels can be combined with editing values.  This is an example of editing all three color channels as well as values:

This mimics the effect of shooting during dawn or dusk, or the effect of the a photo's colors changing over time.

Again, there are alternative tools for editing an image's colors.  Using Levels to do so may or may not yield better results than these alternatives.

GIMP uses an Alpha channel to control transparency levels.  Not all images contain an Alpha channel.  If an image has an Alpha channel, it can be edited using the Levels tool.  This only matters if the image is saved in a format that saves transparency settings or if the image contains more than one layer.  Altering a layer's transparency settings will change how it effects lower layers.