Photoshop Curves Adjustment Tool

The Photoshop curves tool is the most amazing and useful image finishing tool within Photoshop, but is probably the most difficult to master for photographers new to Photoshop.  It is the one single tool in Photoshop I will use on just about 99% of the images I process for printing.  Since the art of photography is chasing the light at the time of capture and painting with light at the point of finishing the image, the curves tool is a critical tool in the photographer's toolkt to finish and enhance and even transform your images.  The tonal qualities are what give the unique character to each image and curves is the tool to allow the photographer to mimic any film type.

The two most common curves used in curves are the S-curve and the inverted S-curve.  The S curve adds contrast to the midtones whereas the inverted S curve does the opposite.  Notice the changes in the histogram as well as how these changes influence the tonal qualities of the image.  Curves lets the photographer better utilize the limited dynamic range you had to work with at the time of capture.

As Shot, Linear Curve

While our camera estimates the amount of light hitting each aspect of our subject(s) being captured, our eyes make their own adjustment to optimize visual sensitivity over the largest range of light.  The camera has to apply its own tonal curve to the RAW file format in that each digital sensor has its own set of tonal curves when displaying images (gamma) that are proprietary to each camera manufacturer.  Thus, using the curves tool, we can choose on our own to change what the camera sensor selected for what we believe creates a more realistic image that we actually saw with our own eyes.  Thus, tonal curves are a must to properly finish each image prior to printing.

Keep in mind, that you can not add contrast in one tonal region of an image without decreasing it in another part of the image.  Thus, to increase tonal contrast in the midtones, you decrease the contrast in the shadows and highlights.  The photographer gets the final say to choose to spread contrast evenly (straight line diagonal curve) or by varying the slope in some manner, i.e. S-curve or inverted S-curve.

S Curve without Color Correction

In using the curves tool, there are 3 anchor points, one for shadows, midtones and highlights, in addition to the selection of the black and white points.  Notice even minor movement of any of the anchor points makes major changes in the final image's tonal quality.  Thus, small adjustments which produce smooth curves are how you should make changes in working with curves.  You can also enlarge the curves window with the tab in the lower right hand corner which will allow fine tuning capability.

You can also choose various selections of tonal curves from the drop down box at the top as a starting place to see what you like about the impact on your image by each curve selection.  If the histogram edges do not go all the way to the end of the total distribution, you will have unused tonal range that will minimize the trade off when trying to increase contrast by adjusting the black and white anchor points as done with the levels tool shown in a previous blog and this example.  If gaps occur between tonal peaks, with curves you can decrease contrast in these unused tones, freeing up contrast to be spent on the midtones.

S Curve with Color Correction

Digital images may abruptly clip the highlights once the brightness reaches its maximum (255 for 8-bit images).  This will create an unrealistic appearance in your image.  Usually a smoother transition to white is preferred.  The highlight transition can be made more gradual by decreasing the curve's slope at the upper right hand corner.

Color saturation can be greatly decreased by using the inverted S-curve and increased by using the S-curve. Changes in saturation may be desirable when brightening shadows, but in all other instances will be destructive to your image tonal quality.

Strong Contrast Drop Down

Another very important aspect of curves is to correct color balance.  All the discussion prior to now have been using curve adjustments applied to RGB values, but they can also be used on each individual color channels to correct color casts.  Changing the white balance or adjusting the overall color of one region of the image, would inadvertently be destructive to the other tonal qualities.  The easiest way to color correct your images is to use the black eye dropper on the left of the curves module.  Just click on the black eye dropper and touch it to a black portion in the image and you immediately color correct the image.  You can also do the same with the white eye dropper if you actually have real white pixels to choose from.  The most accurate of all to color correct is to have your subject hold a white-black-gray card image in one of your images to use in photoshop to color correct your images.  Then use the eye dropper for each of the white, black and gray and just touch each respective eye dropper to the appropriate color on the WBG card in the image.  Then you can save that setting to apply to all other images taken in the same lighting conditions.

Strong Contrast Drop Down

This one finishing adjustment is probably the only tool you must use to properly prepare each image for printing. You will see with each image an amazing difference and improvement in the tonal quality of your images.  This is the one key element used by all professionals to obtain the tonal quality you see in their work. 

Keep chasing the light for that Perfect Tyme!