Digital imaging and digital post software have enabled more of us to consider color grading as a creative option. When shooting we make decisions regarding white balance along the Kelvin scale, with tungsten at 3200k and daylight at 5600k. In post production we can further tweak or adjust the color of an image.
This process used to be done photo-chemically and was commonly refereed to as color timing (source). Now it is most often done digitally either with a DI, digital intermediate, or working with the digital files straight from a digital camera. Processes like Bleach-Bypass refer specifically to the technique of skipping a typical bleach bath in order to leave more silver in an image. This effectively increases the contrast and desaturates the image a bit. Now this same look, or nearly the same look, can be achieved digitally and is still referred to as “bleach bypass.” Below you can see a frame from Saving Private Ryan (1998), a film that popularized the bleach bypass look. Notice that the blacks are “bunched up” or “crunched” meaning that the darks transition to solid black quickly.
While color correction in digital photography is often done through the adjustment of the curves for the various color channels and luma channel, much color grading in the world of film and video is done through the kind of three color wheel interface you see above. These there color wheels refer to the darks, mids, and highlights but are typically called Light, Gamma, and Gain. With these controls, adding a little blue in the highlights is as easy as moving the selector on the far right toward blue. Doing this, however, can have an impact on the mids and darks. So a counter move is sometimes necessary in the darks to balance out the initial choice. This is called pushing and pulling, e.g. if we push up the blue in the highlights, we may need to pull out the unwanted blue in the darks. Colorists also control what is color corrected through the use of mattes that define what region is affected by a particular adjustment.
The use of mattes or masks allows for secondary color correction, or in others, a different set of color choices to be made on a defined portion of the image. In Adobe Premiere, you can select what part of the image will receive your color correction by first applying a 3-color correction effect to a clip, then changing the effect display from “composite” to “mask.” The image will become white, until you scroll down to the “secondary color” section, expand it and use the eyedropper tool to select a color for inclusion in the effect. With the eyedropper chosen, the image returns to allow you to choose a color. Once chosen, the image displays the selected region in white and the rest in black. You can add to your selected color by using the eyedropper tool with the plus symbol. For example, you might select different areas of the face to build up a continuous selection of the skintones. This is a typical operation as often one wants to control the skintones and background color seperately. With a section chosen you should also blur slightly the mask so that the transition between different color corrections is not obvious. Now it is possible to return to the display settings of the effect and set it back to “composite.” All of this is explained in this video tutorial from Adobe, http://tv.adobe.com/watch/adobe-evangelists-jason-levine/basic-secondary-color-correctiongrading-in-premiere-pro-cs55/
Additional info:
A video tutorial on how to achieve the “summer blockbuster film look”
Two free tools for color correction
