Compositing is the art of assembling dis-associated images into a single composition. The most common version of this is chroma keyed backgrounds (‘green screen’ or maybe ‘blue screen’). However chroma key is primarily a technique for film and video. For photography, compositing is more often the inserting of objects into a picture (unless you are Dave Hill, then it is an art form of collages on steroids).
While adding something (or someone) to an image with Photoshop seems easy, the results are usually less than desired. The item looks pasted in and the effect is obvious.
Even when the objects don’t match type (a bikini babe in the middle of Times Square), a successful composite can be accomplished if the lighting matches for all objects. Light is our visual clues. Light has several properties (intensity, hue, and direction being the big three) and if they match then our eyes see a single location.
There are a lot of ways to adjust for this depending on a particular situation. The better your mastery of Photoshop, the more you will be able to adjust and tweak the final outcome, but understanding the role of light is the key to ‘making it work’.
This is a fun thing to do and I encourage everyone to play with this a lot. In the process you will become much more aware of scene lighting and that will have a direct impact on all of your photography!
Have a great week of picture making!