The following contains some of my loosely organized thoughts about image manipulation, digital and otherwise. I do not expect these ramblings to provide answers to any profound questions, but I hope they can open a few avenues for thought on the topic.

Photography forums today brim with emphatic ravings from people who seem pathologically concerned about any sort of image manipulation. Some ask how much is too much, as if they might be arrested by the Image Police if they make that final tweak. Just when I thought I had seen every conceivable comment on the topic, I read how one poor fellow worried that basic adjustments might make his images look "better than they really are". Many of these writings hold the opinion that film is somehow the standard for image faithfulness, while any and all digital manipulations are somehow deceptive. I feel these notions are critically flawed.

Many years ago I read a book called "The Print" by Ansel Adams. Although it was written before digital imaging existed, its most fundamental lessons are as valuable now as they ever were. In the book Adams states "...the negative is similar to a musician's score, and the print to the print to the performance of that score". Whether we work with negatives, positives, or RAW digital files, the image as captured realizes only part of its potential. Realizing its full potential requires refinement on many levels for many reasons, some technical and some artistic.

What is digital image manipulation?
Digital image manipulation is not confined to files that originate in digital cameras. Today film is usually scanned to create a digital file for printing, and the resulting file is subject to the same manipulations as any other digital image file. Further, many digital manipulations are simply digital versions of darkroom techniques that are nearly as old as photography itself. Digital image manipulations range from simple color balance and level adjustments to the addition of compositional elements that were not present when the photograph was taken. In between are techniques that can improve the quality of nearly any printed image. Their usefulness of each depends on the image in question.

For discussion purposes I have grouped the myriad of available digital manipulations into three groups. I call them adjustments, subtractive manipulations, and additive manipulations. I have included techniques like burning, dodging, and contrast masking, in the adjustments group. These are often used, can have a large impact on image quality, and do not fit into any of the other groupings. There are of course many other techniques, but I feel these are representative.

Adjustments
This grouping contains items like brightness, contrast, color balance, color saturation, curves, and levels adjustments. Nearly any image ever captured on film or digital media can benefit by the manipulations contained in this group. Without these most basic manipulations, creating fine prints is usually a hopeless exercise. For instance, many images look dull or flat because they use only a portion of the available dynamic range. Since the extreme ends of the dynamic range are mapped to white and black respectively, these images contain only middle tones, making them look dull and lifeless. A basic level adjustment can simply move the very brightest pixel (or pixels) in an image to pure white, and the very darkest pixels to pure black. Pixels with levels between these extremes move proportionately. This adjustment can bring life to lifeless images, making them look more like a human eye would see the original scene. In the black and white darkroom the same thing is accomplished by matching the contrast grade of the the paper to the negative, using different developers, and varying development time of the exposed paper. Things get more complex with color media, but the principals are the same.

In addition, level adjustments on individual color channels can eliminate false color casts in an image, such as the blue cast always present when film is used in open shade on a sunny day. Level and color balance adjustments can also reproduce the effect of certain lens mounted filters without the disadvantages these filters present in the field. In the darkroom, using filters on the enlarger can accomplish the same things, but with more difficulty.

Burning and Dodging
I include this technique in the adjustment group because it is essentially a brightness adjustment made to only part of an image. It can also be considered a contrast adjustment, since the brightness of one part of the image is adjusted relative to the other parts. The techniques have their roots in the darkroom, and are nearly as old as photography. In dodging an opaque disk or other shape mounted on a thin wire is simply waved over a part of an image that is to have less exposure while the photographic paper is exposed in an enlarger. Burning entails using a large piece of paper or cardboard with a hole cut in it to allow light from the enlarger to hit only the area of the image needing more exposure. This simple techniques can bring out shadow details or hold back highlights in a restricted area within the image. They can also be used for artistic effect, to emphasize or deemphasize certain parts of an image by making them lighter or darker. The same effect with much more accurate control is easily done in digital processing, and if you make a mistake you can "undo" your work and try again.
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Contrast Masking
Contrast masking is a technique for effectively managing contrast across an entire image. It can be very effective at improving detail in shadow and highlight areas. The technique originated long ago as a rather difficult darkroom process, and can now be done simply using digital processing. In the darkroom, a color transparency is contact printed onto a black and white negative. The result is a black and white negative image of the color positive. Light areas on the positive are dark on the negative and dark areas on the positive are light on the negative. The negative is made “thin” so the blacks are not very opaque. The positive and negative are then “sandwiched” together in exact registration, often with a thin piece of mylar between them to slightly defocus the black and white negative. The resulting sandwich is then printed using a normal enlarger. This keeps the brightest areas of the original image from burning out and gives the darkest areas more exposure to bring out shadow details that would otherwise be lost in blackness. The same technique is applied digitally with much greater control than was ever possible in a darkroom. This technique simply allows more accurate printing of the information already contained in an image.

Subtractive Manipulations or Cloning
Cloning replaces some pixels in an image, usually with other adjacent pixels from the same image. Cloning is often used to remove spots caused by dust on scanned film or a camera’s image sensor. With film scans, cloning is also often used to remove scratches present on the film. Similarly, it can also remove a distracting twig on the edge of an image, power lines cutting across a sky, or even major image components. It is quite difficult to remove objects without a trace unless they are against a plain background such as sky, though. Accomplishing this in a darkroom is more difficult, although scratches in the base layer of film are often “removed” by smearing a light coating of oil on the film to fill the scratch, rendering it invisible on the print. The technique has obvious drawbacks, but is better than the alternative of having visible scratches in the printed image or retouching the final print by hand.

Additive Manipulations
These manipulations typically involve copying an object from one image and pasting it into another. Additive manipulations are obvious unless the light direction, exposure, and scale in both images are exactly the same. These manipulations can put a full moon in the sky of an image taken on a moonless night or add a wolf that didn’t exist in the original scene, howling at a moon that was not originally there either. Many UFO and other “trick” photos have been made in darkrooms, but as is typical, the process is much easier when done on a computer.