What is image processing and why do we need this?
We are often asked ‘what is image processing?’ When an amateur or hobby photographer takes a photograph generally they are happy with the result. The technical restrictions and constraints that professional photographers work to are never considered by the amateur or the hobbyist.
Constraints such as will the darker tones be distinguishable from black in a CMYK print-out, or conversely are the almost white and lighter tones bright enough, or even too bright, to distinguish against paper white for the CMYK process? What type of printing is being used? Are the images likely to be blown up to large posters or are they just for use on the web?
Prior to digital photography these things were controlled by the manipulation of film processing and film scanners. Tonal range and colour were controlled in the lighting and also film processing but mostly through the deft control of lighting. In digital photography considerably more latitude is possible after image capture; that is in the processing stages. Areas too light can be darkened, areas too dark can be lightened, colours can be altered. Processes that used to take enormous amounts of time can now be achieved using just the computer. Photographers often make decisions in the capture stage about whether it is quicker and less expensive to fine tune a shot post photography (in the processing stage) instead of the lighting stage.
As well as all of the above one needs to understand that a professional photographer will mostly shoot in digital “Raw’ format. Raw images are much like a latent image in film, although unlike film, the image is readily visible. It is also important to note here that Raw format offers the highest quality possible in digital capture. Raw format is captured in RGB, representing the colours red, blue and green respectively.
A number of adjustments can be made to improve a Raw image and often the difference between a Raw image and a fully processed image is substantial. And of course then there follows the almost limitless option of photo manipulation, but that is a much broader topic.
So having captured the image for the client in the highest possible quality the files then need to be converted from Raw into a format useable by graphic designers and printers. To maintain the highest quality files are firstly converted from Raw to RGB 16 Bit Tiffs and generally worked on at this stage using photo editing software.
Once the tonal range of the image meets the photographers expectations the file is then converted to CMYK format. CMYK represent the inks used by printers and the letters stand for cyan, magenta, yellow and black respectively. Here problems can occur. Because RGB contains a much broader colour gamut than CMYK, colours are often lost and need to be manipulated to resemble something close to the original RGB capture. Sometimes this is not possible and a compromise follows. Colour problems such as these often occur with dyed media, such as fabrics, paints and plastics.
At this point the photographer may choose to retouch any blemishes in the subject matter, as well as any dust spots. Dust spots are tiny specks of dust, usually invisible to the naked eye. They attach themselves to the surface of the CCD, (Charge-coupled Device). They appear on the image as semi-transparent dark spots and can spoil images if not removed.
The final stage of processing is the sharpening of the image. Sharpening is required on most images and the amount of sharpening can vary from image to image, depending on the number of pixels a subject has. For example a close-up of a coin using most of the CCD’s capture area will have far more pixels defining the edges, texture and colour than the same coin photographed in a pile of let’s say 100 coins. The amount of sharpening will then vary in these two images.
And that is the sum total of image processing. The complexity of an image and the subsequent time required to process individual images can vary, but as a general rule of thumb an average of 3 to 5 minutes per image is normal. In images where colour accuracy is imperative 30 minutes to an hour or more per image is not uncommon.
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