Historical
The camera appeared long before photography itself in the form of the “camera obscura,” literally in Latin a room that is dark. The earliest camera obscura was created by a pin hole in an otherwise opaque window shade, revealing a well focused image of the scene outside on the opposite wall of the room. There are references to this as early as Aristotle in the 4th Century BC. When lenses became available in the 16th and 17th Centuries AD, a lens replaced the pin hole and a mirror was added to reflect the image onto a horizontal surface. This was quickly put to use as a tool to help artists sketch scenes (as seen in the recent film “Girl With a Pearl Earring”). One could actually draw or paint on a canvas inside a suitable camera obscura.
The jump to photography was made when artists imagined the possibility of capturing the projected image in some kind of medium that would not require the artist’s hand. Various light-sensitive media were tried and, as we know, the silver-halide emulsion in gelatin proved to be efficient and economical. The idea of using light-sensitive media to capture an image required some changes in the camera itself; in particular, the exposure to light had to be limited. When media were relatively insensitive, this could be achieved by simply removing and then replacing a lens cap; however, as more sensitive media were developed, the light path was controlled by a “shutter” which admitted light in fractions of seconds. The actual amount of light admitted per unit of time could be controlled by incorporation of a variable width diaphragm, making the lens opening variable.
During the 20th Century, camera and film technology developed rapidly, leading to panchromatic film media and roll film that replaced single plates. While cameras were originally aimed at a subject with a simple external sighting device, some manufacturers developed a viewing lens parallel to the exposure lens. As lens development moved toward expensive compound lenses, however, this design proved decreasingly effective and the “single lens reflex” camera was developed. The 35 mm SLR camera allowed the subject to be composed through the same lens that exposes the film. This was done simply by putting a mirror in the light path so that the scene can be examined in a viewfinder. When the shutter is released, the mirror flips up out of the light path and the light is admitted to the film over a time measured exactly by the shutter speed.
The image pattern of light is “saved” on a section of film because microcrystalline silver halide is selectively “activated” in proportion to the light intensity striking it. In the film development process, the activated microcrystals react more rapidly with the chemical developer (a reducing agent), depositing silver in the emulsion. So long as the development process is stopped (with the traditional acid "stop bath") before very much un-activated silver halide is reduced, the film “negative” is an accurate replica of the light image, though inverted --- dark to light. Excess silver halide is removed by chemically complexing it with a thiosulfate solution ("fixer"). The print process in the darkroom is a chemically identical process that re-inverts the image --- light to dark --- so that the image is captured in silver crystals embedded in an emulsion on an opaque sheet of paper.
The appearance of digital photography is simply a natural evolution of image-capturing technology. The light-sensitive emulsion was replaced by light-measuring sensors. Certain changes in camera construction had to follow but the optical principles remained in tact.
The Digital Camera
A digital SLR camera is a straightforward extension of the 35 mm SLR cameras already widely available. These cameras had already risen to a high degree of electronic sophistication, including internal light-measuring devices, range-finding lenses, automatic focus, and electronically controlled shutters and apertures. The basic adjustments necessary were to remove the film plain back and insert a charged couple device (CCD), provide for a removable memory card, and expand the electronics to save light measurements to memory. The front end of the camera remained essentially unchanged. [Complementary Metal Oxide Semiconductor (CMOS) technology is an alternative to CCDs but the differences are subtle.]
The CCD is an array of micro light-sensing devices each one of which is covered by a colored filter. The filters represent the primary colors --- red, green, and blue (RGB) --- and are grouped in neighborhoods which will allow interpretation as one point of colored light hew and brightness (a "pixel"). Most digital SLR cameras allow the exposure information to be saved on the camera’s memory medium in “raw” format. This is a simple data dump, giving the measured light intensity at every sensor. This is not an image or even an image replica. As a result of this design --- the fact that each pixel is a neighborhood-averaged hew and brightness --- a digital image is inherently soft-focused and inexact.
Images are constructed digitally as arrays of “pixels.” Each pixel is a square of uniform light hew and brightness. When the pixels are arranged in a rectangular array and viewed from sufficient distance, the appearance is an image (or picture) with continuous variations of color and intensity. This theory of imaging was already well developed by physiological psychologists in the late 19th Century and was applied to painting by the French Impressionists at the turn of the century.
The computational problem in digital photography is to convert the CCD data to pixels. In order to do this, the light measured at each sensor in an RGB neighborhood must be pooled and represented as a pixel. When the data is saved in raw format, this task is performed in your personal computer using the camera manufacturer’s “capture software” or a generic capture software like Adobe PhotoShop’s raw-file converter. Once a pixel array has been calculated for an image, the file can be saved in any of the common image formats --- tiff (uncompressed) or jpeg (variably compressed). [When a file is compressed, a certain amount of data is thrown away because it is judged as redundant --- e.g., a series of pixels that are judged as the same hew and brightness. Once thrown away, these cannot be recovered.] Digital cameras will also perform this computation inside the camera and save a pixel file in tiff or jpeg formats.
A few years ago Nikon released its most up to date F-series 35 mm SLR camera, the F100, at a street price of $1000. At the same time, relatively fine SLR cameras like Nikon’s N80 and N65 could be purchased for half of that or less. In contrast, Nikon’s first digital SLR, the D1, was priced at around $5000. Later on, the D100 was released at $2000 (now $1500) and the D70 at $1000. Canon digital cameras have followed a similar path.
The high cost of digital SLR cameras forces most amateur photographers and students to use so-called point-and-shoot cameras. There are huge differences in this market from one camera to another. One is best advised to buy from one of the high-quality manufacturers --- Nikon, Canon, Olympus, etc --- and to spend as much as possible within that category to secure features and quality reproduction. Desirable features should include the overall design of the body. (Is the camera comfortable to hold and operate?) The features should also include control of lens aperture, control of shutter speed, various sensitivity settings (equivalent to film ISO), shooting modes including “manual” mode, and a variety of file saving formats.
Virtually all point-and-shoot cameras are equipped with small lenses that allow very limited aperture adjustment. Point-and-shoots also have “electronic shutters” rather than mechanical ones. That is, the light image is focused on the CCD sensor while the picture is being composed and immediately afterwards. The shutter release merely tells the camera’s electronics when to save a copy of this image. This is why you can see the image in the camera’s video screen throughout the process. This produces an annoying lag in the shutter release process, unfamiliar to 35 mm photographers, since the camera’s computer does not copy the image immediately when the shutter release tells it to. (The amount of lag varies from camera to camera.) This can be very frustrating if you are trying to take pictures of moving objects!
Digital Files
Once CCD data is converted to an image file, either by the camera or by capture software in your computer, you have a serial array of bits that computer software can interpret into a rectangular array of pixels for display on a monitor or send to a printer. The quality of an image is directly proportional to the quantity and quality of pixels in it. The quantity of pixels is measured by "resolution" (pixels-per-inch). The quality depends on the camera's optics and the accuracy of the CCD.
The Nikon D100 and D70 CCDs produce files with up to 6.1 effective megapixels. Because the CCD itself is 23.7 mm wide and 15.6 mm high, the image file measures 3034 pixels wide by 2024 pixels high (the same proportions as a 35 mm negative). Photographers normally print files at a minimum resolution of 200 pixels per inch (ppi) so the full image from a Nikon D100 would print out at 15.1 inches wide by 10.1 inches high. If the camera produces fewer megapixels, the print size must be reduced in order to retain the same resolution. [You would need about one megapixel for a 4X6 print.]
Removing the memory chip from a digital camera is like removing the film; it is ready for processing. The first step in processing is to transfer the data to a computer and this will depend on the particular kind of medium the camera uses. From this point onward, the computer and printer play the role of the traditional darkroom.
Professional and serious amateur photographers archive their work and it is good to adopt this habit. All original files are copied into an archive folder that will not be used for regular processing. Images are manipulated using "working copies" only. Printable perfected images are saved in yet another folder so that duplicate prints can be made later. [One of the huge advantages of digital processing to professionals is the possibility of printing numerous copies of one image, all essentially identical.] Since image files are large and take up considerable space on the computer’s hard drive, archive files and printable files are periodically written to CDs for long-term storage.
Normal image files in tiff and jpeg formats are like negatives and you can return to these years later to process other prints. Like film negatives, however, there is a limited amount that you can do with them. For instance, if the camera exposure settings were not correct or if the white balance of the sensors was not correctly set (similar to using the wrong color film in your camera), there is no way to overcome these problems with computer software. Raw format files, on the other hand, are better than negatives and are more like undeveloped film. With film, you have one shot to develop into a printable negative. With raw format, you archive the “film” itself and have the opportunity of re-working the digital equivalent of the negative every time you go back to the original data. The capture software allows you to change white balance, exposure, and other features of the image. In this sense, digital photography achieves a flexibility that was never available in traditional photography. (The analogy would be shooting color slides inside a warehouse with "daylight" film and producing orangy colored prints. With raw format files, the photographer can "go back" and shoot the same scenes with "flourescent light" film simply by changing the white balance (or color temperature) in the capture software.)
Professional photographer have always kept extensive camera notes so that they could re-trace how particular images were shot. The digital camera records these notes automatically as a part of the image file. When you open the file in capture or viewing software, you can examine this extensive record of camera data. Nevertheless, a field notebook may still be useful in order to describe the subject and other conditions.
The "Digital Darkroom"
The final phase of digital photography occurs in two distinct operations --- using image software to manipulate the digital image and printing the image. The most up-to-date version of Adobe PhotoShop is the "CS" or version 8.0. PhotoShop was designed with photography in mind and most of its tools are directly analogous to darkroom manipulations. The jump to printing is less intuitive to photographers and requires learning something about the complexities of color printing in its numerous forms.
Image Manipulation. One of the nice attributes of the CS version is its File Browser which can browse all image files in a given folder. It places thumbnail pictures in an array on the right side of the screen; and when one image is selected, it displays a larger picture on the left as well as a full range of camera data and personal comments. Files can be rotated in the browser, flagged, organized, and directly opened. Raw files are opened by Adobe's generic capture software.
One of the first operations in typical darkroom work is to crop an image to the particular scene desired. PhotoShop has a cropping tool that easily provides for this. However, when an image is cropped in PhotoShop, pixels are actually being thrown away and cannot be restored except by returning to the original file. For instance, if the original D100 image is cropped to a square 10X10, a rectangle of pixels 1000X2000 has been thrown away. That's 2 megapixels of information.
The Image Size menu allows you to tell the printer what size you want your image to print and is analogous to selecting your print paper and easel adjustments in the darkroom. Width and height are locked together by default so you need change only one. (You can unlock them and distort the image if you wish.) Resolution is linked with size by default as well so, the larger size print selected, the lower the resolution. Photographers generally print at resolutions between 200 and 300 ppi. So long as width, height, and resolution are locked by default, the number of pixels remains unchanged and, hence, the file size remains constant. This can be a problem if you want to use the image as a Web photo or send it to someone via e-mail. In both applications a resolution closer to 80 ppi is more than adequate and a file size under one megabyte would be preferred. In order to do this, the default locking is removed by checking the "re-sample" box so that resolution and size can be separately selected. This, of course, throws away a large amount of the original information. Re-sampling also allows one to create larger prints while retaining resolution at 200 ppi, but in order to achieve that the software must actually invent pixels that did not exist before by modeling those that do exist.
PhotoShop allows you to manipulate brightness and contrast much as you would have done in the standard darkroom with enlargement time and variable contrast filters. The simplest way is to use Adjust>Levels in the Image menu. This produces a histogram of the image --- essentially, a graph telling you how many pixels there are of each brightness from black to white. Using the tools in this menu, you can fit the image to fill the black-to-white spectrum and then adjust the total contrast balance within. Color balance and saturation can also be adjusted.
Finally, PhotoShop also has a filter called "Unsharp Mask" that allows you to sharpen the image before printing it. As mentioned earlier, the CCD construction has an inherent blurring feature built into it. Without sharpening, digital photos will always appear softly focused. In this tool, the software examines pixels looking for contrast (edges) and adds a variable amount of "halo" where it finds them. To our perception, the image looks in better focus.
Printing.
When you have an image that you have worked to your satisfaction with the PhotoShop tools, you will want to print it. Printer technology has been evolving at a great rate of speed and color InkJet printers have become especially good for photo reproduction, at a reasonable cost. The Humanities Department uses an Epson 900 InkJet printer for this course. While it was once impossible to produce an acceptable black&white print on an InkJet printer, that situation has changed dramatically. The evolution of high quality inks has also changed the perspective on archival lifetimes. If InkJet printers are not yet competitive with classic photo-processing, for archival prints, the gap is closing very rapidly.
Print papers have been evolving almost as rapidly as the printers themselves. You can print photos on virtually every kind of medium imaginable --- postcards, greeting cards, personal cards, scrapbook sheets, etc. For fine art photography, you can find a number of fine photo-quality print papers in matte, silk, and glossy surfaces. Furthermore, printer-control software can be set to the character of the surface in use and will decrease or increase the amount of ink being sprayed accordingly.
An inkjet printer moves a sheet of photo print paper at variable speed through its length. A rack-mounted set of ink cartridges and micro-spray jets moves across the width of the paper at a constant speed. The jets lay down several overlapping dots of color for each pixel. So a six-jet printer actually places six dots of color over each other on a single pixel. [You can see why each paper surface will have a different effect on the perceived pixel quality.] While the computer is sending an image at 200 pixels-per-inch, the printer jets are actually laying down 1200 dots-per-inch. When judging the quality of a printer, it is important to keep this in mind. "Dpi" and "ppi" are not the same thing!
.Unlike photographic prints, InkJet prints can be damaged in the drying process and ought to be laid out separately for at least a few hours so that the inks can finish flowing and drying. Inked media also cannot tolerate certain atmospheric conditions as well as photographic media. If you want to maintain the quality of a fine-art photograph printed on an InkJet printer, it is best to keep it in a plastic sleave or frame it behind glass.