The Theory of Nothing

How does a digital camera work?

How does a digital camera work?

Once upon a time the Eastman Kodak Company ruled the photography realm. George Eastman didn’t invent photography but he made it popular by introducing an inexpensive camera for the masses in the 1890’s. He did invent roll film, which was also necessary for the movie camera. His still camera was simple to operate, and all one had to do was mail it back to Kodak and receive the developed prints and the camera back loaded with a new roll of film. What could be easier than that? The fact is that this became very popular and made photography available to everyone.

All photography in the first half of the 20th century involved silver, the precious metal. The process for making photographic film and paper is more an art than a science. One starts with Silver Nitrate, a soluble salt of silver, and Potassium Chloride or Potassium Bromide. The Silver Nitrate and the Chloride or Bromide salts are dissolved in separate solutions. These are added simultaneously into a solution of gelatin, which is made by soaking high purity gelatin in water and then heating the solution until it’s dissolves. The Silver Nitrate reacts with the Chloride or Bromide to make insoluble Silver Chloride or Bromide crystals emulsified (dispersed as tiny crystals) into the gelatin solution. This emulsion is mixed with other materials to stabilize and make the gelatin able to harden once it’s coated on a cellulose acetate plastic film base or special photographic paper. The gelatin emulsion must be noodled (cut up into small chunks), washed to remove nitrate salts, and then heated back up to be ripened (made more sensitive by a process of making the silver grains larger). Heat and stirring ripen the emulsion, which is more art than science--sort of like making a good soup or gumbo.

When exposed to light, the silver halide (chloride or bromide) crystals are activated so that when the film or paper is developed with a photographic reducing solution (Metol or the hydrogen sulfate salt of N-methylaminophenol, for example), the silver ions in the halide salt crystals are reduced to black metallic silver grains, thus forming the image. All silver negative film and paper images are made from tiny grains of silver at high resolution. After development, the film is bathed in acetic acid to neutralize the base that helps the developer work and halt the developing process. The next step is called fixing, and this is the job of Sodium Thiosulfate, which acts by removing (dissolving) any unconverted silver halide. Thorough washing with water removes any traces of the fixer. Without this fixation step the negative or print would darken with time because the unconverted silver halide is still sensitive to light and heat.

Color photography involves the same process except that dyes are added to form color dye crystals where the silver grains were. Cyan, Magenta and Yellow separate dye layers create an absorptive color image when viewed. This is a very simple explanation for a very complicated process, but the main point is that color involves three separate layers of silver halide, each layer made sensitive to a narrow band of visible light.

Digital cameras changed all of this. No more chemicals for processing--an inkjet or laser printer can be used to make a print. Although the concept of a digital camera goes back to the seventies, the first attempt to make one was attributed to Steven Sasson in 1975. He worked at--would you believe--Kodak. However, it wasn’t until the late eighties that commercial digital cameras began to be marketed. Kodak didn’t start getting into digital photography until the nineties, probably when they realized that the handwriting was on the wall. It said that silver is dying!

The digital camera uses an image sensor chip instead of film. They’re either a CCD (charge-coupled-device) or a CMOS (Complementary metal-oxide-semiconductor) circuit. Both consist of many tiny photo cells that are light sensitive, which makes them equivalent to the grains of silver halide in a film emulsion coating. However, the digital sensors work quite differently than silver film. The individual CCD photocells store the effect of the light as an electrical charge. In the case of the CMOS detector, circuitry next to each photo sensor converts the light to a voltage. All that is needed is a computer processor chip and associated circuitry to scan these individual cells to create a digital image.

It’s not that simple, especially when you are working with color. There are several rather complicated ways to record color. One is by use of beam splitters that moves the sensor to record the three primary colors separately. Some cameras use three filters and shoot three times in rapid succession. Many cameras use an anti-aliasing filter that restricts the bandwidth of the light signals from the pixel array to given wavelengths of light. These schemes work by different methods but end up producing a digital word, a number that conveys the RBG (red, blue, green) values, and intensity for each image pixel in the sensor chip. Image resolution in digital format is given as the number of mega-pixels in the image. That usually corresponds to the number of individual photocells in the image sensor. A typical 35mm ISO100 film negative has an equivalent resolution of 4 to 16 million pixels. Large format, 4 X 5, would have a 200-megapixel resolution.

The bottom line is that present day digital cameras are pretty close, but there are many other factors involved such as dynamic range, color replication, and noise. All of these factors are affected by image sensor size. A good DSLR (digital single lens reflex) camera would contain a sensor the same size as a 35 mm film negative, which is 36mm X 24 mm. Most digital cameras have sensors with fractions of that size in order to keep the price down. The larger the sensor, the better the image quality but the higher the cost. So, your smart phone is sub par when compared to the professional digital cameras, but they still take a quality picture compared to the earliest digital cameras.

This is only a cursory summary of a very complicated technical subject, but I didn’t want to write a book.

Thanks for reading.