digital image sensors

Do todays digital image sensors out perform photographic film?

The sensors found in todays high end digital cameras are said to produce better resolution and finer tones than film! How are these conclusions reached, and do digital images actually look better than

those on film?

 

With digital capture slowly becoming the norm across the photographic industry, are we actually getting better images or are  we sacrificing quality for convenience? This question is not easily answered. Images have different subjective effects on different people and in different environments. In this

article I will examine how we compare imaging systems and try to come to some conclusions that are useful to the average working photographer.

 

The Technologies

Photographic films still use the same basic process as used by the pioneers. That being that silver halide crystals will form metallic silver when exposed to light. The size of the crystals, the intensity of the light and the duration of the exposure being the main factors effecting the amount of silver produced. Modern emulsions contain specially sensitized materials that are more sensitive and respond to a wider spectrum of light (untreated silver halide responds only

to blue light) but the same basic chemistry is at work.!

 

In the case of black and white film, a layer of this emulsion is held between a protective ‘super coat’ and the films clear base. When the film is exposed to light a ‘latent image’ is formed in the emulsion layer. As the film is developed this image is amplified chemically to produce a visible negative image.

Colour film is slightly more complicated in that it uses three separate layers of emulsion with differing sensitivities and separated by filters. These each record information for one of the primary colours. The silver is then bleached out of the film during development, leaving behind a coloured dye which forms the negative image for that colour. The three layers combined form a full colour negative image of the subject. Focusing light though this negative onto more film or photographic paper will then form a positive image, hopefully containing all the tonal information found in the original scene.  Reversal or slide films use a similar process to produce the latent image but it is the unexposed halides that are developed to form the dyes. This produces a positive image in the film that can be used for direct projection.

 

 

Enter digital

 

Digital sensors include two basic types, CCDs which were originally developed for analogue video cameras and the more recent and natively digital CMOS sensors. Both are arrays of tiny light sensitive capacitors that each form a separate picture element or pixel. During exposure they each accumulate a charge proportional to the amount of light that falls on them. What happens after exposure depends on the type.

 

CCDs (or charge coupled devices) are specially manufactured so each capacitor passes it’s charge to the next in line, while the last one feeds an amplifier circuit. This process is repeated until all the capacitors are empty. The amplifier circuit outputs a voltage which is then converted to a digital value by an analogue to digital converter (ADC). CMOS (or complementary metal oxide semiconductor) sensors are manufactured using the same process as computer CPUs.

Thus they need a voltage converter at each pixel to feed the ADC via more conventional microprocessor wiring. The ADC and other functions are often included on the same chip as the pixel array. Most sensors of both types achieve colour using a system known as a