Overview of Film, Digital Cameras and Scanning

Film

There is a fundamental difference between images captured with a film camera and those taken with a digital camera and that is the storage medium. Film is a physical medium coated with a light sensitive Silver Halide (salt crystal) emulsion and divided into frames. When these frames are exposed to light through the camera lens, a chemical reaction affects the salt crystals, effectlively trapping the image invisibly on the film. That frame of film is now used so the next frame is wound on in order to capture another image. The developing process later reveals and secures the invisible images.

Depending upon the film grain (salt crystal) size, an incredible amount of detail can be resolved from these tiny images. Films are rated at different speeds usually denoted by an ASA/DIN or ISO rating, the higher the number, the faster the film can react to the available light. The faster the film, the less exposure it needs in order to get a stable picture, ideal for people who take action photos and need to avoid motion blurring. An ISO rating of 50 is quite slow compared to ISO 800, although the picture quality is better due to the smaller film grain. The film grain on fast film is usually much larger than that of slow film and so cannot resolve as much detail. Finally, the image has to be developed in a lab before you can use it and even then you can't do much else with it in its native form.

Digital

Digital cameras expose the image using the same initial process and mostly use the same terminology to help film users understand digital equipment, however this is where the similarity ends. Instead of film, the digital camera uses two components. First a light sensitive electronic device called a CCD (Charge Coupled Device) or CMOS (Complementary Metal Oxide Semiconductor) captures the electronic image with a series of dots called pixels (electronic equivalent of the salt crystals on film), each of which produces an electronic signal. Each of these signals is then processed in camera to produce a usable image file, a bit like a Polaroid camera does with its special film (no lab developing required). The image file is then transferred to the second component for storage - a memory card, and the sensor is ready for the next shot.

The dots or pixels (abbreviation of Picture Elements) on the CCD or CMOS can be used again and again without replacing any film, unlike the salt crystals on a film which can only be used the once. When the memory card is full it can be emptied onto a PC or other storage device and reused. The other difference with regards to film speed is that although digital cameras use the ISO rating system for the way that the sensor reacts to light, rather than increasing the size of the dots (as with larger crystals or grain on film), the sensitivity of the dots is increased, like turning up the brightness. The downside of this, especially on dark images is the increase in visual noise or static in the final image.

Different manufacturers use different technologies both for the sensor and storage, but the end result is the same as far as the photographer is concerned. This image is effectively multimedia straight out of the camera, meaning it is ready to be used in a variety of scenarios. It can be copied onto a PC, viewed on a television, projected onto a screen, inserted into a publication, loaded on to a web site or strange as it may sound, processed into a photographic print!

Scanners

Scanners work in a similar fashion to digital cameras. They use a lens and a built-in light source to expose the original item line by line and build up a digital image to be loaded into a computer. Scanners have a very narrow depth of field which is only really suitable for taking pictures of very flat objects. Anything with too much depth will produce out of focus images. Scanners can convert the somewhat limited original film image into a fully multimedia capable file as if it had been taken with a digital camera.

Resolution

Film was always designed to be enlarged into prints in the photo processing lab and as such it works very well for the purposes of digital scanning at very high resolutions. Modern digital scanners are now able to out-resolve film, exposing the film grain at very high resolutions. Try to think in terms of magnification - the greater the magnification, the more detail you can see. A speck in the distance might be seen as a bird at high enough magnification. However, eventually you will start to see the material that the image is caught on, in the same way that if you look closely at a piece of fine art you may see the weave in the canvas and the individual paint daubs.

Digital image resolution is measured in pixels or dots per inch (ppi or dpi). A digital camera's resolution, one of its main selling features, is measured in millions of dots or Mega Pixels. Simplistically, the higher the number, the more detail is resolved and the larger the image file. Basically, a digital image file is made up of dots, all with different colour and brightness signals. If you magnify a digital image you eventually come across a phenomenon known as pixelisation where the dots get so big you can see them as squares. Being perfectly aligned in a grid format, this is more noticeable than the random appearance of film grain at great enlargements. This is why some professionals still prefer to use film.

Small images such as negatives or slides need to be scanned at much higher resolutions than prints in order to produce acceptable enlargements. This is not a problem as slides and negatives usually contain a lot of very fine detail as they were designed to be enlarged and printed. Prints however are not designed to be enlarged any further so you can already see most of the detail that is available. Scanning prints at a very high resolution produces no more detail but it does produce a very large file. There is usually little point scanning a print at anything higher than 600dpi, whereas a negative might be scanned at resolutions in excess of 3200dpi, depending upon the final size of print required.

Depending upon the quality of the film used and the photo taken, some great conversions can be made from film to digital images. A 35mm slide or negative scanned at 2400dpi could produce a digital image equivalent to that taken by a 6 or 7 Mega Pixel digital camera. 3200dpi could be nearer to a 10MP camera. Medium and Large Format film can be scanned at a lower resolution to achieve the same results due to the larger size of the film.

This topic is huge and if you need further information there are much more detailed examples out there both in print and digitally via the web!