When you look at the photographic word, you see a great change, the change from analog to digital. Today most amateur and professional photographers are using digital cameras, but some are still using film. Also Some studio work is still done with slides and film, due to the cost of large digital backs. The following article by Dov Klein sheds some light about the terms used when evaluating film, though most of the terms are relevant for digital sensors.

Film

Film is the media used to record a photograph. The media is composed from a flexible celluloid sheet (that brownish-orengish plastic, where the holes in the film are), covered with emulsion (A gel-like chemical substance).

The emulsion layer holds millions of microscopic light sensitive, silver halide, particles. When a film is exposed to light, a chemical reaction is formed and the silver halide particles bond together, to create an "unseen" image. That unseen image, will be visible after processing the film.

When we are talking about black and white film, the exposed silver halide particles are turned black. The "thickness" of this black, depends upon the amount of light that hit the film. When the film is developed, the silver halide from all the areas that were not exposed is washed away, to reveal the celluloid base. It is in that time, that you can first see the image on the film.

With color film or "negative", the same principle applies, only instead of one emulsion layer, you have three layers. Red, Green and Blue (yap – RGB). In addition to the silver halide, each layer also holds chemicals known as dye. The dye makes the silver halide sensitive to different colors. So different layers are sensitive to different colors. During the development of the film, each layer creates a color depending on the dye it had. For example, if the object photographed is read, the red layer will be "dyed black", and the color seen after developing the film will be cyan (result of blue and green).

The slide film or "positive", is allot like the color negative. When light hits the film, silver halide particles also cling to each other. The dyes used and the development process is different. This is why the color on the slide is exactly the same as the color of the photographed object. For example, a red ball in real life will be seen as a red ball in the positive.

Before we get to know the parameters of films, there are several term, that are worth knowing.

Shadows – The dark regions of a picture

Highlights – The brightest regions in the picture

Midtones – Regions with average brightness – not too dark and not too bright.

Film speed – A quality of the film, which the film maker determined during manufacturing of the film. The film speed is the time needed to expose the film to create a good chemical reaction.
The Film speed is measured in units called ASA (or ISO). The higher the ASA value is, the more sensitive the film is. A 200 ASA film is twice as sensitive then a 100 ASA film. Or in other words, a 200 ASA film would need half the amount of light to create the same exposure.
If, for example, 100 ASA film needs to be exposed with f/16 for 1/125 seconds, a 200 ASA film will need only 1/250 seconds with f/16 to achieve the same exposure, and a 400 ASA film will only need 1/1000 of a second with the same F stop.
They call it the film speed because the ASA value determines "how fast" the film reacts to light.
It is usually said that films with ASA higher then 400 ASA are fast, because the respond fast to light. And it is also usually said that films with ASA value lower then 100 are slow, because they take longer to respond to light. The 200 ASA film is considered of medium speed.

The lower the speed of the film is, the finer its granularity, so it will be better for making huge prints, 50cmX70cm, for example. Granularity is the term used to specify the thickness of the silver halide grains on the film.
Low speed films are usually sharper, has richer colors and better contrast.
Today’s 400ASA films are made with top quality, and you can print a 20cmX30cm, almost without noticing the grains.

Usually we will use films up to 100 ASA when we have very good light conditions, on a sunny day. When shooting outdoors. If we know we want to make a large print, and the light conditions are not so good, we can use a tripod. The tripod will allow us to shoot at low speed need for slow films.
200 ASA films are usually used when the light source is a big window, on a partially cloudy day, or for general purpose / family shots and trips.
400 ASA and up, are used for sports, or low lighting conditions, like stage lights, street light and so on.

Some basic terms:

Resolution – The ability of a film to display the finer details shot in a clear way. Resolution is measured in lines per millimeter, the more line you can see per millimeter, the higher the resolution is.
Resolution checks are performed using a specially drawn target with spaced lines. After developing the film, a check of the number and location of lines seen, and determines the resolution of the film.

Sharpness – The film’s ability to display the edges of small details in a clear way. The more lines per millimeter you can distinguish, the higher the sharpness is.

Color saturation – The film’s ability to display color intensity with respect to the original colors. The more intense the color is on the film, the higher the saturation is.

Granularity – Granularity refers to the size of the silver halide grains on the film. The grains are just like sand grains. The rains are a byproduct of the development process, and each manufacturer uses different techniques to reduce the size of the grains. Granularity is measured by RMS (Root Mean Squares).
Larger granularity will make a picture composed of large dots, and finer granularity will make for a smoother picture. Granularity is usually bigger for faster films.

Contrast – Contrast is the film’s ability to show differences between shades of color: shades of highlights, shades of midtones and shades of shadows.

High contrast films – will emphasize the difference between different shades in the picture. The changes between highlights and shadows are very clear, and there are almost no midtones in the picture. The result of shooting with a high contrast film will be emphasizing a subject with low contrast.

Medium-high contrast films – will show very little midtones. They will show the gradient of highlights and shadows in the picture.

Medium contrast films – will show very good details both in the highlights regions of the picture and in the shaded regions of the picture. Medium contrast films will have very good gradient in the midtones. Most films are Medium contrast films.

Low contrast films – will show a wide spread of shades, all over the picture – starting with shadows, through midtones ending with highlights. Low contrast films will make a contrasty view to look like average view. Also low contrast films has the tendency to "flatten out" medium contrast subjects.

Exposure range – is the films ability to show a reasonably exposed picture, with good amount of details in all the regions of the picture: highlights, shadows and midtones.
The Exposure range is measured in stops, and it is bigger if the film can decently show details in more stops. The average negative film has a range of 5 stops, from -2 stops to +3 stops. The average slide positive has a lesser exposure range, from -1.5 stops to +1 stop.

Reciprocity failure – (or the Schwarzschild effect) is a decrease in light sensitivity (speed) with increased length of exposure. That means that the longer you expose the film, the effect of new light hitting the film will have less influence. Usually reciprocity failure data is indicated on the data sheet of the professional film.