by Philip Greenspun; revised January 2007
You've settled on the light and on your position relative to your subject and the magnification (i.e., the lens). In the good old days, people such as Leonardo da Vinci would simply put a piece of paper where an image was formed and start drawing. The mediocre results that he achieved aren't likely to satisfy a serious-minded visual artist today. So perhaps it is time to think about putting some film behind the lens to record the image.
What is film? A chemist would say that it was "silver halide crystals suspended in gelatin on a base manufactured by extruding heated polyester material through a slot, thus orienting the molecules." For the purposes of this article, we need only understand that slide film is magic stuff that starts out opaque and turns transparent as more light hits it. Negative film is magic stuff that starts out transparent and turns opaque as more light hits it.
Let's start out by thinking about the simplest possible kind of film: black and white positive (slide) film. Only one product has been manufactured in this category within recent memory, Agfa Scala. Here's a graph showing Agfa Scala's response to light:
The amount of light (lux-seconds) is increasing on the x-axis. The opacity of the film is increasing on the y-axis. Note that there is a limit to "how opaque" this film can get. No matter how little light strikes the film, it will never be denser than "3" (see the notes for nerds at the bottom of this page to see why this means 1/1000th of the light gets through; this is actually roughly equivalent to the maximum density that typical desktop CCD slide scanners can read). If you look at the right of the graph, you'll notice that the film doesn't ever become perfectly clear. More than +1.0 (log lux-seconds) of light doesn't really make the film any more clear.
Staring us in the face is the first thing that we need to know about film: it has its limits. Only over a small range of light intensity does this film react differently to different amounts of light. Anything less than -1.0 and the film is "simply black". Anything more than +1.0 exposure and the film is "simply clear". Since this is a log chart, this means that the film will register detail over a 100:1 range. In the next chapter, I'll show you how to use your camera's shutter speed and aperture controls to make sure that this 100:1 covers the most important tones in the scene you've chosen to depict. Right now, let's think how this information is presented to consumers.
If you go to the camera shop, the Agfa Scala box does not have a curve on it like the above. The only information on the box about how the film will react to light is "ISO 200". What does that mean? Let's look at charts for two color slide films.
Agfa RSX 100 (ISO 100)
Agfa RSX 200 (ISO 200)
Note first that each chart shows three curves, one for each color-sensitive layer. Note second that the ISO 200 film (at right) requires less light to reach a certain level of clarity. If you ask "how much light must fall on the film before it is transmitting 1/100th of the light (density 2.0)?", you can see that this happens for the ISO 200 film at about a log lux-seconds of -1.8 (that's 10^(-1.8) or 0.016). For the ISO 100 film, this requires -1.5 log lux-seconds (10^(-1.5) or 0.032 lux-seconds). Hmmm... the ISO 100 film took twice as many lux seconds to reach the same level of transmittance. So it is perhaps half as sensitive to light.
Bing! The ISO speed on the side of the film box tells you roughly how sensitive to light a film is. But it is merely a summary of the information that is available in a sensitivity curve. Why does the sensitivity curve matter? It tells you the range of tones that you can represent with a film and the range of exposure that will result in distinguishable tones. It also tells you about the film contrast, i.e., how a given change in exposure will affect the density. A film with a flatter curve will hold more detail in shadows and highlights than a film with a steeper curve.
Let's switch to color negative films now. Density will rise rather than fall with increasing exposure. Use your browser to open up the curves for Kodak Pro400 and those for Kodak Pro400MC in separate windows. These films have the same ISO ratings but note how much less steeply the density for Pro400MC rises. The "MC" in this case stands for "medium contrast", a slightly lower contrast film than their standard Pro 400 product. Kodak expects that the lower contrast film will be more useful at a wedding, for example, where the groom is dressed in black and the bride is in white and you want to see some detail in both outfits.
With black & white film, you can easy change a film's contrast-recording characteristics by changing the development time. See the figure below for how Kodak Tri-X Pan Professional, a legendary ISO 320 black and white negative film, reacts to changes in development time.
Note that the longer the development time, the steeper the curve and therefore the higher the contrast. Also, the longer the development time, the less exposure is required to reach a given density. Increasing development time increases the film's speed and the film's contrast.
The most serious black & white photographers use big cameras that expose film in sheets that must be processed one at a time. Since they are going to have to process exposures individually, these photographers figure they might as well take advantage of the flexibility offered by changing the development time. If they have a high contrast scene and want detail in a wide range of areas, they'll plan to develop the film for less time than Kodak might suggest. This will reduce contrast. But it will also reduce sensitivity so they'll increase the exposure a bit. Ansel Adams systematized this kind of thinking into the Zone System.
With color film, this becomes trickier. Look carefully at the curves for the Agfachrome slide film. Notice how they diverge a bit when the light is scanty. The different layers of the film are reacting differently. Designing color film is a delicate process and almost anything you do that diverges from what the manufacturer expected is likely to affect one layer more than the others. This will result in a color shift. Nonetheless, there are photographers who use overdevelop (push) and underdevelop (pull) color slide film specifically to increase or decrease the contrast.
The higher the ISO, the more sensitive the film is to light. Double the ISO and you halve the amount of light necessary for a photo. All cameras have ways to throttle back the amount of light reaching the film (e.g., by exposing for only 1/8000th of a second). So it would seem that the best approach to photography would be to buy the highest speed film available, e.g., ISO 3200, and use that all the time. Then you can take photos when it is nearly dark or take photos when it is bright and sunny.
Kodak is in fact trying to convince people to do just this. They've got an ISO 800 film that they call "Gold MAX". Like all color negative films, it tolerates exposure errors very well and especially tolerates overexposure. Like all high-speed films, it is much grainier than low-speed films. It also has reduced color saturation and reduced color accuracy. So why is Kodak telling people to shoot it? First, because so many people have bought zoom point & shoot cameras with lenses that gather almost no light. Second, because most people are apparently practically blind and can't tell the difference between a print from an ISO 100 negative and a print from an ISO 800 negative. It helps that people seldom enlarge beyond 4x6".
If you were just looking for a way to put a 4x6" print on your fridge, you probably wouldn't have read through all those graphs up top. So let's assume that you're a little more discriminating than Kodak expects the average person to be. If so, you want slow film. Slow film has finer grain and better color. Slow film resolves finer resolution than fast film. It requires longer exposure times, so you might need a tripod (see below). At some point, it becomes wasteful to use slow film. If you don't plan to enlarge very much, the fine grain and high resolution of the slow film won't do you any good. If you have a cheap low-resolution zoom lens, then improving film resolution beyond a certain point isn't of any practical value.
It turns out that the best modern films are rated between ISO 25 and ISO 50. A typical example is Fuji Velvia, ISO 50 color slide film. It requires 8 times as much light (a lens opening 3 f-stops larger or a shutter opening 8 times as long) as ISO 400 film. But it rewards the user with super fine grain, high-resolution, and beautiful colors.
Fuji Velvia comes in a little green box that says "ISO 50" on the side. You'll have to read Basic Photographic Materials and Processes if you want to know how Fuji came up with that "50" number. A lot of photographers set their cameras on automatic and found that they'd lost detail in shadows. The photos looked a bit dark. They started "rating Velvia at ISO 40." This means that they turned the little knob on their cameras to 40 instead of 50, resulting in a camera on automatic giving the film 1/3 f-stop (about 25%) more exposure.
Does this mean that they've got a correct exposure at ISO 40? No. There is no such thing as a correct exposure. The real world generally contains a wider range of tones than you can represent on film. You have to make an artistic decision about where you place those tones. Some detail will inevitably be lost as tones that are distinguishable in the real world are mapped by the sensitivity curves to the same density on film.
If you care about details in the shadows and are using a high-contrast film like Velvia, you have to be careful to give the film enough exposure so that the tones you care about are mapped onto the linear portion of the curve.
You're in the camera shop. You have to pick something. You don't have time to read all those curves. You just need to pick a film speed and a film type. Let's do the film speed first. Here are the questions you need to answer:
Let's gloss over the subtle issues and assume that all you want to avoid is a blurry picture. Faster film helps you avoid blurry pictures in two ways. First, you can close the lens down more ("stop down to a smaller aperture"). This results in a greater depth of field. More objects in the scene will be in focus. Your subject can move a little bit forward or back and will still be in focus. You or the camera can be a little bit inaccurate in focussing and your subject will still be acceptably focussed. A second way that faster film helps you avoid blur is by letting you use a faster shutter speed. The camera is taking a shorter snapshot of the world. This gives your subject less time in which to move. This gives you (holding the camera) less time in which to move. This gives the wind less time in which to shake leaves on trees. This gives the wind less time in which to shake your camera on its tripod.
If there is a tremendous amount of light available, e.g., high noon on a bright sunny day, then you can probably use even the lowest speed film for most purposes. You can handhold the camera and still use fast enough shutter speeds. If there isn't much light available, e.g., flash from the puny strobe on a point and shoot camera, then you will need fast film.
If you don't need much depth of field and you have a big professional-grade lens then you'll be able to use a large aperture, e.g., f/2.8. This will gather a lot of light and let you use short enough shutter speeds to freeze your movement (holding the camera) or your subject's movement.
If your subject is lying down asleep then you can use slow shutter speeds (and therefore slow film) without worrying about subject blur.
If you have bolted your camera down on a tripod then you can use slow shutter speeds (and therefore slow film).
It is tough to talk about film speed before talking about exposure and vice versa. So I'm sorry if this chapter has seemed confusing up until now.
Your basic options are color negative, color positive, and black & white negative.
Color neg film is good for making prints because that's what it is designed for. Neg film is also good for scanning with cheap scanners because it never gets too dense. Neg film is good when you can't be too sure about your exposure and/or where the scene contains important detail in a wide range of tones. Neg film is good for when you need 1-hour processing in obscure places.
Color slide film is good for making you feel good about yourself as a photographer. It costs about 50 cents/image to get beautiful slides (versus $100/image to get beautiful prints from color neg film). Slide film is good for selling to magazines. Slide film is good for scanning with expensive drum scanners because the service bureaus understand it. Slide film is good for printing because the printers understand it and the slide itself serves as a witness to what it should look like.
Black and white negative film is good for giving you control over contrast and density without your having to invest $200,000 in digital imaging hardware. B&W film is also archival, unlike color film which can fade alarmingly fast.
Try to buy film from a professional camera shop. These shops have fresh inventory and keep most of their stock in large refrigerators. If you want to save money, don't try doing so by bulk loading your own rolls. It is too difficult to avoid getting dust inside the canisters.
Transmittance (T) is defined as the ratio of transmitted light over incident light. It is usually expressed as a percent. So 1% transmittance means that 1/100th of the light got through a piece of film or a lens or whatever. Opacity is 1/T, the reciprocal of transmittance. An opacity of 100 is equivalent to a transmittance of 1%. Density (D) is the log (base 10) of opacity. If you have a "way black" piece of film that only lets through 1/1000th of the light (0.1%) then you have an opacity of 1000 and a density of 3.