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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
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.
Why not use ISO 3200 film all the time?
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
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.
What about rating Fuji Velvia at ISO 40?
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
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:
how much light is available for your picture
how fast is the lens that you're using (i.e., what aperture do you
expect to use)
how fast is your subject going to be moving
how fast is your camera going to be moving
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
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
Your basic options are color negative, color positive, and black & white
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.
Where to Buy
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.
However, buying gray market film from one of the large New York
is a reasonable way to economize.
For up-to-date recommendations for specific emulsions, see
the photo.net film recommendations page.
Notes for Nerds
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.