If you ask a professional for some exposure advice, the typical answer
is "f/8 and be there." This is a bit of an in joke. The "f/8" part of
it sounds vaguely technical and useful, since f/8 is an actual aperture
that you can set on most lenses. But it doesn't mean anything without
an accompanying shutter speed or film ISO. The "be there" reminds you
that ultimately exposure is pretty easy. The most important thing to
have is patience and dedication so that you're around when a great
photograph is happening.
There is no correct exposure
As I noted in the chapter on film, the real world
generally contains a wider range of tones than you can represent on
paper, film, or even with the best digital sensors. 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 to the same number out of a digital sensor or density on film.
This chapter will teach you how to control and predict which details are
Single-lens reflex cameras have an intimidating array of buttons. It will
please you to know that there are only three controls that affect the
imaged: focus, aperture, and shutter speed. The two controls
that affect exposure are aperture and shutter speed.
If neither the subject nor the camera are moving, the shutter speed is
not very important. Aperture, however, affects the depth of
field and therefore which portions of the image will be in focus.
What is aperture and why is it useful to change it? Aperture is the
degree to which the iris or diaphragm inside the lens is opened. Lenses
are designed for maximum light-gathering capability. The diaphragm is
just like the iris in your eye; it can be closed or stopped down
to block off a portion of the light coming through the lens. A lot of
expense and weight went into making your lens fast or good at
gathering light. Why would you want to throw away some of that
The first reason to stop down a lens is that the world might simply be
too bright. If you're using high-speed (sensitive) film and have a slow
shutter that must expose the film for at least 1/500th of a second,
using a smaller aperture is the only way to prevent too much light from
striking the film and overexposing it.
A more interesting reason is for aesthetic control of sharpness.
Suppose the lens has a maximum aperture of f/2. The f-number is the
lens length divided by the diameter of the aperture opening. So for a
100mm lens, this would be a 50mm opening. The depth of field will be
shallow. Only the object on which you focussed will be sharp. Things
closer or farther from the camera will be out of focus. The range of
distances for which objects are acceptably sharp is called the
"depth of field". Notice the word "acceptably" in the definition. What
is acceptable in an 8x10 print viewed from across the room may not be
acceptable in the same print viewed at arm's length. What is acceptable
in an 8x10 print viewed at arm's length may not be acceptable in a
30x40 print viewed at arm's length.
If you want more objects in the scene to be acceptably focussed, you
have to stop down the lens to a smaller aperture, e.g., f/16 or f/22.
This nomenclature is a bit confusing at first for beginners because a
smaller aperture means that the lens length divided by the aperture
diameter gets larger, yielding a larger f-number. Even more confusing
is the fact that lenses are calibrated with a strange succession of
apertures: 1.4, 2.0, 2.8, 4.0, 5.6, 8.0, 11, 16, 22, 32, 45, 64. Each
step represents a halving of the amount of light that comes through the
lens. Why? The area of the aperture is proportional to half the
diameter squared. So multiplying the f-number by the square root of 2
halves the amount of light coming through the lens.
Let's look at some example images.
With a long lens and a wide aperture, the depth of field is very narrow.
Only those objects exactly at the focussed distance will be sharp. For
example, here are a couple of images taken with
a 600mm lens at f/4 or
Notice that only the birds are sharp and the backgrounds are soft. The
effect may seem rather extreme given that f/4 and f/5.6 are not
ordinarily considered super wide apertures. Depth of field is related
to the absolute size of the aperture not the f-number (lens length
divided by aperture diameter). A 600mm lens is a big honker and an
f-number of 4 implies an aperture 150mm across. I.e., the depth of
field at f/4 on a 600mm lens will be shallower than at f/1.0 on a 50mm
One way to achieve overall image sharpness is to choose a composition
where everything is roughly the same distance from the lens (50mm):
Another approach is to stop the lens down to a small aperture. Note
here the leaves in the upper right corner of the frame and the trees at
infinity. Both are sharp thanks to the f/16 aperture used on this 50mm
The best way to learn about depth of field is to put your camera on
tripod and expose the same image at different apertures. In these
examples, note how much clearer the background is at f/22 than wide open
If you're using a single-lens reflex camera, where what you see through
the viewfinder is what the film will see after the mirror flips up, you
might be confused at this point. You turn the aperture ring on the lens
and the image remains just as bright in the viewfinder. Moreover, out
of focus objects don't get any sharper as you stop down. You're using a
lens with an automatic diaphragm, introduced in the 1960s. The
lens will be stopped down by the camera an instant before exposure, just
as the mirror is flipping up. If you're just viewing and composing
pictures, the lens is kept wide open for maximum brightness. To see
what the film will see, you press the depth of field preview
button. This lets you visualize in the viewfinder the focus effects of
stopping down the aperture but it takes some practice to adjust to the
extreme dimming that occurs by f/11 or f/16.
For a given amount of exposure on the film, the shutter speed can be
determined by the aperture that you set for aesthetic purposes. If you
are taking a portrait and want to throw the background out of focus,
choose a wide-open f/2.8 aperture. Suppose that implies a shutter speed
of 1/125th of a second. If you change your mind and want to ensure that
the background is sharp, stop down to f/22, 6 f-stops less light. The
film will need to be exposed for 2^6 times as long. Two raised to the
6th power is 64 so you'll need a shutter speed of 1/2 second to achieve
the same density of exposure on film.
A camera with built-in meter can do this calculation for you.
Professional photographers most typically use an exposure mode called
"aperture-priority autoexposure". The photographer picks the aperture
and the camera picks the shutter speed. Does it matter what shutter
speed the camera picks? Not as long as neither the camera nor subject
is moving. If they are standing up, most subjects won't be able to hold
acceptably still for the 1/2 second exposure mentioned above. The
photographer will be advised to open the aperture until the shutter
speed is 1/15th second or faster. If the photographer is handholding
the camera, i.e., not using a tripod, the 1/15th of a second exposure
will very likely result in an unacceptable amount of camera shake
being recorded on film. When using a normal lens, the general rule is
to use shutter speeds of 1/60th or faster. Longer lenses magnify the
subject but they also magnify camera shake. The traditional rule for
handheld photography is to use shutter speeds of at least
1/focal-length. So if you've got a 250mm lens you'd use shutter speeds
of 1/250th or faster. You'll be well advised to use faster speeds if
you intend to make big enlargements from your originals. You can get
away with slower shutter speeds if you either (1) brace yourself against
a solid object, (2) rest the camera/lens on a solid object, or (3) use a
lens with electronic image stabilization, as explained in
the photo.net review of the Canon 600/4 IS
There are sometimes aesthetic reasons to use different shutter speeds.
If you are taking a picture of something moving and want to show the
motion, you'll need a slowish shutter speed. If you're taking a picture
of something moving and want to freeze the motion, you'll need
a fastish shutter speed, the exact speed depending on the velocity with
which your subject is moving and whether the direction of moving is
towards the camera or sideways across the frame (note: the best way to
freeze motion is with an electronic flash, which is actually a kind of
strobe light; a cheap on-camera flash may have a duration as short as
1/30,000th of a second).
The shutter speed here (Berlin) isn't fast enough to
freeze the camera shake induced by the photographer panning to follow
the bike rider. Note the blurred arm. The image remains successful
because the blurring suggests motion.
This image was taken from a moving car at 1/15th or 1/30th of a second,
slow enough to blur the background but fast enough to keep
NSX reasonably sharp (since the photographer's car and the subject
car were moving at approximately the same speed).
On a bright sunny day with a handheld camera and therefore a fast
shutter speed (1/125th?), the water looks more or less as you might see
it with your eyes.
Some softening of the waves breaking over the rocks due to perhaps a 1/4
second exposure (from Hawaii).
Given the information and examples above you ought to have some idea of
the aesthetic results you're trying to achieve. If you're interested in
the blurring or stopping of motion on film, set the shutter speed
first. If you're interested in what will be in focus, set the aperture
first. If you can't get a combination that suits you, look for a
different speed of film or put a neutral density filter over the lens
to reduce the amount of light coming through without changing what is in
How do you know that you're send the right number of photons through to
the film so that your result won't be completely black (underexposed
slide) or completely white (overexposed slide)? Old-timers using
negative film would simply estimate the exposure from their experience,
then fix up any minor errors in the darkroom. A somewhat more accurate
technique is to RTFM. Here are the instructions included with Kodak
Tri-X, a name shared by two confusingly different films (Tri-X Pan is
ISO 400 and has good midtone separation; Tri-X Pan Professional
is ISO 320 and has more highlight separation):
"Use the exposures in the table below for frontlighted subjects from 2
hours after sunrise to 2 hours before sunset."
Shutter Speed (Second) and Lens Opening
Tri-X Pan Professional TXP, TXT
Tri-X Pan TX
Bright or Hazy Sun on Light Sand or Snow Sand or Snow
1/500 f /16
1/500 f /22
Bright or Hazy Sun (Distinct Shadows)
1/500 f /11*
1/500 f /16¶
Weak, Hazy Sun (Soft Shadows)
1/500 f /8
1/500 f /11
Cloudy Bright (No Shadows)
1/500 f /5.6
1/500 f /8
Heavy Overcast or Open Shade§
1/500 f /4
1/500 f /5.6
* Use f/5.6 at 1/500 for backlighted close-up subjects.
¶ Use f /8 at 1/500 for backlighted close-up subjects.
§ Subject shaded from the sun but lighted by a large area of clear sky.
How well does it work to simply read Kodak's instructions and follow
them as best you can? Quite well with negative film; not well enough
with slide film; not at all when using electronic flash.
Fundamentally, an exposure meter can be built in two ways. The first is
to measure the light falling on the subject that you intend to
photograph: incident metering. The second is to measure the
light coming off the subject in the direction of the camera lens:
reflected metering. The typical handheld accessory lightmeter
gives the photographer a choice between these two methods. The typical
in-camera meter can only measure reflected light. Both kinds of meters
recommend a combination of aperture and shutter speed to the
photographer who will then use that recommendation as a starting point
when actually exposing film.
When using an incident light meter, the most important source of error
of which the photographer must be aware occurs when the light is highly
directional. The incident dome may not catch the light exactly the way
the combination of the subject and camera lens.
When using a reflected light meter, the most important source of error
is that the subject's reflectance may not match the meter's assumption
about the subject's reflectance. Suppose that you're taking individual
portraits of Alex and Mia (at right). You measure the light being
reflected off Alex's white fur and set the camera to whatever the meter
recommends. Repeating the image with Mia as the subject you find that
much less light is reflected by her black and brown fur. So the
reflected light meter recommends a wider aperture or a slower shutter
speed than it did for Alex.
Does this make sense? With negative film, perhaps. Mia is darker and
if you want to get her tones into the linear portion of the film's curve
you'll need a longer exposure. But consider that if you'd used an
incident light meter it would have recommended the same exposure for
both dogs. After all, the same amount of light was falling on them. If
you'd used color slide film and the incident meter's recommendation
you'd get one slide with a white dog in it and one slide with a black
dog in it. What if you'd used the reflected meter's recommendation
with the slide film? You'd get two slides exposed with an identical
amount of light and therefore both would be the same shade.
Exactly what shade do you get when you follow a reflective meter's
recommendation? 18% gray. This is a tone midway between 0% gray
(white) and 100% gray (black). Reflected meters are calibrated to
assume that the average scene is 18% gray. The reflected meter couldn't
know that Alex is a white dog and that Mia is a black dog. When you
pointed it at Alex it assumed that the day had gotten brighter. When
you pointed it at Mia it assumed that the sky had become cloudier.
Is this 18% gray assumption reasonable? If you take portraits of
Caucasian people and meter off their facial skin you'll probably find
that your slides come out a bit too dark. Typical Caucasian skin is
about 1/2 f-stop lighter than 18% gray. So the reflected meter thinks
that the subject is lit somewhat brighter than in reality.
Here are some examples:
A dream scene! All roughly the same tone. All roughly 18% gray.
A nightmare. The snow is white but the meter might also pick up on
some of the dark trees. And would we really want to add exposure until
the white was super white on film? We're trying to suggest evening
here. This is a good occasion for bracketing!
Slightly challenging. The key here is to make sure to meter only the
central (illuminated) portion of the frame so that the black sky does
not get averaged into the exposure calculation. Then open up 1/2 to 1
f-stop over the meter's recommendation so that the builder is rendered
white rather than gray.
Same challenge. Use the in-camera spot meter of the Rollei 6008 to measure only the
brightly illuminated cliff face, then open up 1/2 stop over the meter's
recommendation to render it bright on film. Then try another exposure
at 1 stop over because it is tough to get back to
the bottom of the Grand Canyon.
Point, meter, open 1 stop to move the shells from gray to white, click.
From Cape Cod.
Point, meter, close 1 stop to move the lava from gray to black, click.
There are some details that can make life painful when setting exposure.
As you focus closer to a subject you are moving the lens farther from
the film. The lens is throwing the same amount of light in a larger and
larger circle of which the film intercepts a smaller and smaller
fraction. For small format (35mm) cameras this effect is not
significant until you get a macro lens and
start taking pictures of things comparable in size to the 24x36mm frame
itself. However, if you are taking macro photographs and following the
recommendations of a handheld light meter you will find that your
pictures are underexposed by 1 or 2 f-stops.
The handheld meter, whether reflected or incident, can't know what
impediments there are to light reaching the film. The meter
manufacturers assume an ideal lens. Your lens may be covered with a
fine coating of dust. Your lens's internal elements will not be
perfectly transmissive; some light will be lost each time it goes
through a piece of glass within your lens. You may have stuck a filter
in front of the lens.
A good way of sweeping away all of these details is the
through-the-lens meter. Necessarily a reflected light meter, the
metering cells are placed behind the lens and in front of the film,
oftentimes built into the viewing system. These cells see what the film
will see and therefore if light is getting blocked for any reason the
meter simply sets the exposure as if there were less overall scene illumination.