Exposure
Right Image: 35mm SLR, 100mm Macro Lens, Fuji Provia 100, 1/30 sec at f-11, aperture priority, multi-segment metering. Bogen tripod and ball head. To take the shot above, I just selected my aperture of f-11 on aperture priority and let the camera choose the shutter speed. Since everything in this image is of average tonality, no exposure compensation was required.
Shutter
speeds and apertures
With built-in
multisegment, spot, and centerweighted metering in modern SLRs, most photographers don't
pay much attention to exposure settings when taking pictures. They believe their cameras
will set the correct exposure and all they have to do is press the shutter release button.
If you want to be in total control of your photography you must understand exposure.
First step is to learn about shutter speeds and apertures (f-stops), as together they
control exposure. Shutter speeds control the length of time you keep the shutter open to
allow light to expose the film, while f-stops control the amount of light reaching the
film. Some standard shutter speeds are: 1,1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250,
1/500, 1/1000 sec. Most today's cameras offer longer and shorter shutter speeds going as
long as 30 sec. to a blazing fast 1/12000 sec. All these numbers are called stops. 1/30
sec. is one stop faster than 1/15 sec. but half as fast as 1/60 sec. Shutter speeds
control motion in a photograph. To freeze a runner you need a faster shutter speed. To
create blur in a waterfall you select a slower speed.
Aperture settings are marked on lenses or selected with the camera in later
35mm and digital SLRs. They also work in stops. Some f-stops are: f-1.4,
2, 2.8, 4, 5.6, 8, 11, 16, 22 and 32.
Not all lenses have all these f-stops. Smaller
numbers represent larger apertures and allow in more light. Larger numbers represent
smaller apertures and allow in less light. f-8 is one stop larger than f-11 and allows
twice as much light, but one stop smaller than f-5.6 and allows half as much light.
Aperture settings control depth-of-field. Depth -of- field is the area in a photograph
which is in sharp focus. Smaller f-stops provide a greater depth-of-field than larger
f-stops. If you want to photograph a scene such as a landscape and want everything from
foreground to background to be in sharp focus, you stop down to a smaller f-stop. When
photographing a model and want the background to be blurred you open up to a larger
aperture.
Shutter speeds and f-stops work together. It's a partnership. For example,
if the proper
exposure for a scene is 1/60 sec. at f-16. You would get the same exposure by using 1/125
sec. at f-11 or 1/30 sec. at f-22. What settings you choose depends on what you are
photographing. If depth-of-field is your concern choose an f-stop and adjust your shutter
speed accordingly. If you need to control motion in your photograph do the opposite.
| Shutter speed | Aperture |
|---|---|
| 1/1000 | 2 |
| 1/500 | 2.8 |
| 1/250 | 4 |
| 1/125 | 5.6 |
| 1/60 | 8 |
| 1/30 | 11 |
| 1/15 | 16 |
| 1/8 | 22 |
Above example shows different aperture/shutter speed combinations. All above settings will provide identical exposure. Depth-of-field and motion will differ, but overall exposure remains the same.
Calculating Exposure
You can calculate exposure several ways. The easiest and fastest way is your built-in camera meter if you know how they work. All meters such as multi-segments, spots, or Centerweighted meters are designed to produce a middle tone exposure. However, they each do this differently. Multisegment meters calculate a middle tone exposure by dividing the scene into several sections. Spot meters read a small portion of the subject to produce middle tone. Centerweighted meters take an average reading of the whole scene for a middle tone result. A middle tone has an average tonality. It's neither white nor black, neither light nor dark but half way in between. The main gray border background of this page is a good example of a middle tone subject. With a middle tone subject you simply meter and shoot. As always you can use different shutter speed and f- stop combinations.
This is an 18% gray color
If your subject is lighter or darker than middle tone you need to make adjustments. First take a reading of your subject using a spot meter if your camera has one or attach a longer lens. Set the recommended shutter speed and f-stop. If your subject is lighter than middle tone you must open up from the meter reading. If your subject is darker than middle tone you must close down. How much to open up or close down? That depends how light or dark the subject is. By opening up one stop from your meter reading you make any color light. Two stops makes colors very light. By closing down you can do the opposite. Close down one stop and any color you have metered will become dark. Close down two stops and you get very dark colors. 1/2 stop adjustments make finer tuning of any color. Slide shooters beware! slide films have a range of +/- 2 stops. By going 2 stops over or under meter reading you will loose all details by severe underexposure or overexposure.
Use the following chart as a guide when metering different colors.
|
 |
Depending on the tonality
of your subject you can use the above chart to meter any color. Meter your subject first.
If your subject is medium color use the camera meter reading. If it is very dark, dark,
light, or very light use the chart to set proper exposure.
Metering white and black subjects
Metering white and black subjects are a bit tricky. White reflects a lot of light
confusing the meter. In bright sun when metering a white subject take a reading using a
spot meter or a longer lens and then open up by 1-1/2 to 2 stops. If you shoot at what the
meter recommends the final result will be a grayish subject. When photographing white
subjects in shade or on overcast days you can open up 1/2 to 1 stop. Black subjects don't
reflect that much light. If you meter black your final result will be an over exposed
subject by several stops. The best way to photograph black subjects is to meter a middle
tone area which is in the same light and then opening up by 1/2 to one stop. This method
is another way to meter any color. Find something middle tone such as green grass, tree
trunks, rocks or something else with average tonality which is in the same light as your
subject. Meter the middle tone. Use the setting from the middle tone reading and then recompose to
photograph your subject.
Right Image: To meter this dull's sheep, I took a reading off its face and opened up 1 stop.
35mm SLR, 300mm f-2.8 lens, Fujichrome Velvia 50. Bogen tripod and ball head.
18% gray cards
A third method is to use an 18% gray
card. Gray cards being medium tone in color are used for metering. You meter the gray card
in the same light as your subject and then use the setting obtained by the gray card to
take the picture. The problem with gray cards is that you cannot always place the card in
the same light as your subject. For example, when photographing a wild animal that you
can't get close to. As an alternative to gray cards, you can take a reading from the palm
of your hand and open up by 1 stop. Just make sure to meter your palm in the same light as
your subject.
Sunny f-16 rule
In bright sun light you can totally ignore your camera meter regardless of how
sophisticated it might be by using the Sunny f-16 rule. In bright sunlight the correct
exposure for a middle tone, frontlit subject is the shutter speed which is closest to your
film's ISO number at F-16. With an ISO 50 film the correct exposure would be 1/60 sec. at
f-16. For ISO 100 films 1/125 sec at f-16 ; ISO 200 1/250 sec at f-16. You can use any
shutter speed / f-stop combinations which is equivalent exposure. For example with ISO 50
film you can use 1/125 sec at f-11 or 1/30 sec at f-22 . Keep in mind that this method
works in bright sunlight for middle tone, frontlit subjects only. For sidelit subjects
with medium tonality open up one stop from the Sunny f-16 rule and for backlit subjects
open up two stops. For example, using ISO 50 with a sidelit middle tone subject your
setting would be 1/60 at f-11, and with backlit subjects 1/60sec at f-8 or any other
combinations of shutter speed and f-stop with same exposure value.
Sunny f-16 rule when used to photograph white and black subjects don't work. Whites become
washed out and loose detail and blacks become underexposed. In bright sun for white
frontlit subjects close down one stop from Sunny f-16 and for black frontlit subjects
close down half stop. In other words Sunny f-16 becomes Sunny f-22 for white and Sunny
f-13 for black subjects.
Right Image: To photograph this Verreaux's eagle I spot metered the background vegetation and set my exposure based on this reading. 35mm SLR, 300mm f-2.8 lens, 1.4x converter, Fuji Velvia 50. Bogen tripod and Ball head.
Incident light meters
The last method of metering is by using
an incident light meter. Unlike camera meters which measure the light reflected by the
subject, incident light meters measure the light that falls on your subject. You simply
point the spherical diffuser at the light source and take a reading and transfer this
setting into your camera set on manual mode. Incident light meters are a great tool for
measuring light as they are not fooled by a subject's brightness or tonality.
Recommended reading:
Understanding Exposure: How to Shoot Great Photographs with a Film or
Digital Camera
More than 100 vivid, graphic comparison pictures illustrate every point in this classic and can help any photographer maximize the creative impact of his or her exposure decisions. Peterson stresses the importance of metering the subject for a starting exposure and then explains how to use various exposure meters and different kinds of lighting. The book contains lessons on each element of the triangle and how it relates to the other two in terms of depth of field, freezing and blurring action, and shooting in low light or at night. A section on special techniques explores such options as deliberate under-and over-exposures, how to produce double exposures, bracketing, shooting the moon, and the use of filters. Understanding Exposure demonstrates that there are always creative choices about how to expose a picture - and that the decision is up to the photographer, not the camera.