Exposure


 

Right Image: Minolta Maxxum 7, 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.

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. Very dark colors (-2 stops) /Dark colors (-1 stop)/Medium (0) / Light colors (+1 stop) Very light colors (+2 stops) Very dark red /Dark red/Medium red/Light red/Very light red Very dark blue/Dark blue/Medium blue/Light blue/Very light blue Very dark green/Dark green/Medium green/Light green/Very light green Very dark brown/Dark brown/Medium brown/Light brown/Very light brown Very dark orange/Dark orange/Medium orange/Light orange/Very light orange Very dark yellow/Dark yellow/Medium yellow/Light yellow/Very light yellow Very dark purple/Dark purple/Medium purple/Light purple/Very light purple Very dark pink/Dark pink/Medium pink/Light pink/Very light pink

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.
Minolta Maxxum 9xi, 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. Minolta Maxxum 9xi, 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.