Labradoodle & Goldendoodle Forum
Photography Lesson: Aperture
Aperture defines the size of the opening in the lens, which in advanced cameras can be adjusted to control the amount of light reaching the film or digital sensor (CCD or CMOS). In combination with variation of the shutter speed, and variation in film speed (ISO), this will regulate the photograph's degree of exposure to light.
A device called a diaphragm usually serves as the aperture stop, and controls the aperture. The diaphragm functions much like the iris of the eye - it controls the effective diameter of the lens opening.
The lens aperture is usually specified as an f-number, the ratio of focal length to effective aperture diameter. A lens typically has a set of marked "f-stops" that the f-number can be set to. A lower f-number denotes a greater aperture opening which allows more light to reach the film or image sensor.
Aperture and shutter speed are closely linked. For instance, in a given situation an exposure of 1/15 s and f22 is needed to get a perfectly exposed image. Opening the aperture to f16, only 1/30 second speed is needed to get correct exposure. 1/15 used originally would result in over-exposure, because now more light enters the camera.
The other thing the aperture directly controls is the Depth Of Field (DoF).
Controlling Depth Of Field
A definition of Depth of Field is that it is the distance in front of and beyond the picture's sharply focused main subject. Taking a picture of a subject 2 yards from the camera with aperture f1.8 would have a very narrow depth of field. The point we focused on will be very sharp, but closely in front and everything behind it will be blurred - out of focus. If the lens is stopped down to f22, a majority of the picture will be sharp. The following diagram shows how the aperture opening affects the depth of field; dark green shows the sharp area of the subject, light green is out of focus:
Using a telephoto lens, the depth of field would be narrow, which nicely isolates image from it's environment. Narrow DoF is used in portraits where we want the distracting background supressed and to focus the viewer's attention on the subject. On the other hand, a wide-angle lens has greater DoF, which makes it good for wide shots like nature and architecture.
Note how changing the aperture in the following example changes the depth of field. See how the background is changing? This is how you can beautifully isolate your subject from a background (last pic) or keep the overall sharpness (first pic).
Aperture f22
Aperture f16
Aperture f8
Aperture f5.6
Aperture f3.5
Aperture f1.8
Depth of field is an optical figure that is influenced by 3 factors:
1. The distance of the object from the camera: This is easy to understand when taken to the extreme. Shooting an object 10 feet from the camera will result in a smaller depth of field than shooting an object 100 feet from the camera. Obviously shooting an object, which for all practical purposes is an infinite distance from the camera, will result in the camera being focused on infinity and the depth of field being infinite too.
2. The aperture: The wider the aperture (f number decreases) the smaller the depth of field and vice versa: the narrower the aperture (f number increases) the greater the depth of field. Just remember that the depth of field changes along with the f number: when the f number increases the depth of field increases and vice-versa. Unfortunately you cannot just change the aperture as you wish. Changing the aperture results in a need to change the shutter speed (either manually or the camera can most likely do that for you automatically in shutter priority mode). The wider the aperture the more light that penetrates the lens and the faster the shutter speed needs to be so there is a limit to how wide you can set the aperture.
3. The focal length of the lens: The shorter the focal length the greater the depth of field and vice versa the longer the focal length the smaller the depth of field. For example if you use normal lenses at somewhere around 50mm your depth of field will be much greater than if you use zoom lenses at 200mm. With long focal lengths the depth of field can be extremely small. For that reason zoom lenses are a good choice in order to shoot a good blurry photo that is practically focused only on the object.
After understanding the theory it is best to go to the field for some experimentation. You will get a sense of how your photo will look and how blurry the background will be after playing with different lenses, different apertures and a variety of distances from your object. If you own zoom lenses start with taking photos of a relatively close object while using your zoom lenses. You will find out that it is extremely easy to get a blurry background photo using this method even without setting the aperture. Once you feel comfortable with that setup start changing the aperture and also change to your normal 18mm-55mm or similar lenses.
Remember this rule of thumb for controlling what is sharp on your picture:
Small aperture settings (f16, F22, f32) + lower focal lengths + subject in distance = greater DoF where everything is sharp
Large aperture settings (f1.8 - f2.8) + zoomed in on subject + subject is close = narrow DoF where background is blurred
Aperture Priority
Most compact and SLR cameras have a mode called Aperture Priority. You set the aperture and the camera automatically adjusts the shutter speed. This can be extremely useful in almost any situation. You just concentrate on composition and what you want sharp in your picture, and the camera does the rest. Because as we've learned above, shutter speed and aperture are linked, if we set a large aperture (eg. f/1.8), more light enters the camera, therefore the system decides that a faster shutter speed is needed for correct exposure and vice-versa. Many photographers use this mode 90% of the time.
Tip!
When using aperture priority, always check what shutter speed the system has set for you at any given aperture. Keep in mind that for smaller apertures the system sets a lower shutter speed. If not enough light is available at the moment, the exposure could be too long which may result in visible camera shake. If that is the case, use a tripod.
Exposure Measuring
A good combination of aperture and shutter speed will give a good exposure. But how do we know what is good in a certain situation? Most cameras have built-in exposure metering systems. They should give you correct readings most of the time, but not always. Sometimes the metering system gets fooled by reflections, big light or dark areas in the background. If such a situation occurs and we rely on the exposure meter, we get under or over-exposure unless we compensate for it.
Automatic Exposure Lock
Controls may be slightly different on different camera models (please find the Auto Exposure Lock controls for your camera), but the principle is the same. For instance if you shoot a portrait, you want a correct reading for the model's skin tones. Get closer to your subject, take the reading and activate the auto exposure lock control. Get back to your shooting position while holding the lock button and take the picture.
Spot Metering
If your subject is surrounded by too much light, normal metering systems would measure average light being much higher than what is needed for correct exposure. the result would be an under-exposure. If your camera has a spot metering system, it is sometimes good to use it. It restricts your meter to measuring the light only on a small spot in the center of a viewfinder. Point it to the spot you want to expose for and half press the shutter release. Then re-compose if needed while still holding the shutter release half pressed, and fully press when you're happy with the composition.
The biggest advantage of a spot meter is that it allows you to measure the brightness of small areas in a scene from the camera position without walking in to make a close-up reading. Since a spot meter measures only the specific area you point it at, the reading is not influenced by large light or dark surroundings. This makes a spot meter especially useful when the principal subject is a relatively small part of the overall scene and the background is either much lighter or darker than the subject. Spot meters are also helpful for determining the scene brightness range.
Hand-Held Exposure Meter
If you're using a handheld meter, read the information on your meter and set the camera controls accordingly. An overall exposure reading taken from the camera position will give good results for an average scene with an even distribution of light and dark areas. For many subjects, then, exposure-meter operation is mostly mechanical; all you do is point the meter at the scene and set the aperture and shutter speed as indicated. But your meter does not know if you need a fast shutter speed to stop action or a small aperture to extend depth of field. You will have to select the appropriate aperture and shutter combination for the effect you want. There will be other situations where either the lighting conditions or the reflective properties of the subject will require you to make additional judgments about the exposure information the meter provides, and you may have to adjust the camera controls accordingly.
"The Sunny 16" Rule
In photography, the sunny 16 rule (or, less often, the "sunny f/16 rule") is a method to estimate correct daylight exposures without using a light meter.
The basic sunny 16 rule, applicable on a sunny day, is this:
Set aperture to f/16 and shutter speed (reciprocal seconds) to ISO film speed.
For example, for ISO 100, choose shutter speed of 1/100 second (or 1/125 second)
The elaborated form of the sunny 16 rule for more general situations is:
1. Set the shutter speed to the setting nearest to the ISO film speed
2. Set the f-number according to the table below:
Aperture Lighting Conditions Shadow Detail
f/16 Sunny Distinct
f/11 Slight Overcast Soft around edges
f/8 Overcast Barely visible
f/5.6 Heavy Overcast No shadows
f/4 Sunset
For example, to shoot ISO 100 film in sunny conditions, set the shutter speed to 1/100 or 1/125 and the f-stop to f/16. With ISO 200 film, set the speed to 1/200 or 1/250. For ISO 400 film, 1/400 or 1/500. As with other light readings, the shutter speed can be changed, as long as the f-number is compensated. For example, 1/250th of a second at f/11 would be the equivalent to 1/125th at f/16.
Bracketing
Bracketing is the general technique of taking several shots of the same subject using different exposure settings. It is useful and often recommended in situations that make it difficult to obtain a satisfactory image with a single shot, especially when a small variation in exposure parameters has a comparatively large effect on the resulting image. It can apply to flash or available light exposures. When used it will creative 3 separate photos of the same scene at slightly varying exposures. One photo is exposed by the cameras meter automatically, one under-exposed and one over-exposed by a predetermined number of stops.
Bracketing doesn't simply mean guessing. It is a calculated technique to obtain a few additional insurance exposures that are based on your best estimate for correct results. The key is to recognize those unusual situations or unorthodox lighting conditions where a single exposure may not assure you of success. Because slide films have less exposure latitude than negative films, bracketing is a technique that is more useful for slide films.
Bracketing is a simple procedure. First take a reflected-light meter reading of the subject from the camera position or use the recommendation in an exposure table. This will give you an approximate exposure and provide a starting point for making any subsequent adjustments. Take one picture using this exposure. Then bracket your exposure by using one stop larger and then one stop smaller than the indicated exposure. For extra assurance, you can use a bracketing range of ± 2 stops. For more subtle control, particularly with slide films, you can bracket in 1/2-stop increments.
Negative films for either black-and-white or color prints, are generally more tolerant of overexposure than underexposure, so bracket toward more rather than less exposure unless there is a significant risk of badly overexposing important elements of the scene. Usually, one-stop increments are enough because of the film's exposure latitude. Whatever you do, it's a good idea to write down the meter reading and the settings used for each exposure so you can compare them with your results to help you judge similar situations in the future. Of course, you don't have to write anything down with a digital camera. All settings will be recorded in the EXIF data as the pictures are taken.
A device called a diaphragm usually serves as the aperture stop, and controls the aperture. The diaphragm functions much like the iris of the eye - it controls the effective diameter of the lens opening.
The lens aperture is usually specified as an f-number, the ratio of focal length to effective aperture diameter. A lens typically has a set of marked "f-stops" that the f-number can be set to. A lower f-number denotes a greater aperture opening which allows more light to reach the film or image sensor.
Aperture and shutter speed are closely linked. For instance, in a given situation an exposure of 1/15 s and f22 is needed to get a perfectly exposed image. Opening the aperture to f16, only 1/30 second speed is needed to get correct exposure. 1/15 used originally would result in over-exposure, because now more light enters the camera.
The other thing the aperture directly controls is the Depth Of Field (DoF).
Controlling Depth Of Field
A definition of Depth of Field is that it is the distance in front of and beyond the picture's sharply focused main subject. Taking a picture of a subject 2 yards from the camera with aperture f1.8 would have a very narrow depth of field. The point we focused on will be very sharp, but closely in front and everything behind it will be blurred - out of focus. If the lens is stopped down to f22, a majority of the picture will be sharp. The following diagram shows how the aperture opening affects the depth of field; dark green shows the sharp area of the subject, light green is out of focus:
Using a telephoto lens, the depth of field would be narrow, which nicely isolates image from it's environment. Narrow DoF is used in portraits where we want the distracting background supressed and to focus the viewer's attention on the subject. On the other hand, a wide-angle lens has greater DoF, which makes it good for wide shots like nature and architecture.
Note how changing the aperture in the following example changes the depth of field. See how the background is changing? This is how you can beautifully isolate your subject from a background (last pic) or keep the overall sharpness (first pic).
Aperture f22
Aperture f16
Aperture f8
Aperture f5.6
Aperture f3.5
Aperture f1.8
Depth of field is an optical figure that is influenced by 3 factors:
1. The distance of the object from the camera: This is easy to understand when taken to the extreme. Shooting an object 10 feet from the camera will result in a smaller depth of field than shooting an object 100 feet from the camera. Obviously shooting an object, which for all practical purposes is an infinite distance from the camera, will result in the camera being focused on infinity and the depth of field being infinite too.
2. The aperture: The wider the aperture (f number decreases) the smaller the depth of field and vice versa: the narrower the aperture (f number increases) the greater the depth of field. Just remember that the depth of field changes along with the f number: when the f number increases the depth of field increases and vice-versa. Unfortunately you cannot just change the aperture as you wish. Changing the aperture results in a need to change the shutter speed (either manually or the camera can most likely do that for you automatically in shutter priority mode). The wider the aperture the more light that penetrates the lens and the faster the shutter speed needs to be so there is a limit to how wide you can set the aperture.
3. The focal length of the lens: The shorter the focal length the greater the depth of field and vice versa the longer the focal length the smaller the depth of field. For example if you use normal lenses at somewhere around 50mm your depth of field will be much greater than if you use zoom lenses at 200mm. With long focal lengths the depth of field can be extremely small. For that reason zoom lenses are a good choice in order to shoot a good blurry photo that is practically focused only on the object.
After understanding the theory it is best to go to the field for some experimentation. You will get a sense of how your photo will look and how blurry the background will be after playing with different lenses, different apertures and a variety of distances from your object. If you own zoom lenses start with taking photos of a relatively close object while using your zoom lenses. You will find out that it is extremely easy to get a blurry background photo using this method even without setting the aperture. Once you feel comfortable with that setup start changing the aperture and also change to your normal 18mm-55mm or similar lenses.
Remember this rule of thumb for controlling what is sharp on your picture:
Small aperture settings (f16, F22, f32) + lower focal lengths + subject in distance = greater DoF where everything is sharp
Large aperture settings (f1.8 - f2.8) + zoomed in on subject + subject is close = narrow DoF where background is blurred
Aperture Priority
Most compact and SLR cameras have a mode called Aperture Priority. You set the aperture and the camera automatically adjusts the shutter speed. This can be extremely useful in almost any situation. You just concentrate on composition and what you want sharp in your picture, and the camera does the rest. Because as we've learned above, shutter speed and aperture are linked, if we set a large aperture (eg. f/1.8), more light enters the camera, therefore the system decides that a faster shutter speed is needed for correct exposure and vice-versa. Many photographers use this mode 90% of the time.
Tip!
When using aperture priority, always check what shutter speed the system has set for you at any given aperture. Keep in mind that for smaller apertures the system sets a lower shutter speed. If not enough light is available at the moment, the exposure could be too long which may result in visible camera shake. If that is the case, use a tripod.
Exposure Measuring
A good combination of aperture and shutter speed will give a good exposure. But how do we know what is good in a certain situation? Most cameras have built-in exposure metering systems. They should give you correct readings most of the time, but not always. Sometimes the metering system gets fooled by reflections, big light or dark areas in the background. If such a situation occurs and we rely on the exposure meter, we get under or over-exposure unless we compensate for it.
Automatic Exposure Lock
Controls may be slightly different on different camera models (please find the Auto Exposure Lock controls for your camera), but the principle is the same. For instance if you shoot a portrait, you want a correct reading for the model's skin tones. Get closer to your subject, take the reading and activate the auto exposure lock control. Get back to your shooting position while holding the lock button and take the picture.
Spot Metering
If your subject is surrounded by too much light, normal metering systems would measure average light being much higher than what is needed for correct exposure. the result would be an under-exposure. If your camera has a spot metering system, it is sometimes good to use it. It restricts your meter to measuring the light only on a small spot in the center of a viewfinder. Point it to the spot you want to expose for and half press the shutter release. Then re-compose if needed while still holding the shutter release half pressed, and fully press when you're happy with the composition.
The biggest advantage of a spot meter is that it allows you to measure the brightness of small areas in a scene from the camera position without walking in to make a close-up reading. Since a spot meter measures only the specific area you point it at, the reading is not influenced by large light or dark surroundings. This makes a spot meter especially useful when the principal subject is a relatively small part of the overall scene and the background is either much lighter or darker than the subject. Spot meters are also helpful for determining the scene brightness range.
Hand-Held Exposure Meter
If you're using a handheld meter, read the information on your meter and set the camera controls accordingly. An overall exposure reading taken from the camera position will give good results for an average scene with an even distribution of light and dark areas. For many subjects, then, exposure-meter operation is mostly mechanical; all you do is point the meter at the scene and set the aperture and shutter speed as indicated. But your meter does not know if you need a fast shutter speed to stop action or a small aperture to extend depth of field. You will have to select the appropriate aperture and shutter combination for the effect you want. There will be other situations where either the lighting conditions or the reflective properties of the subject will require you to make additional judgments about the exposure information the meter provides, and you may have to adjust the camera controls accordingly.
"The Sunny 16" Rule
In photography, the sunny 16 rule (or, less often, the "sunny f/16 rule") is a method to estimate correct daylight exposures without using a light meter.
The basic sunny 16 rule, applicable on a sunny day, is this:
Set aperture to f/16 and shutter speed (reciprocal seconds) to ISO film speed.
For example, for ISO 100, choose shutter speed of 1/100 second (or 1/125 second)
The elaborated form of the sunny 16 rule for more general situations is:
1. Set the shutter speed to the setting nearest to the ISO film speed
2. Set the f-number according to the table below:
Aperture Lighting Conditions Shadow Detail
f/16 Sunny Distinct
f/11 Slight Overcast Soft around edges
f/8 Overcast Barely visible
f/5.6 Heavy Overcast No shadows
f/4 Sunset
For example, to shoot ISO 100 film in sunny conditions, set the shutter speed to 1/100 or 1/125 and the f-stop to f/16. With ISO 200 film, set the speed to 1/200 or 1/250. For ISO 400 film, 1/400 or 1/500. As with other light readings, the shutter speed can be changed, as long as the f-number is compensated. For example, 1/250th of a second at f/11 would be the equivalent to 1/125th at f/16.
Bracketing
Bracketing is the general technique of taking several shots of the same subject using different exposure settings. It is useful and often recommended in situations that make it difficult to obtain a satisfactory image with a single shot, especially when a small variation in exposure parameters has a comparatively large effect on the resulting image. It can apply to flash or available light exposures. When used it will creative 3 separate photos of the same scene at slightly varying exposures. One photo is exposed by the cameras meter automatically, one under-exposed and one over-exposed by a predetermined number of stops.
Bracketing doesn't simply mean guessing. It is a calculated technique to obtain a few additional insurance exposures that are based on your best estimate for correct results. The key is to recognize those unusual situations or unorthodox lighting conditions where a single exposure may not assure you of success. Because slide films have less exposure latitude than negative films, bracketing is a technique that is more useful for slide films.
Bracketing is a simple procedure. First take a reflected-light meter reading of the subject from the camera position or use the recommendation in an exposure table. This will give you an approximate exposure and provide a starting point for making any subsequent adjustments. Take one picture using this exposure. Then bracket your exposure by using one stop larger and then one stop smaller than the indicated exposure. For extra assurance, you can use a bracketing range of ± 2 stops. For more subtle control, particularly with slide films, you can bracket in 1/2-stop increments.
Negative films for either black-and-white or color prints, are generally more tolerant of overexposure than underexposure, so bracket toward more rather than less exposure unless there is a significant risk of badly overexposing important elements of the scene. Usually, one-stop increments are enough because of the film's exposure latitude. Whatever you do, it's a good idea to write down the meter reading and the settings used for each exposure so you can compare them with your results to help you judge similar situations in the future. Of course, you don't have to write anything down with a digital camera. All settings will be recorded in the EXIF data as the pictures are taken.
Comment
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Comment by Debbie Penny on February 2, 2010 at 7:46pm
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Thanks Diane.
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Comment by F, Calla & Luca on February 2, 2010 at 7:08pm
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Yes, thanks indeed.
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Comment by Nancy, Ned, Clancy, and Charlie on February 2, 2010 at 6:51pm
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Thanks very much.
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