The two primary controls that you employ on-camera are the shutter speed and the aperture. this article will describe what they are, how you can use them to control exposure and the effects that each might have on your photographs.
SHUTTER SPEED
Where the aperture determines the amount of light reaching the film, the shutter speed controls the amount of time that light reaches and exposes the film to the light reflecting from a subject that is in front of the camera. On earlier cameras, exposure was controlled by sliding a plate from the front of the film or by removing a cap from the end of a crude lens. Currently, however, shutter speed can be controlled down to a thousandth of a second either mechanically or through electronic means. Primarily in older film cameras the shutter was of leaf design, a diaphragm comprised of thin blades that would rotate open and closed when the shutter button was depressed. More and more modern film cameras employ shutter curtains. When the shutter button is pressed the first curtain starts moving across the front of the film plane and at a predetermined amount of time the second curtain follows the first allowing light to reach the film evenly for the preset duration. Curtain type shutters have a flash sync speed. The reason for this is that there is a shortest amount of time that the entire amount of the film can be exposed simultaneously without either the first opening curtain or the second closing curtain blocking a portion of the film. If the fastest a curtain can move across the film plane completely and then be followed by the second curtain is about 1/60th of a second then any faster speeds will result in uneven exposure when using flash. This is also the reason that most inexperienced photographers become frustrated with flash photography. They get images where one half or one fourth of the image is lit to proper exposure and the rest of the image is underexposed. This synchronized speed is usually is not the fastest shutter speed setting on the camera. Consult your camera’s documentation to find your camera’s flash sync speed.
The shutter speed on modern film cameras is set with a dial located on the top of the camera body and can generally be set from one second to 1/1000th of a second though different cameras offer faster and slower preset shutter speeds. The shutter speed settings are indicated by the denominator of the fraction of a second that the film is exposed for. A sample of these numbers is listed in the shutter speed table. Most cameras also have a feature called the bulb or “B” setting that allows the shutter to be held open for a longer amount of time by use of a shutter release cable that we will be discussing later. The camera’s shutter speed primarily controls an image’s sharpness (along with focusing) or intentional blur. With a faster shutter speed, a sprinter can be frozen in time, rushing water brought to a standstill or a playful child brought down to a look and a smile. A fast shutter speed can also prevent image blurring due to camera shake (the camera’s mirror moving during exposure) and a photographer’s own motion while holding the camera, even the photographer’s heartbeat. A longer shutter speed can blur the image to indicate motion in a subject such as portraying the motion of a speeding bicyclist, the gentle flowing of water or panning to follow a moving subject and blurring the background.
Shutter Speed (denominator is the shutter speed indicator on camera)
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This series of numbers is geometric as well and its constant is 2. This means that each preceding or following number is a numeric doubling or halving in value. 1/250th is double 1/500th. 1/16th is half that of 1/8th. Shutter speed adjustments either double or half the amount of light reaching the film as its numeric neighbors, thus, helping to maintain that facility in exposure adjustments we alluded to earlier.
There are many ways to prevent camera movement from translating into non-intentionally blurred images. First would be the tripod. This gets the camera out of the photographer’s hands and permits long shutter speeds. Also other devices such as beanbags or even your camera bag on a fence or a car, leaning against a tree or lying on the ground, even simply standing with your legs spread wider apart to stabilize your own stance, there are many things that the photographer can do to help ensure a sharper image.
APERTURE
Most modern lenses have an adjustable opening near the pupil named the aperture. The aperture regulates the amount of light that reaches the film. This may be increased or decreased as may be required to expose your film. On older cameras the aperture may have been a simple board placed behind a simple lens mounted in a shutter. The aperture was cut to a certain size to restrict the amount of light reaching the film and multiple boards provided varying choices in aperture. Now the aperture is similar to the earlier leaf style shutters and is adjusted by moving a ring located on the rear end of the lens that is nearest the camera body to slide thin blades in a circular motion thus opening and closing this aperture. Moving this ring in one direction will decrease the aperture’s size or ‘stop the lens down’. To increase it or ‘open the lens up’, move the ring in the opposite direction. The term ‘stop’ refers to an increment of adjustment to exposure.
An f-stop number is a fraction. It refers to the ratio between a lens’ focal length (f = focal length) and a lens’ aperture diameter. The following equation provides the framework for determining f numbers using the lens’s focal length and the aperture’s physical diameter.
av=f/d (where av=f number, f = focal length of lens and d= physical aperture diameter)
For instance, if the aperture on a 50mm lens has a diameter of 12.5mm then with the preceding equation you can determine that the f number would be 4 (4 = 50/12.5). A 100mm lens’ f/4 setting has an aperture diameter of 25mm (100/4=25). Though the 50mm f/4 aperture diameter is one half that of the 100mm, the 100mm lens requires the light to travel through twice as much lens as with the 50mm. Both lenses set at this setting allow the same amount of light to reach the film because both of these lenses have an opening that the light passes through that is one fourth of the focal length of each lens.
The aperture size used also influences the amount of the subject area that is in reasonably sharp focus. Only a razor sharp plane that is parallel to the film plane will be in perfect focus. As the subject departs this focal plane both toward the photographer and toward infinity, the image will become decreasingly out of focus. The area of reasonably sharp focus is referred to as depth of field. Smaller apertures such as f/22 or f/16 can have the capability of rendering everything from seven feet to infinity in at least relatively sharp focus. Larger apertures such as f/2.8 or f/4 can produce dramatic images by blurring the background and emphasizing or visually isolating the subject by rendering distracting clutter virtually unrecognizable.
The size of the aperture used can have other effects as well. A larger aperture will bring in more light and allow the photographer to use faster shutter speeds to reduce camera shake. A larger aperture will also allow flash photography to be used with subject matter at greater distances from the camera, within the flash's maximum range of course. Read your manufacturer's literature for details. A smaller aperture will require a longer shutter speed which would be very handy in rendering motion in your subject. It would also allow flash photography with closer subjects. Refer to the flash photography section of these Better Photography pages for further discussion on using flash or strobe.
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Now, upon looking at this table you may begin to see a pattern, a repetition from one number to the next. Now, photography is replete with series of numbers that indicate either a doubling or halving of an increment of exposure. This simple ratio makes it much simpler to maintain equal adjustments in exposure. This series of f-numbers is geometric which means that this series of numbers is formed through either multiplication or division. The constant in this series is the square root of 2, or 1.414. Each f-number is the product of the previous value multiplied by 1.414. But most importantly, each numeric increase in this series is a halving in size of the preceding aperture’s geometric area. Conversely, each numeric decrease is a doubling in size of the preceding aperture’s geometric area (see the aperture geometric area table). Try removing your lens from the camera and looking into the end of the lens that mounts up against the camera body. You can then observe the change in aperture size as the aperture ring is turned. What you will see might be similar to the following (admittedly crude) table.
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Seeing the above image you may be wondering something. Why does a larger aperture have a smaller number and a smaller aperture have a larger number? Try to remember the f/x equation mentioned above. A larger number is a smaller aperture. f/8 is 1/8 the lens focal length. f/16, though a larger number, is only 1/16. The smaller the number, the larger the aperture. The larger the number, the smaller the aperture.
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