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A hole in the lens that adjusts in diameter, similar to the way the pupil of an eye works. This controls the amount of light reaching the film or CCD to record an image. Every different diameter opening has a number which indicates the size - it equals the focal length of the lens divided by the diameter of the aperture. These numbers are also called stops. See f/number.

Related Terms

An auto-exposure mode where you select the required lens aperture and the camera sets the necessary shutter speed, to give the correct exposure based on the auto meter reading. This mode is ideal for landscape and still-life photography where maximum depth-of-field is required. It's either indicated on the camera as AP (aperture priority) or AV (aperture value).
This is the smallest aperture that can be selected. A minimum aperture of  f/22 is fine for most photography and the setting only really needs to be smaller to allow exposure at slow shutter speeds when using faster film in brighter conditions.
A ring that is situated near the lens throat on all manual focus lenses and some autofocus models that adjusts to control the amount of light reaching the film or CCD when recording an image. Cameras that don't have an aperture ring often have an electronic control to adjust the aperture either automatically or manually.
A mechanical lever or electronic button used to close the lens aperture down to the actual exposure setting so depth of field can be previewed.
This is the widest aperture that can be selected on the camera and is an indication of the speed of the lens. On zoom lenses two figures are often given. One is for the lens at the wide-angle setting and the other is for the telephoto setting.
This is how the camera adjusts the shutter speed and aperture to ensure the right amount of light reaches the film or CCD. Early cameras only had a manual mode (M) where the user had to select the aperture and shutter speed manually to ensure the correct exposure. Over the years cameras have become more sophisticated and now offer several automated modes including Program (P) - a fully automatic exposure mode that sets the aperture and shutter speed; Aperture priority mode (AP) where the user selects the aperture and the camera sets the necessary shutter speed; and shutter priority (SP, or Tv on some cameras) where the user selects the shutter speed and the camera sets the necessary aperture. Auto bracketing (AB) takes a pre-selected number of photographs, one at the suggested exposure and one to either side, so you can be sure of one accurate result. There are also several subject-based program modes that we haven't listed here that tailor the camera for particular subjects such as sports (action), landscapes, portraits, or flowers (close-ups). Some digital cameras have black & white and sepia modes. Buying advice A full auto program mode is ideal for point-and-shoot photography, but it's also useful to have some control over the exposure. The beauty with digital is that you can see whether the camera has got the shot right by previewing the image on the LCD. If not, you try again. If there is no manual control you can often preset the exposure using an auto-exposure lock or exposure compensation. The subject based program modes are often a waste of time and don't really bring much to the package. Special effects modes on digital cameras are also throw-away because all these can be created using the computer later.
An optical effect which can soften photographs and make them less sharp. As long as light travels in straight lines, this phenomenon will not occur, but as soon as it starts to bend - disperse or "diffract" - when it has to travel through a hole so small that it has to squeeze through, it will begin to interfere with the quality of the final result. Although a negligible effect in most situations, it actually increases with smaller apertures. There is a break-even point at which the disadvantage of the diffraction of the light captured is still compensated by the advantage of extra sharpness due to greater depth of field. But beyond that point the softening effect of the diffracted light is only partly compensated by the sharpness due to the greater depth of field. Finding the break-even point can help prevent any negative effects of diffraction. And as a bonus it will limit the length of the exposure or the ISO needed to take a photo with a very small aperture. The difficulty is that the effect isn't the same for different cameras and lenses. The aperture isn't the only critical factor - the size of the film or sensor recording the photo counts as well, and so does the quality and the focal length of the lens. For those who don't want to get into complicated mathematical calculations in order to find the ideal aperture, it is good to remember that the sharpest results for most lenses are found around two or three stops below their maximum aperture. Especially cheaper lenses can give very bad results at full aperture.
To make an exposure, a photographer can, for example, choose a combination of a small aperture and a slow shutter speed or a large aperture and a fast shutter speed. Each movement of the aperture is classed as one stop – go from f/8 to f/11 and you close the aperture by one stop. Similarly by adjusting the shutter speed from 1/125sec to 1/500sec you reduce the exposure by one stop. If you adjust the aperture by a stop and counteract this by also adjusting the shutter speed by one stop you will produce the same exposure value. Therefore an exposure of f/8 at 1/60th could be changed to any of the following combinations: f/11 at 1/30th, f/16 at 1/15th, f/5.6 at 1/125th or f/4 at 1/250th. This is called the law of reciprocity (if one value increases the other will decrease proportionally and visa versa).
Called 'extender', 'lens extender' or 'telephoto extender' by some manufacturers. (Not to be confused with an 'extension tube'!). A teleconverter is an accessory that fits between the camera lens and body to increase the focal length of a lens by 1.4x, 1.7x, 2x or 3x. When coupled with a 200mm lens, for instance, teleconverters would give these results: a 1.4x teleconverter gives an effective focal length of 280mm, while a 1.7x teleconverter increases this to 340mm, a 2x teleconverter to 400mm and a 3x to no less than 600mm. Teleconverters are compatible only with selected lenses, so always check with the manufacturer or retailer before buying. Although they will work with some zoom lenses, they're best used with (fast) prime lenses, since there usually is a drop in quality, and many prime lenses give a higher level of quality to start with. Besides changing the effective focal length, the effective aperture of the attached lens is increased by one or more stops as well. Autofocusing usually does not operate if the effective maximum aperture is greater than f/5.6 with the lenses/cameras from most manufacturers (in some cases greater than f/8, like on the Canon EOS 1V and EOS 3 cameras). And not all teleconverters support autofocus in the first place!
The lens focal length divided by effective diameter of the aperture gives the f/number that's used to indicate the aperture value. Each full f/number, also called a stop or f-stop, halves or doubles image brightness and some lenses can also be controlled in half or third steps. The most common f/numbers are 1.4, 2, 2.8, 4, 5.6, 8, 11, 16 and 22. They are usually preceded by an "f". The larger the f-number, the smaller the lens opening. In the aforementioned series of numbers, f/1.4 is the largest opening and f/22 the smallest. The smaller stops (larger f numbers) give the greater depth of field in a photograph, and vice versa.
This is technically known as a catadioptric lens and has an unusual construction of mirrors and lens elements. As well as glass elements there are mirrors at the front and rear to fold the light as it enters the lens. Although this results in a body that's a little wider than normal, advantages are: 1. The lens is usually only half the physical length a regular lens of the same focal length would be, and 2. It's much lighter. Disadvantages are: 1. There is no adjustable aperture, so the user is forced to take all of his photos at a permanent aperture setting, usually f8, but sometimes even f11, which means you need plenty of light for taking photos; 2. Practically all mirror lenses use manual focus; 3. Highlights that are out of focus are, in some situations, shown as doughnut shapes (although some actually like this characteristic and consider it an advantage, not a disadvantage).
Reciprocity law states that as you increase the intensity of light reaching the film you also need to decrease the speed it reaches the film by the equivalent amount. Most films work quite happily between exposures of 1/2sec and 1/1000sec, but go beyond these extremes with a very low intensity of light and a long exposure or a very high intensity of light and a correspondingly short exposure and the law fails an exposure increase may be required when the shutter speed is beyond these limits. At these extremes the law fails. Compensation is required to adjust for this, but there is no strict rule to correct the error. Most film and paper manufacturers provide technical details on request with a rough guide to exposure adjustments. As a rough guide for an exposure of one second you would increase the speed to two seconds, or open the aperture by one stop. A speed of 10sec would need to be increased to about 50sec or open the aperture up two stops. With black & white film you only have to worry about this exposure correction, but with colour film does not only suffer from exposure problems but also colour casts. A colour film is made up of three individual colour layers, each layer suffers from reciprocity failure at different levels. On an uncorrected film the shadows may have a magenta colour cast but the highlights may suffer from a cyan cast. To correct the cast not only would a longer exposure be needed but also the inclusion of a colour correction filter of a low value, care would have to be taken in choosing the correct filter otherwise an over corrected result may appear.
A button or lever on the camera that either stops the lens aperture down manually or electronically. This enables you to view the image at the aperture that will be used to take the picture. The view will be darker, but you will be able to see exactly what will and won't be sharp or in focus. See "Using the depth of field button" in the Techniques section.
This is the eye of the camera and is used to capture the image it sees onto the camera's light sensitive film, or CCD in digital camera. The size of lens is measured and indicated as a focal length. Cameras come with either a fixed lens or zoom lens with a range of focal lengths (see lens range) and on some SLR and rangefinder cameras the lens detaches so others can be attached to increase versatility. With a detachable lens camera it's often possible to buy just the body with a lens of your choice.The amount of light passing through the lens is controlled by an aperture, which is often quoted with its maximum aperture setting.Buying adviceSLRs: If you intend buying a specific lens in the future make sure that it's available in the same mount as the camera you are considering or own. Also if you're upgrading cameras, buy one with the same lens fitting or one that can be adapted to save the cost of replacing all the lenses you own.Digital and compact cameras: A fixed lens camera can often be much smaller so could be selected if you need to travel light. It's also less expensive if your budget is tight. It's better, though, to choose a camera with a zoom if you can afford to, rather than using a digital zoom or cropping the picture later. Go for one with a wider angle zoom if most of your pictures will be landscapes, interiors or family group shots.Choose a version with a longer telephoto setting if you want to shoot long distance subjects, portraits and wildlife.
A shutter with blades that is positioned near to the aperture within a lens (also known as a leaf shutter).
Program mode on some Canon cameras that sets the optimum aperture to ensure enough depth of field to make the whole of the subject sharp.
The technical name for the lens aperture that is made from overlapping blades.
The combined shutter and aperture values used on older cameras and light meters.
The amount of light reaching light sensitive material such as film, printing paper or a CCD. Exposure is controlled by the shutter speed (time) and aperture (intensity).
Indicates the output, or power, of an electronic flash. You can use the guide number to work out the aperture or flash to subject distance by dividing either into the guide number.
A shutter with blades that is positioned near to the aperture within a lens (also known as a between-lens shutter).
An full override to a camera's automatic exposure where you select the aperture and shutter speed.
An automatic exposure mode that sets the camera's aperture and shutter speed often referred to on camera dials and panels as P. What sets it apart from Auto mode, is that all the other functions, including exposure compensation, can still be adjusted.
A program mode thats used to adjust the shutter and aperture combinations that were set automatically by the camera in its program mode.