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Method of focusing the lens, using either a rotating mechanical action or a motorised method, where you enter a distance on the camera. Cameras with mechanical actions often have an aid to help you focus correctly. On some cameras you see the image split in half and as you adjust focus the two halves come together. When the images are lined up the focusing is achieved. Others have an LED indicator that blinks when out of focus and either goes out or stays on when focusing is correct.
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The action of adjusting the lens so that the subject appears sharp on the film or CCD. Most cameras now have automatic focus (AF), some have manual focus override (MF) and some have a fixed focus (F) lens that ensures things from about 1.5 meters to infinity are relatively sharp. A few cameras have a power focus (PF), which is a manual method but using a motorised focusing ring. Buying advice: Autofocus is essential if you want to use the camera effortlessly, but occasionally the system is fooled, so make sure the camera has a focus lock or infinity setting or, better still, full manual control. Also check what the closest range is. Some only go down to about one meter which is useless if you want to fill the frame with a small object such as a flower or piece of jewellery.
The ring used to adjust the lens so that the subject appears sharp on the film or CCD. Most cameras now have automatic focus (AF), some have manual focus override (MF) and some have a fixed focus (F) lens that ensures things from about 1.5 meters to infinity are relatively sharp. A few cameras have a power focus (PF), which is a manual method, using a motorised focusing ring. Lenses with a ring that allows a good grip make it easier to adjust and focus manually.
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).
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.
One of two popular methods of camera focusing systems, the other is active. Passive has the advantage over active because it isnt fooled by glass in between the camera and the subject. Its also not dependent on subject distance. It works by measuring the subjects contrast and as such its main downfall is when the subject has no contrast. In such situations, such as snow scene or low light, the lens will focus in and out and may never reach sharp focus. Fortunately many cameras with this system have manual override.
Focusing system in some of the latest autofocus cameras that takes measurements of the subject distance from various parts of the image area, rather than just the centre. Some cameras allow manual selection of this point so you could, for example, set it up to focus on a zone to the left of the image if the subjects you shoot are regularly off centre.