Difference between revisions of "Focusing system"
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| − | + | Focusing means setting a lens appropriate to the distance of the subject so as to have a sharp picture. | |
| − | + | The simpler cameras (for example [[Single-use camera|disposable cameras]]) provide no way to alter the focus: this is ''[[fixed focus]]''. Some cameras have auxiliary lenses to introduce into the light path to change the focus to a few fixed distances. A few [[folding camera]]s focus by moving the lens plate back and forth. | |
| − | + | On most cameras, though, you focus by setting the distance on a ring around the lens. How do you know that distance, and how will you focus the image? On the simpler old cameras, you had no indication and had to estimate it: this is ''[[scale focusing]]'' or ''[[guess focusing]]''. The more advanced cameras had some kind of focusing help, the main types are the ''[[ground glass back]]'', the ''[[rangefinder (device)|rangefinder]]'' and the ''[[reflex finder]]'' (in chronological order of appearance). | |
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| + | Normally the entire set of lens elements is moved together when the focusing barrel of lens is turned: this arrangement is sometimes called ''unit focusing''. For a small number of cameras, the same effect is achieved by fixing the lens relative to the body and using a knob on the back of the body to move the film plane instead. In some cameras, only the front element of the lens is moved - ''front-element focusing'': triplet and Tessar-type lenses allow this. In recent, complex designs, one set of elements may be moved while another is not, or different elements may be moved in different ways. | ||
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| + | From the beginning of the 1980s cameras began to incorporate an [[autofocus|autofocusing]] mechanism: the camera determines the distance to the subject automatically. There are two main types of autofocus: ''active autofocus'', where an infrared light (or a sonar pulse) is emitted by the camera and reflected by the subject, and ''passive autofocus'', where the image transmitted by the lens is analysed for its sharpness by a detector inside the camera. Active autofocus is used by the simpler [[point and shoot]] cameras, and passive autofocus is used by the autofocus [[SLR]] cameras. | ||
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| + | The different designs for focus are detailed in separate pages: | ||
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| + | * [[fixed focus]] | ||
| + | * [[scale focusing]] | ||
| + | * [[rangefinder (device)|rangefinder]] | ||
| + | * [[reflex finder]] | ||
| + | * [[ground glass back]] | ||
| + | * [[Autofocus]] | ||
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| + | <!-- This info must be incorporated in the rangefinder page | ||
| + | The '''rangefinder''' is an optical device to set the distance. There are two superimposed images (''superimposed image rangefinder''), or one image split in two parts (''split image rangefinder''). As you turn the focus control one of them is moving, left and right if the camera is hold horizontally, up and down if it is hold vertically. The focus is set once the two images coincide. | ||
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| + | Usually the rangefinder is combined with the viewfinder, the moving image appearing as a central bright patch. So you can focus and frame at the same time. On some older cameras it was independent from the viewfinder, for example on the Leica screw cameras. You had to focus first by looking through the rangefinder, then look through the viewfinder to frame your picture. | ||
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| + | Only one digital camera so far uses this method: the [[Epson RD-1]]. Many famous film cameras use it however, most notably the [[Leica M]] series. There are many rangefinder cameras available on the used market, and these can be a very cheap yet high quality first camera. | ||
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| + | <!-- This info must be incorporated in the SLR and TLR pages | ||
| + | === [[SLR]] === | ||
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| + | SLR means '''Single Lens Reflex'''. Inside the camera is a mirror that reflects the image onto a focusing screen. You view this focusing screen through the viewfinder. This means that you are seeing pretty much exactly what the camera will capture. Note that in most SLRs, there will be some cropping, meaning that the camera will capture slightly more than what shows in the viewfinder. This is usually expressed as a percentage, e.g. 95% meaning that the viewfinder shows 95% of what the camera will capture. Some professional cameras have 100% viewfinders. ([[SLR|more info]]) | ||
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| + | === [[TLR]] === | ||
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| + | A TLR or '''Twin Lens Reflex''' camera uses two lenses, one for viewing and one for taking. The light passing through the viewing lens is refected on an internal mirror and projected onto the ground glass at the top for viewing, focusing, and composing. Due to the use of a single mirror, the viewed image is reversed. ([[TLR|more info]]) | ||
| + | --> | ||
| + | [[Category: Focusing system|*]] | ||
Latest revision as of 19:33, 4 October 2023
Focusing means setting a lens appropriate to the distance of the subject so as to have a sharp picture.
The simpler cameras (for example disposable cameras) provide no way to alter the focus: this is fixed focus. Some cameras have auxiliary lenses to introduce into the light path to change the focus to a few fixed distances. A few folding cameras focus by moving the lens plate back and forth.
On most cameras, though, you focus by setting the distance on a ring around the lens. How do you know that distance, and how will you focus the image? On the simpler old cameras, you had no indication and had to estimate it: this is scale focusing or guess focusing. The more advanced cameras had some kind of focusing help, the main types are the ground glass back, the rangefinder and the reflex finder (in chronological order of appearance).
Normally the entire set of lens elements is moved together when the focusing barrel of lens is turned: this arrangement is sometimes called unit focusing. For a small number of cameras, the same effect is achieved by fixing the lens relative to the body and using a knob on the back of the body to move the film plane instead. In some cameras, only the front element of the lens is moved - front-element focusing: triplet and Tessar-type lenses allow this. In recent, complex designs, one set of elements may be moved while another is not, or different elements may be moved in different ways.
From the beginning of the 1980s cameras began to incorporate an autofocusing mechanism: the camera determines the distance to the subject automatically. There are two main types of autofocus: active autofocus, where an infrared light (or a sonar pulse) is emitted by the camera and reflected by the subject, and passive autofocus, where the image transmitted by the lens is analysed for its sharpness by a detector inside the camera. Active autofocus is used by the simpler point and shoot cameras, and passive autofocus is used by the autofocus SLR cameras.
The different designs for focus are detailed in separate pages:
