Difference between revisions of "Focusing system"

From Camera-wiki.org
Jump to: navigation, search
m (Focusing Mechanisms moved to Focusing system)
(Edited the text about front-element focusing.)
 
(15 intermediate revisions by 6 users not shown)
Line 1: Line 1:
−
== Principles ==
+
Focusing means setting a lens appropriate to the distance of the subject so as to have a sharp picture.
  
−
=== Focusing ===
+
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.
  
−
Focusing means setting the distance of the subject to have a sharp picture.
+
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).
  
−
The simpler cameras (for example disposable cameras) have no focus setting at all: this is ''fixed focusing''.
+
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.  
  
−
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 guess 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''. (Chronologically they appeared in that order.)
+
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.
  
−
From the beginning of the 1980s the cameras began to incorporate an auto focusing mechanism: the camera determines the distance to the subject automatically. There are two main types of autofocus: ''active autofocus'', where an infrared light 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:
  
−
The different focusing systems are detailed below.
+
* [[fixed focus]]
−
 
+
* [[scale focusing]]
−
=== Framing ===
+
* [[rangefinder (device)|rangefinder]]
−
 
+
* [[reflex finder]]
−
You want to know with enough precision what will appear on your picture. This is the role of the [[viewfinder]]. There are different kinds of viewfinder:
+
* [[ground glass back]]
−
 
+
* [[Autofocus]]
−
* '''Frame finders:''' No longer in use. This was the simpler design, with no optics at all, consisting merely of framing devices that the user looks through to frame the scene they wanted to take. These types of viewers were present even on cameras which had conventional viewfinders, such as '''press cameras'''; they were called "sportsfinders" since they could be used to quickly frame action on a field. Typically, such a finder had two parts: a large wire frame that extended above or to the side of the camera, and a smaller frame that the user put their eye up to.
 
−
 
 
−
* '''Simple optical viewfinder:''' This is used on cameras with no zoom lens. It's basically a tiny telescope that you peer through at the scene. It only helps you to frame the image as you want. This is the type found on most low cost cameras.
 
−
 
 
−
* '''Zoom optical viewfinder:''' Similar to the above, but with a zoom feature, so that the view through the viewfinder changes as you zoom the lens. This is used on low cost cameras with a zoom lens and autofocus.
 
−
 
 
−
* '''Optical viewfinder with rangefinder:''' This is the combination of a simple optical viewfinder and a rangefinder. See ''rangefinder'' below.
 
−
 
 
−
* '''Bright finder:''' No longer in use. This was a small finder with a mirror inside. You looked through it from above, with the camera held on your chest or belly. The image was inverted left to right, and it was not really comfortable. It was used on inexpensive cameras until the 1950s.
 
−
 
 
−
* '''Reflex finder:''' See ''SLR'' and ''TLR'' below.
 
−
 
 
−
== Fixed focusing ==
 
−
 
 
−
A camera with '''fixed focusing''' has no focus control. The focus is preset once and for all, at a distance for which sharpness will be correct enough for most subjects, usually from 1 meter (3 feet) to the infinity (far away). These cameras have a lens with a very small aperture (f/5.6 or f/8 for example). If they had a larger aperture they could not be sharp for both medium and long distance.
 
−
 
 
−
== Scale and guess focusing ==
 
−
 
 
−
'''Guess focusing''' was the most common way of focusing during many years. You had to estimate the distance to the subject yourself, and set the focusing ring or focusing lever accordingly. You were somewhat assisted by the [[depth of field scale]]. Usually the lens was fixed and its aperture was not too big, typically f/2.8 as a maximum. It is not possible to guess the distance with enough precision for a larger aperture.
 
−
 
 
−
These cameras were usually more expensive than the fixed focusing ones, but cheaper than the rangefinder or reflex type.
 
−
 
 
−
A variant was '''scale focusing''', used on cheaper cameras, targeted to people who would not take the trouble to guess a distance in meters or feet. Symbols on the focusing ring, (usually "one person", "a group of people" and "a mountain") indicated the most probable setting for your picture depending on the subject.
 
−
 
 
−
This range of camera has evolved to become the point and shoot cameras, equipped with an autofocus device, consisting of an infrared light emitted by the camera and reflected by the subject (note it might differ on the digital ones?). You do not have to guess the distance yourself, and of course the operation is faster.
 
−
 
 
−
== Rangefinder focusing ==
 
  
 +
<!-- 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.
 
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.
  
Line 50: Line 24:
  
 
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.
 
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.
−
 
+
-->
−
''(See [[rangefinder]] for more info)''
+
<!-- This info must be incorporated in the SLR and TLR pages
−
 
 
−
== Reflex focusing ==
 
−
 
 
 
=== [[SLR]] ===
 
=== [[SLR]] ===
  
Line 62: Line 33:
  
 
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]])
 
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]])
−
 
+
-->
−
== Ground Glass focusing ==
+
[[Category: Focusing system|*]]
−
 
 
−
Some cameras have no viewfinder per se, but instead use a piece of ground glass at the focal plane to focus the scene. This type of focusing and viewing system is typically used on so-called '''large-format''' cameras (cameras which use film 4x5 inches or larger). In such a camera, before the picture is taken, the ground-glass viewer is inserted in the back of the camera, and the lens opened to its widest aperture. This projects the scene on the ground glass upside down and backwards. The photographer focuses and composes using this projected image, sometimes with the aid of a magnifying glass or loupe. In order to see the image better, a '''dark cloth''' is used to block out light, hence the image of the old-time photographer with his head stuck under a large black cloth.
 

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:

Retrieved from "http://camera-wiki.org/index.php?title=Focusing_system&oldid=226096"