Definition: Photographic Lens; Camera Lens; Photographic objective; Lens

Definition: Photographic Lens; Camera Lens; Lens | Glossary entry

Photographic Lens; Camera Lens; Photographic objective; Lens

Modern DSLRs and older SLRs are described as Interchangeable photographic lens cameras. This means the photographic lens which focuses the light for capture can be changed with one designed for a different image effect.

The photographic lens is a mechanical and optical unit. It contains one or more optical lenses or groups of optical lenses. Each glass lens forms an element. These may be single lenses or groups of them assembled into a lens body. Singly, or as groups, the lenses are designed to work together to create a precise optical effect. That designed effect creates the character of that lens. Such lenses may be for example: Zoom lens, fish-eye lens, macro lens, standard lenses and others.

An individual lens can vary a lot from another photographic lens made by a different manufacturer. This is the case even if the focal length, or other ways to describe the lens, seem to say the same thing. The design of glass elements and how they work together can introduce imperfections, distortions, colour variations, focus differences and bokeh differences.

Photographic lens Construction

The photographic lens is constructed from a lens body which houses the glass lens elements. The body also contains a focusing unit. This will move the lenses within the body. The movement coordinates the lens elements so the light can create an image circle onto the digital sensor. Inside a single lens body there may be one or many elements. Focus mechanisms only move some of the elements to obtain a focused image.

Lens arrangement will vary with the optical specification and type of lens. In modern photographic lenses, there maybe more than ten individual glass elements arranged in optical groups. The latter are arranged to achieve part of the overall manipulation of light forming the end image. Together the lens elements and groups combine to create the desired image format. The lens body will be the mounting for elements, groups and any mechanisms to move them.

The different elements may be of optical glass, or in cheaper lenses, poorer quality glass. Optical glass is a high quality product with few imperfections. But it is also more expensive. However, the superior quality allows more light through each lens. A poor lens element may reduce the light passing through it by as much as 20%. Good quality optical glass will have much better transmission and so preserve more light for the image capture.

• Theoretical Lens diagram • Theoretical lens layout showing elements and groups.

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Generalised photographic lens layout showing showing the principle features.

Definitions: Photographic lens vs. optical lenses

While it seems the photographic lens focuses light and so does the optical lens the similarities stop there. There are a number of points of difference…

 Photographic lens  Optical lens
 Contains mechanisms in a body  A single glass element
 Focal Length (photographic) defined by the maker and may not be related to focal points.  Focal length defined by the physical properties of the lens.
 Will achieve a range of effects dependent on the design.  Normally will achieve only one specific focal result.
 Many glass elements designed to work together.  Normally one lens designed to produce one type of optical effect (eg. magnification).
Important components in a photographic lens

Beside glass elements, the photographic lens contains a number of mechanisms. Lens designs and the supporting mechanical equipment does too. A modern zoom lens, for example, may house the following mechanisms within the lens body…

  • An anti-vibration system with motors to dampen lens movement.
  • A geared or ‘worm drive’ for the user to change focal length.
  • A focus system to guide the movement of the glass elements.
  • A focus servo-motor for auto-focus.
  • Sensor electronics.
  • The Iris-Diaphragm unit and servo-motor to open/close the Iris
  • Sundry pieces of equipment to balance movement or change the position of components.
  • Various electrical components and circuits to manage and control the mechanical actions.

The inclusion or exclusion of any/all of these component parts, and possibly other items, is dependent on the make and model of the individual camera-lens combination. Although names of components may differ, it is possible to work out what is included in an individual lens from the lens specification available from the manufacturer.

Legacy photographic lens issues

Photographic lenses from old cameras (eg. SLRs) may not fit, or be suitable for, a modern DSLR. An old photographic lens may mount on a modern camera body. However, electrical connections from the lens body to the camera may differ, or be incompatible with it. Mechanical differences may also exist despite an apparent fit. When considering using a legacy lens for a new DSLR body check the combination is suitable before use.

Photographic lens units are made by camera brands and independent manufacturers. The lens body is fitted with a Lens mount to fit into the body of a camera model. Lens mounts vary. Make sure you purchase a lens designed for your camera. Wikipedia has a good list of lens makers  External link - opens new tab/page.

Never attempt to force a fit between the photographic lens and your camera. If it does not fit, assume it is not suitable. Improper fit, or forced fit, can seriously damage both the camera and the photographic lens. However, it is possible that some lenses may be used on a camera with incompatible mounts. Normally this requires specialized adapters or specially designed mounts. Carefully research the comparability for such matches. It is possible that such matches will produce a good photographic result. Equally, such matches often mean that electronic components do not work. So you may have to manually focus the lens or set the aperture.

Photographic lenses are complex mechanisms in the modern DSLR camera. They are extremely delicate and liable to damage with inexpert handling. No attempt should be made to open the lens body. Damage or malfunction of a photographic lens unit should only be repaired by a service agent.

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