Lens

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Revision as of 02:56, 4 November 2005 by Mama60 (talk) (Elements)
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A lens is an optical device through which light is focused in order to form an image inside of a camera either on film or on a digital sensor.

Anatomy of a Lens

Elements

An element is an individual glass structure in a lens that is not permanantly, directly fixed to any other element. In older lenses an element was an individual piece of glass but more modern lenses make use of some composite elements which are two or more pieces of glass cemented together.



Barrel

The barrel is the tube-shaped outer shell that contains the lens elements.

Aperture

An aperture is an opening in a lens that limits the amount of light passing through it to the camera body. The term aperture is also often used to describe the diameter of that opening. It is usually adjustable, most often by turning an "aperture ring" on the outside of the lens body.

Mount

A lens mount is the part of an interchangeable system lens that connects to the camera body.



Geometry of a Lens

Optical Axis

The optical axis is an imaginary straight line which passes through the centers of curvature of the lens elements and meets the focal plane at a 90-degree angle.

Focal Plane

The focal plane is the flat plane onto which a lens focuses its image. The focal plane sits at a 90-degree angle to the optical axis. The film or digital sensor should sit on the focal plane.

Focal Length

The focal length of a lens is defined as the distance in (usually in mm.) from the optical center of the lens to the focal plane, which is located on the sensor or film if the image is "in focus".

Fixed and Interchangeable Lenses

Fixed Lenses

A fixed lens is simply a lens that is permanently fastened to its camera as opposed to a system camera that allows different lenses to be used on the same camera easily. Fixed lenses are commonly found cameras aimed at consumers, from old box, TLR (the Mamiya C systems being the exception) and folding cameras, through the consumer range finders of the '70s and through to today's point and shoot film and digicams. There are certain advantages to having a fixed lens on your camera:

  1. As no mechanism for changing lenses needs to be built into the camera design it can help keep the camera smaller and lighter.
  2. In digital cameras a fixed lens means that there is less chance of introducing dust to the sensor surface.
  3. Fixed lenses are designed for a specific camera model and so fewer compromises have to be made in the lens design.
  4. Cost - if your camera comes with a fixed lens you don't have to worry amount buying a lot of additional glass to build a system.
  5. Portability - a fixed lens should be enough for most situations you encounter so you have less accessories to carry and you will waste less time changing lenses.

Interchangeable Lenses

Interchangeable lenses are more commonly found on cameras aimed at professionals and enthusiasts including large format, SLR (medium format and 35mm) and high-end rangefinder cameras. The advantages to interchangeable lenses include:

  1. A larger range of focal lengths and specialties (shift, macro, etc.) are available than you are likely to find on any fixed lens camera.
  2. Each lens can be designed for a specific kind(s) of working situations and specialties without the compromises a generalist fixed lens has to be designed for.
  3. Longevity - you can upgrade your camera body without losing any investment you have made in additional lenses if your new camera choice is in the same family as your old camera.

A given interchangeable lens body can accept one type of lenses. There are cases of compatibility, when different bodies share the same lens mount. Adapters can exist to put a lens designed with one type of lens mount on a body designed for another.

Auxiliary lenses - if your camera has a fixed lens there are accessories available that allow you to enhance your fixed lenses range. These included close-up lenses that allow your camera to focus closer than it naturally can. They also include wide-angle attachments that allow your fixed lens to capture more of a scene than it otherwise could. They also include popular telephoto attachments that allow your fixed lens to reach further than it otherwise could. Telephoto attachments include extreme digiscoping lenses. Digiscoping is the practice of mounting a digicam on a spotting scope of telescope to create extreme focal lengths.

As with all photographic equipment, auxiliary lenses range in quality from the truly dreadful to the professional. Bear in mind that any auxiliary lens that you attach to your fixed lens is adding more glass between the subject and the film. As such it is bound to affect image quality and the amount of light passing through to the film plane. Cheap auxiliary lenses add horrible distortion and purple fringing to your shots. Auxiliary lenses are a compromise solution to extending the range of a fixed lens that can provide good results but there appear to be no bargains in this niche market place and you will get what you pay for.

Alphabetical list of lens mounts

For a list of lens mounts, see Lens mount.

Zoom and Prime Lenses

Prime

The term prime refers to a lens with a single focal length. Typically, prime lenses have larger maximum apertures, so they are able to let in more light wide open than similar zoom lenses. This makes prime lenses more suitable to low-light photography.

Zoom

A zoom lens is a compound lens with a variable effective focal length. While (contrary to a popular misconception) the perspective does not change, shifting the focal length of a zoom lens does allow the photographer to modify the crop of a photo without moving. Zoom lenses are bulkier than fixed lenses, and they introduce an extra adjustment you must make before taking the picture. The vast majority of digital cameras come equipped with Zoom lenses.

The zoom ratio is the ratio between the shortest focal length and the longest focal length of a given lens. The majority of modern zoom lenses are about 1:3, meaning that their longest focal length is 3 times the shortest. For example, there are many 35-105 lenses available. As the ratio gets bigger, the lens becomes much harder to manufacture, and more expensive. Some modern digital cameras have zoom ratios of 1:10, or even 1:12. It may be that such a camera could lessen the need for interchangeable lenses, and perhaps these will become more of the norm. Currently, they represent the leading edge of consumer optical technology.

Frequently, lenses for digital cameras are labelled with the focal length they would have if they were 35mm cameras. This gives a way of comparing zoom ratios between film and digital cameras. In any case, divide the larger number by the smaller. If the result is less than 3, then it's unimpressive. If it's about 3, then it's a normal, conservative design. If it's much greater than 4, some testing might be in order.

Classifications

Fisheye

Fisheye lenses have the widest field of view of any lens group and they distort the resulting image in a dramatic way. Fisheye lens fall into two categories:

  • Circular fisheyes: produce an image usually with a 180 degree field of view and dramatic vignetting resulting in a circular image in the 35mm format. They usually have a focal length around 8mm.
  • Full frame fisheyes: also have a 180 degree field of view but they stretch the image of the full film plane. Sometimes this category of fisheyes is known as corrected fisheye lenses.

In general fisheye lenses are expensive and little used in everyday photography.

Fisheye adaptors: auxiliary lenses are available that simulate a fisheye field of view. This is cheap way to play with the fisheye effect without investing in a dedicated fisheye lens, although, as with most auxiliary lenses, the quality of your images will not be the same as those taken using a 'real' fisheye.

Wide-angle

A wide-angle lens is a lens with a focal length shorter than normal. Wide-angle lenses have a tendency to introduce visual distortion.

Normal

A normal lens refers to a lens that creates images without visual distortion or compression. In 35mm photography, 50mm is considered to be the normal focal length. For medium format photography, normal is generally at 80mm.

Telephoto

A telephoto lens is any lens with a focal length longer than normal. Telephoto lenses have a tendency to introduce visual compression.

Lens Speed

Lenses that let in a lot of light are called 'fast' lenses. This quality is indicated by a number which is called the maximum aperture or maximum f stop. The smaller the number, the faster the lens. A very fast lens will have a maximum aperture of f2.0 or less. As lenses get faster than about f2.0, they become much more difficult to make, much larger, and much more costly. All very fast lenses are interchangeable lenses, so if this will be important to you, you will need an interchangeable lens camera. Very high speed would be important if you wanted to take photos in dim light without flash, and without a tripod.

Lens Coating

A lens coating applied to the each element of a lens (anywhere an element surface is exposed to air - not just the front element) helps to correct color abnormalities in the glass, reduce flare, improve contrast, and protect exposed glass surfaces.

Multicoating refers to the application of more than one coating on a lens.

All modern, mainstream lenses are coated.

Depth of field

The depth of field is a way of describing how much of your image is in focus. When the camera is focussed at a certain point, it will remain in focus for objects slightly in front of that point, as well as slightly behind. The distance between the closest object that is in focus, and the most distant one, is the depth of field. The depth of field is dependent purely on the geometry of the lens, and cannot be changed by the manufacturer. Generally, the shorter the focal length of the lens, the greater its depth of field. Since all small digital cameras have lenses with very short focal lengths, they tend to have very large depth of field. This has many benefits, and generally makes the job of the autofocus mechanism much easier. On the other hand, certain aesthetic effects become more difficult when the lens has too much depth of field. Finally, the faster the lens, the lower the depth of field. This means that while using a very fast lens will allow you to photograph in dim light, it will be very difficult to adjust the focus when you do this. Moral: if you think you want a very fast lens, you will pay for it in cost, weight, bulk, and poor depth of field. Zoom lenses have more depth of field when set to their shortest focal length, than when set to the longest.

Angle of view

In general, the shorter the lens, the wider it's angle of view. It's much easier to get very wide angle lenses for film cameras than for digital, unless you are talking about very expensive digital cameras. Many digital cameras suffer from a lack of wide angle ability, and if that's important to you, that will affect your choice. If you must have very wide angles, you will need to get a camera with interchangeable lenses, whether film or digital. Some lower cost digital cameras can be fitted with add-on lenses that increase their angle of view. In 35mm cameras, a 50mm lens is said to give an angle of view similar to the human eye, though many folks dispute this. nevertheless, this has come to be called a standard lens, for 35mm cameras.

Distortion

This is when the lens represents straight lines as bent. This can be often seen in zoom lenses at both ends of the zoom range, straight lines at the edge of the frame will appear slightly curved. There seems to have arisen a kind of accepted dogma that this is a bad thing, though nobody seems to explain why. Let's just say that if you think that's a bad thing, then check carefully for that behavior in any camera you are choosing. In the majority of photographs, it simply doesn't matter. Once again, it seems to be difficult for lens manufacturers to achieve very low distortion in conjunction with all the other good features they want their lenses to have.

Barrel Distortion

When straight lines bow out towards the edge of the frame (like the profile of a barrel) it is known as barrel distortion. This is typically found to some extent at the wide end of many zoom lenses.

Pin cushion Distortion

When straigh lines bow in from the frame edge it is known as pin cushioning. This is typically found at the long end of zoom lenses.

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