Difference between revisions of "Lens"
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| image_text=<small>fast lenses from left to right:<br/>[[Konica]] M-Hexanon 1.2/50, Konica Hexanon 1.2/60,<br/>[[Canon]] 1.2/50 [[Leica]] Noctilux 1/50</small>
Revision as of 16:02, 26 October 2011
A lens (here: photographic lens, also known as objective lens or photographic objective) 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.
- 1 Anatomy of a Lens
- 2 Geometry of a Lens
- 3 Fixed and Interchangeable Lenses
- 4 Zoom and Prime Lenses
- 5 Angle of view
- 6 Classifications
- 7 Lens Speed
- 8 Depth of field
- 9 Distortion
- 10 Lens fault corrections
- 11 Lens design
- 12 See also
- 13 Links
Anatomy of a Lens
|lens element's surfaces:|
1. ground, 2. polished, 3. coated
An element of a photographic lens is an optical lens itself, but one consisting of one single round piece of honed glass or plexiglass. Most photographic lenses have several different elements combined in the lens barrel, all having the same optical axis. Two or more elements which are cemented together (without an air space between them) make a group. (Older lenses used Canada balsam, made from pine pitch, for cement, while newer lenses use higher-tech UV-cured cements.)
Making a lens element is not as simple as just having a piece of founded glass. That's just the beginning. After having founded a an appropriate piece of glass its surfaces must be grinded until their plane or spherical concave or spherical convex surfaces are according to the lens elements' mathematically calculated geometry. If a surface has to be aspherical the grinding process becomes more complicated. After grinding the surfaces must be polished since they shall be transparent. Nowadays one, some or even all element surfaces of a lens get a transparent special coating against reflections between the elements. The elements are classified as converging (light-bundling) and diverging (light-spreading) elements.
The basic materials of lens elements is either optical glass or transparent plastic material like acrylic glass or plexiglass. Some microscopes use oildrop lenses, and some lens constructions have elements filled with water. An exception are the mirror elements of some super-tele lenses which can be made of other materials.
crown glass ...
Crown glass is a very old kind of glass, once blown into a "crown" to achieve the round elements of medieval glass windows. Its better variants were also used as optical glass.
... and flint glass
Flint glass is made with a high allotment of lead. Its prismatic division of colors is quite different from that of crown glass. That effect made the construction of color-corrected lenses possible by combining elements of both types of glass so that each neutralizes the prismatic color dispersal effects of the other. The first renowned achromatic lens construction was found 250 years ago by the English optician John Dollond on the basis of his own research and maybe little help of the Swedish expert Samuel Klingenstierna.
|barrel containing a module of a Zeiss lens|
above assembled, below lens elements,
barrel and adjustment rings
The barrel is the tube-shaped outer shell that contains the lens elements.
|10-blade diaphragm as aperture of a Robot|
image by Raphael Borges (Image rights)
The aperture is a round opening in or behind a lens (or between elements of it) that limits the amount of light passing through it into the camera. The lens's optical axis passes in 90 degree angle through the centre of that hole. The term aperture is also often used to describe the amount of light transmitted by the lens, for which the size of this opening is one determining factor. It is usually adjustable, automatically or by turning an aperture control, usually but not always on the lens barrel. A diaphragm is a round aperture of variable size.
A lens mount is the part of an interchangeable system lens that connects to the camera body. Different manufacturers have created many lens mount standards, using a variety of threaded, bayonet, or breech-lock mechanical attachments. A key difference between mount standards can be their flange focal distance.
Geometry of a Lens
The optical axis is an imaginary straight line which passes through the centers of curvature of the lens elements and meets the (untilted) focal plane at a 90-degree angle.
The optical center is the point on the optical axis where it passes through the (theoretical one) plane of refraction of the lens
The focal plane is the plane onto which a lens projects the image of the focused image subject. Usually it's flat but especially some old bakelite cameras with just a simple meniscus lens have a curved image plane since the "curvature of field" of such lenses is stronger than that of more sophisticated multi-element lenses which deliver more or less "planar" images. Usually the middle of a focal plane sits at a 90-degree angle to the optical axis, except when tilt/shift movements cause deviations from a camera's normal geometry of light-pathes. The position of the film's or digital sensor's light sensitive surface should be identical with the focal plane.
The focal length of a lens is the distance on the optical axis from the focal plane to the optical center of the lens when it's focused to infinity. In modern usage, this is expressed in millimeters; but older lenses may state focal lengths in cm or inches instead.
For a subject at a certain distance, doubling the focal length will double the subject's height in the image plane. Likewise, smaller focal lengths mean each part of the subject is reproduced at a smaller scale. Hence a wider view of the scene is included in the photo.
Focal lengths must scale in proportion to the size of the film or digital sensor image format used. That is to say, a 135mm focal length would function as a long telephoto on a Micro Four Thirds camera; while for a 5" x 7" format view camera it would be a wide-angle lens.
Because photographers often move between cameras using very different image-format sizes, it is sometimes useful to express lens coverage/magnification in terms other than optical focal length. These include:
- Diagonal angle of view
- Ratio of focal length to diagonal of image format
- "35mm equivalent" focal length (widely used for digital cameras)
Fixed and Interchangeable 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 (with some exceptions like the Mamiya C) and folding cameras, through the consumer rangefinders 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:
- As no mechanism for changing lenses needs to be built into the camera design it can help keep the camera smaller and lighter.
- In digital cameras a fixed lens means that there is less chance of introducing dust to the sensor surface.
- Fixed lenses are designed for a specific camera model and so fewer compromises have to be made in the lens design.
- 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.
- 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 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:
- A larger range of focal lengths and specialties (shift, macro, etc.) are available than you are likely to find on any fixed lens camera.
- 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.
- 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 marketplace 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
The term prime refers to a lens with a single focal length. Typically, prime lenses (except repro 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. Except for ultra-wide-angle lenses, prime lenses give images with less distortion.
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, but they introduce an extra adjustment you can 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.
Angle of view
In general, the shorter the focal length of 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 people dispute this. Nevertheless, this has come to be called a standard lens for 35mm cameras.
A normal lens has an angle of view that approximates how the human eye sees a scene. A lens is considered normal when its focal length is approximately equal to the diagonal of the film format. Lenses shorter than normal are called wide-angle, while those longer are called telephoto. In 35mm photography, 50mm is considered to be the normal focal length, even though the actual diagonal of the frame (24mm x 36mm) is 43mm. For medium format photography (frame size 2-1/4" square, or 6x6cm), normal is generally 80mm.
A wide-angle lens is a lens with a focal length shorter than normal; that is, smaller than the diagonal of the image format.
A rectilinear wide-angle lens is one reasonably free from geometrical distortion—straight lines appear straight (unlike a fisheye lens). Rectilinear wide-angles may include 65° to 120° coverage as measured on the diagonal. While geometrically accurate, this wide coverage can create an impression of distorted perspective; this increases as the focal length of the lens decreases.
A long-focus lens is any lens with a focal length longer than normal. That is, its focal length will be twice the image diagonal or greater. These lenses have a smaller angle of view than a normal or wide-angle lens.
Focal lengths in the range of 2 to 3 times the image diagonal are often termed portrait lenses. To avoid exaggerating a person's nose, the camera should stay back at least 4 feet/1.25m; a portrait focal length can fill the frame with a face at this distance.
A very long-focus lens brings far subjects closer, like a telescope. The longer the lens, the more likely that camera shake will blur the image; for this reason, longer lenses are frequently used with a tripod or other support to steady the camera.
Originally, a telephoto lens was a particular optical design used in the construction of many long-focus lenses. A simple telephoto lens has a convex lens group at the front and a concave lens group at the rear. This optical design results in a lens with a physical length shorter than the optical focal length.
The distinction between "long lens" and "telephoto" is significant to view camera photographers, since it affects the bellows extension required. Most photographers use "telephoto" to mean any longer-focus lens, regardless of its optical construction.
Fisheye lenses have the widest field of view of any lens group. The geometrical projection is far different from the classic perspective we are used to, as discussed here, and straight lines appear curved if they are near the edge of the image. This creates distortion of the resulting image in a dramatic way. Fisheye lenses fall into two categories:
- Circular fisheyes: have a 180 degree field of view when measured along the smallest dimension of the image, resulting in a circular image with black corners. In 35mm format, they usually have a focal length around 8mm.
- Full frame fisheyes: have a 180 degree field of view when measured along the diagonal, so the image extends on the full film plane. In 35mm format, they usually have a focal length around 16mm.
In general, fisheye lenses are expensive and little used in everyday photography. They are used for measuring, scientific research tasks, especially meteorology, illumination studies, botanic studies on trees. Another important application field is panoramic and VR photography
Fisheye adaptors: auxiliary lenses are available that simulate a fisheye field of view. This is a 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.
Macro lenses are lens heads for SLR's supplementary bellows, or belong to the macro subclasses of wide-angle lenses, normal lenses, mainly zoom lenses, and even tele lenses (telemacros). Macro means that macroscopic exposures are possible since these lenses allow very near image subject distances. Wide-angle lenses may allow distances of 20 cm. On most zoom lenses near distances cannot be chosen directly. Those lenses have to be switched to a special macro mode. Modern digicam zoom lenses have macro modes for minimal image subject distances between 1 (!) and 10 cm. Lenses with a longer tube elongation added may allow shorter distances. Such elongations are usually reached with supplementary macro bellows or macro rings. On top of a macro bellows nearly any sort macro lens head, normal lens, wide-angle lens or zoom lens can be used for macro photography, even if the lens is not explicitly sold as macro lens. For macro lenses or lens/elongation combinations the maximum reproduction scale (or reproduction ratio) is a characteristic parameter. For example a reproduction scale of 1:3 means that the object focused in shortest distance will be reproduced in one third of its original size. Only on a print will it appear enlarged. A reproduction scale of 2:1 means that the object focused at the shortest focusing distance will be reproduced in twice its original size. It will appear enlarged on the negative.
Repro lenses are not constructed for speed but for sharpness. Many are made for making images of frame sizes greater or equal 9×12cm. And many allow short subject distances. These lenses are ideal for reproducing two-dimensional objects. Used on long bellows repro lenses are good macro lenses. Another usage is making images of still objects in studios. Naturally repro cameras need this sort of lens. Many repro lenses, even some for smaller frame formats, are constructed apochromatic.
Some lenses are optical adapters allowing to use cameras with microscopes, endoscopes or telescopes. Others are converters to be mounted between camera and lens to give the lens twice of its original focal length.
|Three 135mm lenses for 24x36mm frame format|
with speeds f2.8, f3.5, and f2: the higher
the speed, the larger the lens diameter.
image by Alle-Dagen-Dromen (Image rights)
|fast lenses from left to right:|
Konica M-Hexanon 1.2/50, Konica Hexanon 1.2/60,
Canon 1.2/50 Leica Noctilux 1/50
image by fotograf@flickr (Image rights)
Virtually all lenses are labeled with a number giving their maximum aperture or lens speed, reflecting the importance of this parameter.
Lenses that, relatively speaking, let in a lot of light are called fast or bright lenses. So for example an f/1.4 lens is one stop faster than an f/2 lens (it admits twice as much light). Fast lenses are important if you want to take photos in dim light without flash or a tripod, or if you desire shallow depth of field. As lenses get faster, they become larger, heavier, more difficult to make, and more costly. Maximum aperture is often a major differentiator between a manufacturer's various lens price/quality levels.
"Fast lens" is a relative description: It depends on the image format and the focal length. A lens's widest f-number is the focal length of the lens divided by the diameter of the widest diaphragm opening (or strictly speaking, its entrance pupil). Longer focal lengths require larger lens diameters to yield the equivalent f-number; and so e.g. the fastest lenses available for large-format view cameras will rarely be faster than f/4.5. In contrast, the short focal lengths used for 8mm and 16mm movie cameras, or video security cameras, may routinely be f/1.4 (even for zoom lens designs).
In the context of 35mm film, (using a 24x36mm image size), the fastest available lenses will be in the focal length range of 35mm to 85mm. In this range, fast lenses of f/1.4 are available—or more rarely, even f/1.2 (one-half stop brighter). For a telephoto of 200mm focal length, f/2.8 would be considered a fast lens; while a maximum aperture of f/2.0 would be exceptional (and dramatically larger, heavier, and more expensive).
Early 35mm cameras were supplied with f/4.5 or f/3.5 lenses; but by the 1970s f/1.8 or f/1.4 normal lenses had become commonplace. The industry's subsequent shift towards zoom lenses sacrifices a stop or two of lens speed, particularly when these are zoomed to the long end of their focal length range. A zoom designed to maintain a constant f/2.8 maximum aperture across its focal length range is "fast" in that context, but at a significant penalty in weight and cost.
Depth of field
The depth of field is a way of describing how much of your image is in focus. When the camera is focused 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. Zoom lenses have more depth of field when set to their shortest focal length, than when set to the longest. 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, for example having a sharp subject emphasized over a blurry background.
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. But of course you will be able to take pictures in dim light, and to detach your subject from the background in more situations.
- Depth of field explained by Ching-Kuang Shene
- In French: Depth-of-field chapter of the Photography wikibook
image by Voxphoto (Image rights)
image by Voxphoto (Image rights)
Geometric distortion is when a lens represents straight lines as curved ones. This can be often seen in zoom lenses, at both extremes of the zoom range: straight lines at the edges of the frame may bulge in or out. Whether this is objectionable depends on the type of pictures you take. It will usually be bad with architectural pictures, or with pictures full of geometrical shapes, and for the reproduction of documents, paintings etc. It will usually not matter for portraits and for landscape shooting. Also modern image-editing software often provides simple tools to counteract lens distortion.
It may be difficult for lens manufacturers to achieve very low distortion in conjunction with all the other desirable features they want their lenses to have. It is a particular challenge to design wide-angle lenses for SLRs which are low in 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. A fish-eye lens is a design where extreme barrel distortion is included deliberately.
Pin cushion Distortion
When straight lines bow in from the frame edge it is known as pin-cushioning. This is typically found at the long end of zoom lenses.
A more complex form of distortion where the degree of barrel or pincushion distortion varies across the field; lines may take on a wavy appearance which simple software tools cannot correct.
Lens fault corrections
Astigmatism (pointlessness) is when a point sending light through a lens cannot be projected as one point behind the lens. It appears as a line on the focal plane. Another explanation is that astigmatic lenses cannot project horizontal lines into the same image plane as vertical lines. That effect mainly appears when biconvex or biconcave lens elements are used. In the case of the biconvex lenses in our eyes astigmatism can be corrected by a lens element with reverse astigmatic effect (cylinder lens). The only possible correction of astigmatism of camera lenses is to combine at least 3 lens elements. The different elements of a well-constructed triplet minimize astigmatism. For more than one hundred years most photographic lenses are anastigmatic, but the term was still used for marketing camera lenses until the 1950s.
Chromatic aberrations are reduced by using elements made of different varieties of glass. Elements made of different glass help to bundle red, green and blue light that is coming from one single point in front of the lens in one single point behind the lens. Lens designs which are projecting blue and green light focused unified are called achromatic lenses. When the focusing correction of the lens includes blue, green and red light it's an apochromatic lens design.
|image by Raphael Borges (Image rights)|
Coating against reflections between lens groups
Modern lenses are coated with a very thin layer of antireflective material (like magnesium fluoride or calcium fluoride). This lens coating is applied to the each element of a lens which has a surface exposed to air. The only purpose of coating lenses is to reduce lens flare by eliminating reflection off the surface of the glass; this has the effect of increasing contrast and giving images more "punch". Lens coating thicknesses are typically of the order of a few wavelengths of light, - a few nanometers (nm). Lens coatings have nothing to do with color correction of lenses, as is widely thought.
Multicoating refers to the application of more than one layer of coating on a lens.
All modern lenses are coated. Coating gives lenses their colored reflective look. Since lens coatings are relatively fragile, one should always be careful when cleaning coated lens surfaces (like the front of the lens), so as not to scratch or smudge them. Many photographers keep a daylight or UV filter on the lens to protect its surface and avoid the necessity of frequent cleaning.
- For an explanation of how lens coatings work, check here.
According to this source  of Rochester University ("Wither Optical Design?", by Douglas Sinclair) lens design reached its heydays in the 20th century. It was the task of the "traditional" lens designers to "balance the aberrations of centered optical systems to achieve a maximum image quality". The optical engineer "worked on layouts and negociated with the marketing and mechanical departments to get enough room for the design to be implemented within the laws of physics." On the one hand Sinclair predicted that the optical engineering will take over the lens designers' tasks in the 21st century. On the other hand he reported from the 1998 lens design contest of the renowned International Optical Design Conference that the top-five designs were made by experienced lens designers. All of them used lens design software. Sinclair's prediction may be wrong: The contest proved that modern lens designers can work with software tools which may be developed by optical engineers, but as Sinclair himself wrote the good lens designs are more than the software can generate. According to his writings it's almost an art to add practical lens design experience to purely technical solutions. What may have changed is that both, optical engineers and lens designers need broader understanding of the whole related fields of physics, geometrical optics, wave optics, algorithms, etc. .
see also Category:Lens designers
- Explanation with photos, Courtesy of toxpose.com
- In Praise of the Standard Prime Lens
- fisheye lens database
- macro lens database
- Simple lens tests are found at this Kiev 60 page
- history of small format film lenses
- Interchangeable non-AF lenses: an attempt of clasification, at Manual Camera
- aberrations, explained on homepage of JML
- lens triplet in wikipedia
- shareware document of Darko Vasiljevic about triplet optimization