THE LENS
Camera lenses are extremely complex devices consisting of a tubular housing (lens barrel) in which pieces of glass (elements) which are precision ground to form a specific shape are placed, some of which are movable for the purpose of focusing the image that they create. Inside the lens is also housed the iris diaphragm, and in the case of lenses for medium and large format cameras, also the a between-the-lens shutter may exist.
Lens which produces an image which conveys the image with the roughly the same perspective as the human eye sees the world is said to be a Normal Lens.
The seperate elements of glass are formed and polished to perform specific functions in the “bending” of light. Imagine the surface of a pool of still water. When light strikes the surface, it is “bent” inward toward the body of water. This is called refraction. The causes of refraction are complex, and we will not discuss them. The elements of glass in a camera lens operate in the same way. When light strikes the surface of the glass, it is refracted, depending upon the angle of the light ray to the angle of the surface of the glass and the shape of the glass element. As the light leaves the element, it leaves at a different angle than the one at which it was traveling when it struck the surface of the glass. The light rays refract when they strike each surface, on both sides of each element within a lens.
There are 3 types of elements in camera lenses.
Convex - curved outward on both sides
Concave - curved inward on both
Complex - curved on one or both sides, or curved on one side and flat on the other
The terms “complex” and “simple” are also used to describe lenses themselves. The following simple illustration demonstrates the way these 3 types of elements refract light. Elements amy take on various shapes, and they may also be various sizes within a lens. In this way, the light may be refracted this way and that to create the image. Focusing of the image is dome by moving certain elements toward the front or rear of the lens in tandem by mechanical means.
When the focusing elements are moved, they move the rear nodal point of the lens forward or backward slightly, and this brings the image into or out of focus. The rear nodal point of a lens is a point in space behind the lens at which the rays of light of the image meet, and where the image is flipped.
From the rear nodal point the image proceeds to the reflex mirror and/or the film plane (focal plane).
Lenses are denoted as having different focal lengths. Simply stated, the focal length of a lens is determined by the length of the lens from the surface of it’s front element to the film plane when it is focused to infinity (maximum distance).
The iris diaphragm is a set of titanium leaves which interleave to form an opening called the apeture. The apeture varies with the amount that these leaves are closed. You’ve probably seen doorways or hatches in scifi movies that operate this way.
(Internet source image)
The size of the apeture of the iris diaphragm determines the intensity of light which enters the camera and strikes the film. The diaphragm, like the shutter, may be controlled manually or automatically by the camera. The smaller the apeture, the larger the number, and the larger the apeture, the smaller the number. I will explain this in a moment!
Below is a scale of apetures common to photography in 1/2 f/stop incriments. Most hand-held camera lenses do not have apetures larger than f/1.8 or smaller than f/32.
f/(stop) Full stops are bolded
1.2 - very large apeture, bright viewing, strong light intensity
1.4
1.8
2.0
2.4
2.8
3.5
4.0 |
4.5 |
5.6 |---- sharpest images are had with these
6.7 |
8.0 |
9.5
11
13
16
19
22
27
32
45
64 - very small apeture, dim viewing, weak light intensity
All photographic lenses produce their sharpest image resolution at medium apetures. This was true in 1940 and it is true today.
The number applied to a particular diameter of the apeture is determined by the number of times that the diameter of that opening may be placed horizontally from the front element of the lens to twice the length of the the rear nodal point of the lens. In other words, it is the number of times it fits into the lens’ focal length. In this way, the size of an apeture of 5.6 will be smaller for a 50mm lens than it is for a 100mm lens, even though both apetures provide exactly the same intensity of light to enter the camera! F/stops are preset as settings of 1/2 incriments, but an iris diaphram in a lens may be set manually to any size you desire, even sizes between these notched settings. It is"infinately" variable between the maximum and minimum apetures.
In the following example, the f/stop (apeture measurement) would be f/5.6 because the diameter of the apeture can be placed end-to-end 5.6 times in the focal length of the lens.
Autofocus lenses for consumer cameras hit the market in the 1980’s. The early models were slow at focusing compared to more recent ones, and they were also much bulkier. One of the earlies models used a weak laser beam to measure the distance to the subject to determine proper focus. Today, autofocus lenses are second nature to cameras and most novice photographers don’t give them a second thought. There is a photographic technique called “selective focus” wherein the photographer uses the focusing of the lens to determine what part of the image will or will not be in sharp focus.
The different kinds of lenses are:
- fixed focal length
- variable focal length (zoom)
Among fixed focal length lenses there are normal lenses, wide-angle lenses, and telephoto lenses. Zoom lenses may provide a wide variety of focal lengths with a single lens. Zoom lenses, when “zoomed in”, have a telephoto focal length. Some zoom lenses are capable of zooming from wide angle to telephoto, while “longer” zoom lenses may zoom from short telephoto to longer telephoto focal lengths.
The focal length of a lens has an effect on the perspective it creates. Short focal length lenses, such as fish-eye lenses and other lesser extreme short focal length lenses produce pincusion distortion. This type of distortion creates an image that seems to expand the space between objects of diferent distances and swell inward toward the viewer, as though you are looking at the top of a pincusion. Long telephoto lenses trend to produce barrel distortion, which creates an image that seems to compress the distance between objects of different distances and makes an image that seems to be a bit flat looking. The effect varies from lens to lens, but it is less profound with telephoto lenses than with wide-angle lenses. These effects can be utilized by the photographer to create a special look in a photograph, although this is best used within limits and not often a desirable technique. Portrait photographers use short telephoto lenses because the slight compression of the image with such a lens helps to minimize their subjects imperfections, such as a long nose or chin.
Fixed focal length lenses have no zoom capability. If you want the subject to be larger or smaller in the image, you must get closer or farther away. Zoom lenses allow for the user to “zoom in” by changing their focal length. There used to be 2 kinds of zoom lenses: true zoom and “two touch” zooms. Two Touch zooms used seperate rings around the lens barrel for focusing and for zooming. These lenses have dissapeared since the mid 1980’s completely because of better technology.
Fixed focal length lenses are also dissapearing, as zoom lenses and auto-zoom lenses are replacing them. Among professional photographers, there is still a need for fixed lenses however. This is because fixed lenses are capable of having a larger maximum apeture. The advantage of this is the ability to see the scene through the camera lens more brightly in dim or limited lighting situations, such as at sports events at night or indoors with low incandescent lighting. Zoom lenses typically do not provide as large a maximum apeture as fixed lenses. The trade-off is the ability to zoom. However, there are elite lines of zoom lenses offered by camera makers which provide the same maximum apeture as most fixed lenses. They are however outrageously expensive, and may cost thousands of dollars a piece.
The ability of a lens to focus exceedingly close is called macro-focusing. Creating photographs using some degree of macro focusing at close distances is called “macro photography”, or more formally, photomacrography. Only some lenses have this special ability. Leses which have this capability are called macro focusing lenses. Macro focusing allows the photographer to photograph something very close and make it fill the image. For example, a macro focusing zoom lens is a good choice for zooming in close on an insect at close distance and having the bug fill the image area. Some specialty accessories for photomacrography include a rail bellows. This is a rail or set of rails with mounts on both ends between which a bellows resides. The lens is attached to one end and the camera to another, and the lens may be moved lengthwise on the rail to increase it’s focal length for extreme photomacrography. Such a device would allow you to make a photograph a small portion of the face on a small coin and have that portion fill the entire image area.
Seperate screw-on glass elements (lenses) called close-up lenses are also available which increase the magnification for a particular lens that they are designed for. I have some of those myself.
Extreme close-up photography is called photomicrography, such as the photographing of things through a microscope or objects exceedingly small, such as might be done for medical/forensic, criminal science, and research into devices and materials which are minute.