The Lens

The lens is the part of the camera which gathers light from the scene to be captured and projects it onto the film or sensor.

Because a single lens element is prone to distortion, additional elements are added to correct for these distortions. These elements are arranged in groups, and make up the “formula” of a lens. These formulas are frequently licensed to third parties, who then manufacture the lens to the original specifications.

The focal length of a lens determines its angle of view, or how much of the scene in front of the camera is captured in terms of width and height. Different focal lengths capture different perspectives, which varies depending on the “format” or dimensions of the captured image when measured diagonally. For example, a 35mm frame has a diagonal measurement of 43mm, and so a lens of this same focal length would be considered “normal” in this format.

Interchangeable Lenses
More sophisticated cameras, such as DSLRs, Mirrorless EVILs and Rangefinders have lenses which can be removed and replaced with lenses of other designs and focal lengths to extend their capabilities. Some of the different lens types are:

Prime lenses are multi-element lenses with only a single focal length. They are relatively faster, lighter in weight, and more compact than zoom lenses.

Zoom lenses are multi-element lenses with a continuously variable focal length within a preset range. They are somewhat slower, heavier and larger than prime lenses, although more convenient. However, because they can take the place of several prime lenses, overall they may conserve space and weight. Technology has advanced lens design such that today’s zoom lenses may produce better quality images than yesterday’s prime lenses. However, individually, prime lenses still have a size and weight advantage.

Normal lenses attempt to capture the natural perspective of an image as seen by a human observer. In the full-frame 35mm format, a lens with a focal length of 43mm would be considered normal. However in the early days of photography, 50mm had become the accepted standard due to the higher degree of sharpness of longer lenses. Today, lenses within a range of about 40mm to 60mm are considered normal.

Wide angle or “shorter” lenses capture a wider than normal angle of view, allowing more of the scene to be captured, or allowing the photographer to shoot in more confined spaces. They have a large depth of field, making them easy to focus. Because they tend not to amplify camera movement, they can be used with slower shutter speeds or smaller apertures. In the 35mm format, lenses within a range of about 28mm to 40mm are considered wide angle. 

Telephoto or “longer” lenses capture a narrower than normal angle of view, allowing objects which are far away to be captured as if they were closer, or to permit more space between the photographer and the subject. Because they tend to amplify the camera’s movement, they are generally used with faster shutter speeds and larger apertures. Telephoto lenses benefit most from “vibration reduction” or “image stabilization”. In the 35mm format, lenses within a range of about 100mm to 300mm are considered telephoto.

Super wide-angle lenses are used to immerse the viewer in the subject or give a sense of vast space. They have enormous depth of field and so are very easy to focus. In the 35mm format, lenses within a range of about 16mm to 24mm are considered super wide-angle.

Super telephotos lenses are used to capture intricate details of distant objects and to create intimate, covert images. They have extremely narrow depth of field and are difficult to focus and to hold steady. Therefore, they are often used with tripods. In the 35mm format, lenses within a range of about 300mm to 600mm are considered super telephoto.

Fisheye lenses are ultra wide angle lenses which generally provide a 180° angle of view, but do so with a distorted (non-rectilinear) projection. They require almost no focusing. Fisheye lenses come in two varieties. Circular fisheyes project a circular image on the negative or sensor, surrounded by black. If pointed upward, you would see a 360° view of the entire horizon around the outside of the circle. Full frame fisheyes crop the image in such a way that the diagonal measurement of the frame constitutes a 180° angle of view. Fisheye lenses can be used to create fun, humorous snapshots, or photos which possess an unusual, intriguing perspective. In the 35mm format, lenses in the range of 16mm are considered super telephoto.

Catadioptric lenses are a special type of super telephoto lens which employs two mirrors in place of refractive lenses to extend the light path, resulting in a shorter physical length. This also has the affect of reducing the number and weight of glass elements. They are smaller, lighter and less expensive than fully refractive telephoto lenses of the same focal length, and have almost no chromatic aberration. However, they produce a distinct, donut-shaped bokeh, are relatively slow, are difficult to focus and have poor contrast and saturation. With a little practice however, they can produce compelling images, not unlike the other extreme, fisheye lenses. Catadioptric lenses are generally available in the range of 500mm to 800mm and have a single, fixed aperture.

Macro lenses are a special type of normal to medium-telephoto lens which possesses closer focusing capability, but retains infinity focus. This allows the image to be magnified to a greater extent than with a standard lens. In many cases, a macro lens can also substitute for a standard lens. However, if infinity focus is not required, any lens can be converted to a macro lens by reversing it or fitting extension rings (short focal length), or close-up lenses (long focal length) to it. 

Perspective Control lenses are a special type of wide to moderately long lens which allow similar “movements” to view cameras. This allow you to shift and/or tilt the lens to increase depth of field without stopping down or to correct perspective distortion created by photographing a large object, such as a building, from a less than ideal vantage point.

Lens Focal Lengths
Which length and type of lens you select depends on the type of imagery you are planning to capture.

24-35mm — architecture, landscapes, photojournalism.

35-75mm — general purpose

75-200mm — portraits

200-1200mm — nature, wildlife, sports

Lens Mounts
There are many different types of lens mounts between brands of camera. Each has its own mechanical properties and “flange focal distance” (the distance between the outermost surface of the lens mount and the film/sensor plane) requirements. Some lenses can be easily adapted to other bodies, while other are more difficult. Lenses with shorter flange focal requirements cannot be used with bodies that have  longer flange focal requirements unless additional optics are employed. This makes them similar to a teleconverter in that light loss and additional aberrations are introduced. Camera/lens combinations that do not require optical elements yield considerably better results. Lens conversions are generally for manual shooting only; aperture coupling and auto focusing are not supported.

This table shows the flange focal distance for popular lens mounts:

Mount	Flange Focal Distance	Year Introduced
Canon EF	44.00 mm	1987
Canon EF-S	44.00 mm	2003
Canon FD	42.00 mm	1971
Canon FL	42.00 mm	1964
Contax/Zeiss M42 Screw (42 x 1mm)	28.8 mm	1949
Contax C/Y	45.50 mm	1975
Contax G	29.00 mm	1994
Fujica X	43.50 mm	1980
Konica AR	40.70 mm	1965
Leica M	27.80 mm	1955
Leica R	47.00 mm	1964
Leica M39 Screw (39 x .977mm)	28.80 mm	1930
Minolta SR	43.50 mm	1958
Minolta/Sony A	44.50 mm	1985
Sony E	18 mm	2010
Nikon F	46.50 mm	1959
Olympus OM	46.00 mm	1972
Olympus Four Thirds	38.67 mm	2003
Olympus/Panasonic Micro Four Thirds	20 mm	2008
Pentax M42 (42 x 1mm)	45.46 mm	1952
Pentax K	45.46 mm	1975
Pentax Q	9.2 mm	2011
Samsung NX	25.50 mm	2010
Sigma SA	44.00 mm	1992
Soviet M39 Screw (39 x 1mm)	45.2	1930
Tamron T2 Screw (42 x .75mm)	55.00 mm	1957

Freelensing
A technique that has gain popularity amongst the “Lo-Fi” or “Toy Camera” movement is called freelensing. This is the practice of removing the lens from an interchangeable lens camera and shooting through a lens of a different mount type by simply holding the lens in front of the camera mount. This introduces light leaks and tilt-shift characteristics which can add a creative flare to the photograph.

Lens Characteristics
Lenses can exhibit certain characteristics in different combinations and proportions, making each one unique. A lens which is very sharp, but has a high degree of chromatic aberration may not look much better than a softer lens with little or no chromatic aberration.

Bokeh is the appearance of the out-of-focus areas, that is, the area beyond the plane of sharp focus. It is more easily achievable with lenses of medium to long focal length, as they naturally possess a narrower depth of field. A smoother bokeh is said to be more appealing as it is less distracting when used to separate the subject from the background or forground. Catadioptric lenses have the most distracting bokeh, the highlights being composed of tiny rings rather than solid disks.

Chromatic aberration is the color fringing that appears along the boundaries of light and dark areas within the image and is corrected by combining elements with different dispersion characteristics. Axial CA occurs across the entire image plane and can be reduced by stopping down. Transverse CA occurs toward the outside of the image, and is unaffected by stopping down. However, transverse CA is relatively easy to correct during post-processing.

Coma manifests itself as a v-shaped or comet-like flare in the image, and is corrected by using multiple lens elements.

Spherical aberration results in a softening or blurring of the image due to light rays not all converging on the same spot. It can be minimized through the use of aspherical lens elements. Lenses bearing the word “Aspherical” or “ASPH” in their designation contain these elements which greatly reduce spherical aberration.

Most of the lenses available today are autofocus, which simply means that the camera senses the sharpness of the image and rotates the focus ring for you to bring the image into sharp focus. It does not however know exactly what you wish to focus on, so a sophisticated zone system is employed to allow you some control in this area. Autofocus is always a compromise, because although it’s faster than a human, it’s not ultimately as accurate.

Many manual focus lenses are available for autofocus interchangeable-lens SLR systems, and they are greatly improved by the autofocus sensor which still tells you when the image is in focus, even though the camera is not controlling the lens. Most systems allow you to focus autofocus lenses manually, but their operation is nowhere near as smooth and precise as a dedicated manual-focus lens.