I remember one sunny afternoon -about fifty years ago- in downtown Mexico City, glued to the window of Foto Rudiger, looking in awe at hundreds of lenses in display. Why are there so many lenses? I asked my father. His answer: "Because lenses are designed for specific purposes, subjects, situations and needs, including budgets."
That afternoon -around several cups of Brazilian coffee- I also learned that all laws have as reason to exist the promotion of a good and/or the avoidance of an evil, and that physics laws of optics are not different. And so, lenses designers look at ways to apply those laws better and better, to promote greater magnification with amazing detail or wider splendid vistas, avoiding distortions and other aberrations as much as glass technology allows at the time.
I also learned that afternoon that the selection process needs not to be a puzzle; once your own set of variables is clear in your mind, choices pop up at you by themselves. That simple. To tackle this subject comprehensibly, although seemingly an enormous task, it is not.
The reasons for the initial overwhelming are, mainly: the wide variety of both lenses offerings and the variety of needs of each individual photographer (the set of variables):
The primary interest (sports, fashion, portraits, landscapes, wildlife, architecture, weddings, photojournalism, macro, travel, etc.)
Which comes immediately into mind, especially if one selects more than one interest; and the seriousness of the photographer, really meaning how much can that budget be stretched.
What camera is in use and with what format (DX or FX); what others are being considered.
What other lenses may already be in the bag.
Film or digital; is there a transition from one media into the other in the near future, and so on.
Let's look first at the basic characteristics of lenses to later match them to our own set of variables.
Lenses can be classified according to focal length, its variability, focusing method, and its relative speed as follows:
By FOCAL LENGTH: Fish-eye; wide angle; normal; short, medium and long telephoto
35mm or FX
Angle of View*
|6 to 16mm
|220° - 180°
|17 to 35mm
|114° - 62°
|45 to 55mm
|51° - 53°
|85 to 135mm
|28° - 18°
|180 to 300mm
|13° - 8°
|400 to 1000mm
|6° - 2.5°
Zoom lenses have now crossed the typical categories above, in an effort to widen the range of applications a single lens can be used for.
By VARIABILITY OF FOCAL LENGTH
- With fixed focal length or "prime" lenses
With variable focal length or "zoom" lenses
- Zoom lenses with variable "speed" or aperture
- Zoom lenses with constant maximum "speed" or aperture
By FOCUSING METHOD
- Manual focusing or "MF" lenses (Rotating a ring in the lens barrel)
- Auto focusing or "AF" lenses (Depressing the shutter button on an automatic camera)
By "SPEED" or "LUMINOSITY" in terms of how much light they allow into the film or digital sensor at its widest aperture
- FAST. With a small f/stop number, wide aperture, (f/1.4, f/2.8), called "fast" in reference to the relatively fast shutter speeds you can use with these lenses.
- SLOW. With a not so small f/stop number, not so wide aperture, (f/4, f/5.6), called "slow" because you cannot use as fast shutter speeds as with "fast" lenses.
*FOCAL LENGTH AND ANGLE OF VIEW
Focal length is the distance between the lens node of emission to the plane at which objects at infinity are brought into focus to form a sharp image; i.e. at the film or sensor plane in a camera.
The focal length determines the size of the subject image on film when the lens is focused at infinity.
Such image size is directly proportional to the focal length. A 100mm lens will render an image size twice as large as that from a 50mm, a 200mm a four times larger one, etc. This also tells us that with focal length the magnification of the subject changes for a fixed lens to object distance, but not the perspective. The perspective changes only when you change the lens to subject distance.
The angle of view (AOV, sometimes referred to also as Field of View) is the angle covered by a lens of given focal length at infinity. Typically it is measured diagonally and for a given film or digital sensor format. In a rectilinear lens, it is possible to approximate it with the formula AOV = 2 arctangent [x / (2 f)], where "x" is the diagonal of the film (43.2666mm for 35mm format) and "f" the focal length in mm1.
In practice, with complex optical formulas employed in modern lenses, the AOV has to be measured with specialized instrumentation. But the important thing is that the angle of view is inversely proportional to the focal length, so the shorter this is, the wider the AOV and viceversa. Also, to increase the focal length to twice, reduces the AOV in half.
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