Lenses are the most fascinating equipment we, as photographers, use. They come in various shapes and sizes. And they have each been fabricated with a specific use in mind. We will break down, in this article some of those fancy-shmancy words you hear associated with the photographic lens. We will also cover types of lenses and what are groups and elements. This article will not delve into aperture too deeply and stick to differentiation between types of lenses. So, without further ado . . .
LENS - noun - a piece of transparent substance, usually glass, having two opposite surfaces either both curved or one curved and one plane, used in an optical device in changing the convergence of light rays, as for magnification, or in correcting defects of vision.
Many times we may hear of a lens being referred to as a 3-group 7-element lens. Huh? Well, lenses consist of glass (the good ones) or plastic focusing material. Really, anything that is transparent. Resins can be used and are used in lens fabrication by some hobbyists. Pieces are placed against each other to direct and correct the light as it enters the lens on its way to the film. These pieces are elements. When an element is by itself (not in conjunction with any other elements in the lens) it is said to be a simple or single element. Simple is usually the correct term unless a singlet lens is being referred to. Where two or more elements or assembled to form a group of elements they may be referred to as, well, a group.
Lenses can be classified into groups (of lenses, not to be confused with groups of elements) such as singlet lenses, duplet lenses, triplet lenses, Petzval-type lenses, telephoto lenses, mirror lenses and zoom lenses.
Singlet lenses are cheap, inexpensive and mass produced lenses that one might find in a disposible camera, older Brownie box cameras and the like. A singlet lens is a lens assembly that has a single lens element or group situated either before or behind the aperture opening or diaphragm.
Duplet lenses cover a wide range of lenses where there are two elements or groups which are either symmetrical or nearly symmetrical on either side of the diaphragm. Perhaps the simplest such example would be an early Steinheil periscopic lens which consisted of a simple curved lens element before and aft of a diaphragm. The Harrison and Schnitzer 'Globe' lens of 1865 is another example where the outer edges of the objectives were placed so that, by following their arc in a circle, the opposing objectives were aligned so as to form the sides of a perfect globe. An extreme example was the Sutton liquid-filled panoramic lens design of 1859 where the curvature of the glass was very extreme and the center of the obtective was filled with water to further accentuate the wideness of the field of view that this lens had.
There are, of course more numerous modern examples of duplet lenses: the rapid rectilinear, the Goerz Dagor, the Zeiss double Protar and the Voigtlander collinear to name a few. The mentioned Zeiss was configured so that the user could use either the fore or aft elements seperately or combined at any desired focal length and this lens was hence referred to as a 'convertible'.
Triplet lenses were first introduced by H. Dennis Taylor of England in 1893. His Cooke lens was composed of three simple seperate elements and could correct for most abberations. It was generally used with wider apertures from 4.5 to 2.0 with the aperture being located just in front of the aft element. H. Harting of the Voigtlander Company came out with the Helair lens in 1901 to follow suit. And the design saw little improvement until after The Great War.
Many tried to design a wide aperture triplet and one man in particular finally led the way out of the objective mire. Of course, it would happen to be Carl Zeiss. His gas filled Sonnar 1.5 series corrected for edge stagmatisms and became the first wide aperture triplet to be widely used on minature cameras. Another successful lens of this design would have to be the f/1.9 Hektor Rapid designed by Berek for the Leica company where the 'Cooke' was improved upon by replacing all of the simple elements for complex, or doubled, elements or groups.
The Petzval lens has been covered in great detail elsewhere so we will cover the basics here. Petzval first created a fantastic portrait lens by using two groups of elements. The first was a telescopic group way up in the front of the lens and widely seperated from a second corrective group back near the aperture/diaphragm. For a 25 degree field of view lens, he found he had an aperture rating of f/3.5 over half of the viewing area, or 12 degrees. So his first lens was a tight and shallow lens, ideal for portraiture.
Any portrait lens suffers the fate of misnomer. The term 'Petzval' lens is no longer in use commonly. However every single portrait lens, including my dreaded Celtic MD mount 135mm lens, owes its existance to its progenitor, the Petzval lens.
A Telephoto lens is constructed so that the front positive element and the rear negative element are placed in such a way as to bring in distant objects and increase the viewing magnification. Usually the physical length of the lens from film plane to front element is significantly lower than the focal length, the opposite truth for wide angle lenses. Early representatives include the Tele Tessar, the Cooke Distortionless Telephoto, the Kodak Ektar f/3.7 the Baker 40" f/5 Aerial Telephoto.
Is it possible to gain greater magnification yet from a reversed telephoto? If the powerful positive element which is now in proximity to the film plane is moved further from the film plane than was its negative partner in the regular telephoto lens, then your answer is yes. Case in point, a Catadioptric (Cat), or Mirror lens. Essentially your mirror becomes your rearward positive element and the smaller negative element further compresses the light incoming to produce extreme telephoto in a very short physical dimension.
ZOOM. Say it again. ZOOM. Z-O-O-O-O-O-O-M-M-M-M-M-M-M. (Me, I just like saying it. Makes the milk come out my nose.) Sorry. Little bit of me comin' out.
All seriousness aside, zoom lenses are what join together light backcountry travel and options. Mobility and possibility. Aside from the first few attempts to develop a zoom lens in the 1930's, all zoom progess can be effectively considered post-war. The first zoom was the Zoomar for movie photography and hit the scene about 1945. TV cameras had some zooms in the early 50's and 8mm cine's inherited them around 1956.
Most zooms have a mobile central element or group that is moved nearer to or further from the film plane thereby adjusting the degree of magnification visible in the viewing area. However, the motion of this central also affects the relative aperture at any focal length. This means that at the shorter focal length you should have a larger aperture than at the longer focal length, normally by at least a full stop. We are familiar with most zooms as, more than likely, we are still using them. 35mm photography didn't really get into the zoom craze until the 60's. I know I have a lens that dates back nearly that far and is likely one of the first.
Other names are attributed to lenses based on their optic correctiveness. An Anistigmat lens is produced being able to correct for the abberation of astagmitism and curvature of field. There are many optical abberations and many corrections of them. We will not dig into these terms here to deeply, only to place their purpose in your mind for future reference.