From so simple a sliver of glass to a complex collection of glass elements within a compact, finely constructed housing, the camera lens has seen an interesting and logical evolution for 450 years; despite the fact that photographic cameras have only been in existence since the 1830’s.

Meniscus Rising

A centuries-old development called the ‘camera obscura’ had been used as a form of entertainment by using a small hole in a wall to project a scene from the outside on to a wall within a room, although, as with cameras, the image was inverted (upside-down). It was in the mid 1500’s when Daniel Barbaro took a convex eyeglass lens, enlarged the hole in his camera obscura, and inserted the lens. This resulted in a brighter (the larger hole) and more detailed image (by no means a high quality image).

The use of a meniscus lens (a simple lens) provided a better image and the use of two lenses, a convex and concave, magnified the image, as was discovered by Johann Kepler in the early 1600’s, yet, both suffered from distortion and softening of the image. Then, in 1812, William H. Wollaston developed a solution to the problem by observing that the central portion of the lens produced a sharper image while the outer parts of the lens produced a softer image and caused distortion. The solution was to block or ‘stop’ the light from passing through the outer portion of the lens by using a metal disc with a small hole in it.

As time passed, we saw the introduction of stops with wider holes, also termed ‘faster’ as they allowed the slow films of the time a shorter exposure time; the development of better lens elements, such as achromatic designs which improved the focus of different colors of light at the film plane; and groupings of elements to make use of the advantages different elements had.

Work it, work it!

In the simplest definition, lenses work by bending light rays (called refraction) to focus as close as possible upon the same plane. Since red, blue, and green light rays are of different wavelengths, they do not naturally do this. The reason we can see these different colors in focus with our eyes is due to the length of the eyeball providing receptors at different points in our retinas which are sensitive to these different colors. That length is a dimension which is missing in capturing an image on the flat surface of a focal plane.

The inability to focus colors at the same point is called chromatic aberration and is corrected by the use of specialized lens elements designed to bend the light rays in such a manner as to bring them as close as possible to being in focus at the same plane. Achromatic lenses, the most common, accomplish this by bringing two colors into focus and one into very close focus. Apochromatic lenses bring all three colors into focus at the focal plane.

It’s getting deep in here!

An important part of the lens is the aperture as this controls the amount of light that can be exposed onto the film within the same amount of time. It acts as a flow valve for the light as a faucet does for water. The faster you have the water running the shorter amount of time it takes to fill a glass with water. Conversely, the slower the water is running the longer an amount of time it takes to fill the glass. The end result in both manners, however, is a full glass. As such the aperture adjusts the amount of time which is needed to properly expose the film.

The aperture additionally controls the depth of field, which is a measure of the distance before and after the point of focus where the image is considered to be of an acceptable sharpness (it is not a measurement of the area of focus). Closing down (reducing the size of) the aperture limits the area of the lens surface used as the center and outer parts of a lens bend light in different manners creating what is called spherical aberration. Closing down the aperture increases the resolution by reducing, if not eliminating, spherical aberration and gives more depth to the perceived area of focus. Keep in mind that a smaller aperture does not mean sharper focus because as the aperture gets smaller, at a point, it causes the light rays to bend away from the lens axis which causes a reduction in sharpness. The aperture at which an image is sharpest is around the mid-point of the aperture range, although it does vary by lens design.

Depth of field does vary with focal length as well. A longer focal length will provide a shorter depth of field providing better separation between subject and background. A shorter focal length provides greater depth of field allowing more subject to be capture within reasonable sharpness. I did not specify wide-angle or telephoto as only focal length determines this, not the relation of the lens to the size of the film. Hence, a large format camera with a 180mm normal lens has less depth of field than a 35mm camera using a 50mm normal lens, even though both cameras may be using a lens providing the same angle of view for its respective film size.

Since aperture is measured in f/stops, the relation of the stops are not immediately obvious as each stop is a factor of 2 relative to each other. A stop at f/4 would allow 2 times as much light as an aperture of f/5.6, conversely, an aperture of f/4 allows 2 times less (half as much) light than an f/2.8. The relation of the stops becomes more obvious when it is realized that each is a square root (rounded) of their ratio to the focal length (a measure from the optical center of the lens to the plane of focus) of the lens. Since the common range of apertures is 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, and 32 (the minimum and maximum vary by lens), the numbers can be squared which gives a range of numbers which vary by a factor of 2 relative to each other, hence the result would be 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, and 1024.

Let’s distort things!

Distortion is a fact of lenses that is inherent to their design and function. When things work properly and the lens has a focal length which approximates the diagonal of the film frame, distortion is relatively low with the primary distortion being as a result of lens aberrations. However, when that focal length changes, it can cause a distortion due to how it changes in relation to the film plane.

When the focal length of the lens is longer than the diagonal of the film plane, a telephoto, it is using a reduced angle of view to produce an image that is full-framed, rather like trying to expand the image which causes lines along the edges to bow into the image. This is called pincushion distortion.

When the focal length of the lens is shorter than the diagonal of the film plane, a wide-angle lens, it is using a greater angle of view to produce an image which is full framed, rather like trying to squeeze something larger into something smaller, which causes lines along the edges to bow out. This is caused barrel distortion.

Although these types of distortion are inherent in lenses, better lens design through the use of special lens elements have reduced these distortions in lenses, but they are still inherent and must be taken into account when photographing a subject in which the distortions will come into play.

Over here, over there!

Lens perspective has been an argued and misunderstood subject which has had many believing that lenses cause a distortion of the appearance of objects in a photo in their relative size to each other. This is actually a cause of the position of the photographer in relation to their subject.

Using wide-angle, normal, and telephoto lenses at the same distance from a subject will produce images which show the same relation of foreground and background objects. However, moving closer to the foreground subject will make the background object appear to be smaller in relation to the size of the foreground subject. Conversely, moving farther from the foreground subject will make the background object appear to be larger in relation to the subject.

Since, however, we are dealing with a flat focal plane which only captures an image in two of the three dimensions in which it exists, exaggeration of distance can happen within a photo due to the loss of that dimension. This is, however, due to image compression and not lens distortion.

Who are you calling normal?

A ‘normal’ lens is of the earliest lens design as it represents the easiest lens to design (theoretically speaking) providing the highest resolution with the least amount of distortion. A common misconception about the normal lens, however, is that it represents the normal field of vision of the human eye. In reality, the normal lens represents two things: the attention of focus of human vision, as the actual field of vision is much, much wider; and, most notably, the approximation of the diagonal of a frame of film.

In a 35mm camera, the frame used measures 36mm x 24mm. This gives a diagonal measurement of approximately 44mm. Normal lenses, however, have been made (for 35mm) anywhere from 40mm to 58mm (which is a little long for a normal lens). Being that the normal lens represents the attention of focus of the human eye, a normal lens is said to represent a ‘natural view’. That, however, is not near as important as the normal lens approximating the diagonal measurement of the film frame which causes less distortion than a lens which has an angle of view greater than or less than the diagonal. Hence, less distortion also equals better resolution (by theoretical design).

Let’s get close baby… ow!!!

The next step in lens development was the long-focus lens, which was basically a lens using two elements and a distance between the two lenses (focal length) to magnify an image to make it appear closer. As the actual focal length of the lens was necessary for the result, a 500mm lens (they used inches at the time) would physically be 500mm in length. Then along came the telephoto lens.

Telephoto lens design shortens the physical length of the lens by the addition of other lens elements magnifying and correcting the image in place of actual length. Hence, when a telephoto lens has a focal length of 200mm, for example, that is a representative of its magnification capabilities and not its actual physical length.

Looking wide baby… ow!!!

The wide-angle lens took longer in its development due to a few factors: distortion which was inherent in optic design which could be overcome in normal and telephoto lens design, but was more noticeable in wide angle lens design; the need for a wide-angle lens was less perceived when photographers were using slower films with longer set-up times for their equipment, as compared to the introduction of faster films and smaller cameras calling into need more flexibility.

Although true wide-angle (short-focus) lenses have been and are available, there are few of them made this way as the advent of SLR’s with flip up mirrors introduced a problem. Many cameras could not use them due to the mirror interfering with the rear of the lens, due to the close proximity to the film plane required by a lens with such a short focal length. Hence, wide-angle lenses were designed as a reverse telephoto lens (retrofocus) allowing them to be mounted further from the film plane.

Mirror, mirror, on the… it cracked!!!

Another development, taken from telescope design, was the mirror lens. The mirror lens uses two mirrors in place of some elements in a standard telephoto lens (refractive). This causes the light to be bounced off the mirrors within the lens, effectively shortening the lens by replacing length with reflection. This does create some problems though as the lens is more fragile than a regular refractive design lens and the doughnut shaped rear mirror and front element can show in out-of-focus highlights in a picture (looks like ghostly Spaghetti-O’s). Additionally, due to the design, mirror lenses are also made with only one aperture (a fixed diaphragm) which means that the only way to adjust exposure is through shutter speed or the use of neutral density filters (filters which drop the light level but do not affect the color balance of the light).

Mirror lenses are also called catadioptric lenses and have a unique property which is inherent to the design. The idea of a lens is to bring three primary colors into focus at the film plane (focal plane). Standard refractive lenses (by design) bring two colors into focus at the film plane and one color very close to being in focus. The mirror lens, due to the light bending properties, brings all three colors into focus at the film plane thus offering a potentially higher degree of sharpness (depending, of course, on the quality of build).

Zoom… wasn’t that a TV show for kids?

Zoom lenses were actually around in the 1930’s, but mostly on 16mm movie cameras and then later on television cameras as they provided convenience in providing a range of focal lengths and enough resolution for their respective smaller formats on which they were used. The first zoom lens for a 35mm still camera was introduced in 1959.

Zoom lenses basically came in two flavors: a varifocal zoom which allowed the focal length to be adjusted but had to be re-focused when this was done; and the parfocal which maintained its focus as the focal length was adjusted (within reason). The parfocal variety is the standard for today as the improvement in optics have made zooms a choice lens for all but the most stringent of photographers and those using larger format cameras as the single-focal length provides better resolution for larger images than 35mm or digital.

Fisheyes and macros and shifts, oh my!

There are a variety of lenses which fall into a category of special purpose lenses. Although lenses will be defined by their focal length, special features can be designed into them which make them more useful for particular types of work or for special effects.

FISHEYE LENS: Fisheye lenses are very short focal length lenses (down to 6mm) which have such a wide angle of coverage that they produce a circular (the shape of the lens) image. Unlike other wide-angle lenses, they are not corrected for barrel distortion (the bending of lines in an outward direction due to compressing an image onto a frame with a narrower field of view than the lens). Hence, they can produce exaggerated effects both in lines and perspective (due to the photographer’s proximity to their subject). There are fisheye lenses with longer focal lengths (16mm for example) in which a squared or rectangular image is cut out of the fisheye in order to provide a full-frame rather than circular image.

MACRO LENS: A macro lens is specially designed to be able to focus closely on a object, provide a 1:2 to 1:1 magnification ratio (the relation of the physical dimensions of the object on film in relation to its real size), and to provide a flat-field of focus which maintains a center to edge sharpness. Macro lenses are very special purpose lenses and as such are expensive compared to regular lenses of equivalent focal length. Some close focusing zooms are often referred to as macro or having a macro mode, although they are not true macro lenses as they need to be able to focus close enough to provide a minimum of a 1:2 magnification and provide a flat field of focus across a flat surface such as a stamp to function as a macro lens.

SHIFT LENS: A shift lens, also called a perspective control or PC lens, is a specially designed lens which allows the front elements to be shifted independently of the rest of the lens. This allows for the lens to be used to correct having to shoot an object (like a building) with the camera at an angle. When the film plane of a camera in not parallel to the subject, there is an unevenness created in the resultant image which can cause lines to bend and make buildings appear as though they are leaning. The shift lens allows the lens to be tilted (to a small degree) so that the camera does not need to be angled (ideally). Shift lenses are even more expensive than macro lenses.

How long is it?

Shorter focal lengths give a greater angle of view (field of vision) than longer focal lengths, which give a narrow angle of view. What is considered a short or long focal length is respective to the film format being used, or, in the case of digital, the size of the sensor being employed. I have listed in the chart below comparisons between the same focal length in its relation to a 35mm frame and a 645 medium format frame. Since there is little standardization among manufacturers of digital cameras in the size of sensors they use and their 35mm equivalency, I cannot include digital cameras in the comparison, but, if you know the multiplier for your specific camera’s equivalency, you can do a conversion, if such interests you.

Put it in the bag

This article, as usual for me, has been a collection of random wanderings on the construction of lenses. I have not presented this as a complete examination of lenses; rather as an introduction to the subject. If you seek more information, one of the best books written about lenses is simply called The Lens Book which was co-written by Roger Hicks and Franz Shultz. Kodak has additionally published a book called Photographic Lenses which can provide useful information for those wanting more on lens design.