U S I N G T H E L E N S

THE PHOTOGRAPHIC LENS The quality of your lens sets the upper l imi t on the image quality that you can achieve with your camera. The best of lenses can outperform virtually any f i l m that it is used wi th , while low-quality lenses wi l l l imi t the image quality irrespective of how superb the capture surface is, be it sensor or f i l m . Understanding how lenses are put together and how they work, as well as appreciating their limitations and their individual qualities, wi l l help you obtain the best f rom your lenses.

LENS CONSTRUCTION Modern camera lenses are supreme examples of today's technology at work. The optical elements are made of the finest glass and highest-quality plastics. I n a compact digital camera, the individual elements are much smaller than a contact lens. And a lens can consist of more than a dozen separate elements. There are far more than just the glass elements, however: there are mechanical parts of the highest precision, which must be strong enough to protect the elements against shock, yet some must be light enough to be easily moved by an autofocus mechanism. Then there are electronic elements, f rom tiny processors that help control autofocus settings to electromagnetic elements that stop down the lens or aid in focusing. The vast majority of lenses share the same basic structure. Lenses for large-format cameras differ in lacking a focusing mechanism.

Lens barrel The barrel encloses the optical and other components of the lens and provides a foundation for mechanical parts, such as the focusing and lens mount. It can be made very substantial, such as classic lenses for the Leica or Hasselblad, or be constructed of very light materials, such as for compact digital cameras.

Front mount The front mount is often designed to accommodate filters or a lens hood. I f the mount rotates as the lens is focused, this is an inconvenience for filters— polarizing or graduated (see page 168)—which need to be oriented in a particular direction. Lenses on compact cameras may use an adaptor that fits over the lens barrel to carry lens accessories.

Focusing ring focusing rings are normally found on interchangeable lenses but may be missing f rom autofocus lenses. They may drive the optics for focusing or actuate a servo motor, which drives the lens elements. The latter II i d hod is used on digital cameras and can make lot u . i i i f , feel indistinct.

• Lens structure This cross section of a Zeiss 40mm f /4 wide-angle lens for the 120 format shows its complexity, requiring ultra-high-precision design, manufacture, and assembly.

< Alpha rear The rear of a typical modern SLR camera shows the complex electrical contacts (left) and the aperture stop-down lever (center), as well as the socket (right), which connects with a lug to drive the autofocus mechanism.

< Front mount The front mount of a medium-format lens shows the inner screw thread, which accepts filters and similar accessories. On the outer rim of the front of the lens are the lugs that accept a lens hood or bayonet-mount filters.

Rear mount The rear mount interacts between the lens and camera, so it must be manufactured to the highest precision. In addition, the mount must be extremely durable, as it receives much wear when lenses are changed. The modern lens mount also transmits data between the lens and camera body. In the past, the data about lens aperture were sent mechanically; now the contacts are electrical and transmit digital data. These contacts must be kept clean and free of grease or oil . Some mounts still use a mechanical coupling, through which the camera drives the focusing mechanism, or a lever is pushed to stop down the lens aperture.

T H E P H O T O G R A P H I C L E N S 133

Aperture The control for the aperture is commonly found on the lens barrel of interchangeable lenses, but it is also frequently controlled f rom a dial located on the camera. Al l lenses can change apertures in steps of 1 stop, but for professional work it is important to be able to vary aperture by ^2-stop steps or smaller.

Zoom control There are two types of lens-based zoom control; the

commonest is to turn a knurled ring, such as the

focusing ring. The other type is to push or pull on a collar. Both types have their advocates, but the latter is more likely to suck air into the camera, which may attract dust into the innards. Autofocus compacts control the zoom through a switch on the camera body.

OTHER COMPONENTS Some lenses, such as those used in medium-and large-format cameras, incorporate a shutter mechanism set in between the lens elements. Settings for this may be made on the lens or via controls on the camera. The majority, such as those in the Hasselblad and Mamiya systems, are mechanically actuated and governed (timed), but those in the Rollei system are actuated and controlled electronically.

T Vignetting

This view of rice harvesters, taken with a compact zoom lens set to 135mm, shows vignetting. A constriction along the length of the lens barrel cuts into the edge of the light bundle, causing the corners of the image to darken. A larger lens would solve the problem, but it would cost and weigh much more.

A Lack of flare Well-controlled flare from (areful lens construction and design, as demonstrated in this image, does not cause multiple images of the light source—strictly the lens's exit pupil—and does not reduce • "iitrast in the shadows.

• Veiling flare (|uate lens design can

• i l l n w light to reflect on the n l e of the optics and cause

v e i l i n g flare, shown as

1 ts representing the :• v; 111 \ource in the image. This •• i ides the overall contrast,

s o introduces usually ed elements, but they

• h e used to communicate '•.Illness of the light.

136 U S I N G T H E L E N S

LENS SPECIFICATIONS • IN UHI.HJJJNN.HU>

The specifications of a photographic lens describe

the lens's optical features, that is, those that are

designed and built into the optic. The main feature

is the focal length of a lens: this measures the

distance between a point in space, f rom which the

image appears to be projected (the rear principal

point), and the sharp image of an object, where the

object is at infinity. In simple lenses, the image

appears to be projected f rom the physical center of

the lens. Not so wi th multielement photographic

optics: wi th a wide-angle lens, the image may

appear to be projected f rom a point behind the lens,

and with telephoto lenses, the image appears to be

projected f rom a point that is in front of the lens.

EQUIVALENT FOCAL LENGTH In modem lenses, the focal length has very little to do with the physical length of the lens. In fact, many zoom lenses are longer at the wide-angle setting than at the longest setting.

• Image circle This image represents the full image circle projected by a 24mm lens. Ordinarily we see only a rectangle cut out of the middle of this image.

And there is a further factor that may cause confusion: this is because the photographic effect of focal length depends on the size of the film or sensor being used.

Given the popularity of the 35mm format, it has become common practice to relate the focal length of

digital camera lenses to the 35mm equivalent focal length (efl). For

example, the 3sefl of 35mm is 5.4mm on one camera but could be 7mm on another with a larger sensor. To work out the efl, the calculation is as follows: multiply the actual focal length by the ratio of the

length of the diagonal of the 35mm with the length of

diagonal of the sensor. However, digital cameras use sensors with

different proportions (3:2, 3:4, or even 16:9). Long, thin proportions tend to look wider than squarer ones, even if the angle of view is identical. (See also Focal-length factor, page 145.)

ANGLE OF VIEW The main photographic effect of change of focal length is to vary the angle of view—how much the lens sees. With a greater angle of view, the larger amount of the scene we can see. Taking the field of view of a human eye as the standard, we say that a lens's field of view is wide-angle if it is wider than that of the human eye. And if the field of view is less, the lens is commonly said to be "tele" or "telephoto". For a given format, lenses with focal lengths shorter than that of the length of the diagonal of the format will give a wider field of view: the shorter the focal length, the greater the field of view. Here, objects appear smaller because more of the scene must be fitted into the image frame, so very short focal lengths giving wide-angle views greatly reduce the size of objects. We use a long-focal-length lens, i.e. the focal length is greater than the length of the format diagonal, to magnify a narrow field of view to fill the capture surface.

• 4:3 image rectangle •• r»•< tangle ni /,: j p ropo r t i ons is popu la r

a s il i . i k i " , in 111111 c d e p t h . Not ice there is

n i n e -.1 v . m i l l o reg round than w i t h the

• u n . i K ' ' , I 'ni il 1 ' in appea l less w i d e .

M

• 3:2 image rectangle A rectangle of 3:2 proportions, such as the 36 x 24mm of the 35mm format, is a good compromise between an image that is too square or too narrow. The diagonal is the same as the diameter of the image circle.

A 16:9 image rectangle The high-definition TV format of 16:9 (4:3 is equivalent to 12:9) looks wide but at the expense of less depth—there is less sky, for example. Of course, its diagonal is the same as that of the other formats.

L E N S S P E C I F I C A T I O N S 137

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•< Fields of view These images show the approximate fields of view seen by 12mm, 24mm, 50mm, and 120mm lenses on the 35mm format. Note that field of view reduces very rapidly when the focal length increases. The difference of 12mm focal length between 12mm and 24mm lenses is far larger than the 85mm between 50mm and 135mm.

As there is less of the scene—say, just an animal's head—so this must be enlarged in order to fill the frame. The longer the focal length, the narrower the field of view and the greater the enlargement. Angle of view is normally measured as the angle at the camera over the diagonal of the view. Note that the perception of wide-angle also depends on the proportions of the frame: high aspect ratios look wider than frames with square proportions.

MAXIMUM APERTURE The maximum aperture of a lens relates the effective diameter of the front element to its focal length. A lens with large front element for its focal length—a "fast" lens—offers .1 large maximum aperture: //1.4 is considered fast for normal focal lengths, f/2 is fast for wide angle, f/2.8 is fast for supertelephoto.

Fast lenses are invaluable for working in low light. Large maximum apertures also project a bright tinder image in SLR cameras, increase the effective rangefinder base of focusing aids, and may help autofocus systems.

Another advantage of fast lenses has become increasingly

appreciated as sensors have become smaller, resulting in a large depth of field. The best way to reduce depth of field effectively is to use the fastest lenses. The 50mm //1.4 lens is coming back into fashion with small-sensor DSLRs because of its maximum aperture and useful 75mm effective focal length.

MINIMUM APERTURE Theoretically, very small minimum apertures promise extensive depth of field, but it is important to bear in mind that at very small apertures, lens performance will drop (see page 165).

MINIMUM OBJECT DISTANCE All general-purpose lenses focus to infinity, but there is great variation in how close they can focus, measured by the minimum object distance. This is usually measured from the focal plane to the object. In earlier lens designs, focusing involved moving the lens closer to or further away from the film, but now focusing is often achieved by moving internal elements. Note that lens performance may drop significantly at close focus ranges, particularly with zoom lenses (see pages 138-9).

TABLE OF ANGLE OF VIEW

This table shows how diagonal angle of view varies with focal length and with size of sensor. Broadly, wide angle is coverage greater than about 45°; normal is 45-55°; telephoto is less than 55°; extreme telephoto is less than io°.

80

D I A G O N A L MN

42

1 1

27.3

15mm fisheye 175° 112°

12mm 122° 97°

17mm 104° 78°

20mm 95° 69°

24mm 84° 59°

28mm 75° 52°

35mm 64° 43°

50mm 77° 47° 31°

70mm 59° 42° 22°

85mm 50° 28° 180

100mm 43° 24° 16°

135mm 33° 18° 12°

200mm 22° 12° 7-8°

300mm 15° 8.2" 5-2°

500mm 4-9" 3-1°