Most lenses have parts that slide or turn
or shift. Internal movement is governed by external rings and switches
so that you can control the way a lens takes a picture. The control
mechanisms are accompanied by sets of numbers that enable you to exert
control with precision. No one lens has all the following features, but
most lenses have some of them.
The most common set of number on any lens is the one designating a
series of f-stops, for almost all lenses have a built in iris
diaphragm. The numbers may be on a movable ring which turns to line up
with an index mark on the lens barrel. Or, the numbers may be on the
barrel and the index on the ring. The f-stop ring is an especially
important control on cameras that choose a shutter speed automatically.
Most cameras taking interchangeable lenses have focal plane shutters
built into the camera body and therefore they have no need for shutters
in the lenses. However, many medium format and all large format cameras
use lenses with leaf type shutters located between the lens elements. A
control ring to set the shutter speeds accompanied by numbers
designating the amount of time the shutter will be open is located
around the lens barrel. Detents (click stops) help to position the ring
exactly. Many modern cameras display f-stops, shutter speeds, and other
useful data in the viewfinder as well.
Cameras that focus by means of a movable bellows generally take lenses
that have no independent focusing mechanism of their own. However, all
other cameras must use lenses with the focusing ability built into the
lens itself. This necessitates another ring around the lens barrel and
the numbered scale to mark the distance set. Most of the world uses the
metric system, but the United States still measures distance by feet
and inches, so two series of numbers usually appear on the focusing
scales, one indicating meters and the other in feet.
In conjunction with the distance scale there is usually a depth of
field scale. Starting with a central index mark, numbers representing
the f-stops spread out in both directions. On either side the largest
f-stop will be nearest the mark and the smallest will be farthest away.
After you have focused on a subject, you can check the number on each
side of the index mark representing the f-stop you are using and, on
the distance scale next to it, read off the nearest and farthest
distances that will be in acceptable focus.
Zoom lenses have a special control ring of their own. It changes the
focal length between the shortest and the longest positions. These two
extremes plus several intermediate focal lengths are clearly marked on
the ring or on the lens barrel.
On close focusing macro lenses there is often a special scale giving
the reproduction ratio of the image such as 1:4 or 1:10. These numbers
mean that the image is 1/4 or 1/10 as big as the real life subject.
Often the scale extends to 1:1 which indicates a full life size image.
Auxiliary lenses such as tele extenders also reduce the amount of light
reaching the sensor. This usually have a scale showing the real
(effective) aperture being used as opposed to the one you have set on
the prime lens.