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Lenses: How they capture and control light

The linguistic roots of the word "photography" are the Greek words meaning "light" and "drawing." Photography is "drawing

with light," and lenses are the brushes. After their imagination, lenses are the photographer's primary creative tools. The way

a lens captures and presents an image to the camera's sensor determines the visual outcome more than any other factor.

The ability to choose the right lens and use it well is one of the most important skills an aspiring photographer should acquire.

In this brief guide we'll look at some of the basics that will help you to choose lenses that are suited to your needs, and

make the most out of them to create truly satisfying photographs.

Projecting an image

Our eyes do it, cameras do it, even a simple

the pinhole camera. In a pinhole camera a tiny

light-tight box with a tiny hole in one end will

hole is all that's needed to project an image.

do it: the feat of turning light into an image

can only be accomplished by first capturing

To make this easier to understand, remember

the light from a scene and projecting it onto

that light normally travels in straight lines, then try

a surface. That surface, the "image plane,"

to imagine the subject being photographed as

can be a wall, a piece of film, a sensor, or

being made up of a multitude of points of light of

the retina in our eye. In all cases the image

appropriate brightness and color.

is projected upside-down and horizontally

In the example in Figure 1, light from a point at

reversed. Let's take a look at the precursor of

modern cameras, the simplest camera of all:

the top of the tree travels in a straight line

A pinhole camera is basically a light-tight box

Figure 1. A simple pinhole of appropriate size

with a small hole in one end

is capable of projecting a sharp but dim image

If a little hole can do all of this, why do we need lenses?

Pinholes can "project" images, but they are limited and inflexible. In

order for the projected image to be sufficiently sharp, the hole must be

very small, but this also means that the projected image is very dim.

In principle, lenses work similarly to the pinhole, but they are capable

of capturing more light from each point on the subject, and therefore

project a much brighter image. A lens can also bring more light into

sharp focus. That's helpful because it means we can use short sub-

A simplified cross section of a modern lens and a typical SLR (Single Lens Reflex) type digital camera

Pentaprism (flips the image so it can viewed in proper orientation)

Focal point

Light

Lens element

Subject

Optical axis

Interchangeable-lens

(objective lens)

Light reflected by the subject is effectively collected

and focused by the lens elements to project an

image on the camera's image sensor plane.

through the pinhole and reaches a point at

the bottom of the image plane, whereas light

from a point at the bottom of the tree ends up

at the top of the image plane after passing

through the pinhole.

The real-world scene becomes an image

projected on the image plane, upside-down

and reversed left-to-right.

Figure 2. A lens uses the principle of "refraction"

to gather more light from the subject and project

a sharp, bright image

second exposures rather than having to make sure that both the

camera and subject stay perfectly still for many minutes or even hours,

which is usually the case with a pinhole camera. Other advantages are

that lenses can be made in a variety of focal lengths from wide-angle to

capture expansive scenes or telephoto to photograph distant subjects.

Modern lenses are precision optical devices that give photographers

boundless freedom to realize their creative vision by "drawing with light."

Refraction: bending light

Viewfinder

The physical principle that allows lenses to

Camera

gather and focus light is called "refraction."

Refraction causes lightwaves to change speed

and direction when they pass from one medium

Image

(air, for example) to another (glass, for example),

sensor plane

and allows lenses to be designed to "bend" light

in a controlled way. The "refractive index" of an

Mirror

optically transparent medium is a measure of

the speed of light in that medium, and therefore

the degree to which light will be "bent" by that

Focal length

medium. Optical materials that have different

refractive indices—conventional optical glass and

ED glass, for example—are sometimes combined

in lenses to achieve the desired characteristics.

A look inside

Elements and groups

All modern photographic lenses are "compound"

Fixed focal length lenses, also known as "prime"

lenses that use a number of lens "elements"

lenses, generally have the simplest construction

precisely mounted along the same optical axis.

with the fewest groups and elements. Zoom

The use of multiple elements allows lens designers

lenses require a larger number of groups/

to effectively reduce optical aberrations so you

elements to support the zoom functionality.

get nice sharp, clean images.

While most lens elements are "spherical,"

"Elements" are the individual pieces of specially

meaning that one or more surfaces form part

shaped glass that make up the lens. A "group"

of a sphere, some lenses include "aspherical"

consists of two or three elements that have been

elements. Aspherical elements have more

glued together to function as a unit. Sometimes

complex shapes than simple spherical elements,

groups consist of different types of glass that

and are much more difficult and more expensive

have been combined in order to control some

to produce. Aspherical elements are sometimes

form of aberration. Lenses are sometimes

used in wide-angle and fast standard lenses,

described in terms of the number of elements

where they can be effective in reducing certain

and groups they contain. You'll hear terms such

types of aberration.

as "7-group 9-element lens."

Zoom and focus mechanisms

The job of varying focal length in a zoom lens

Focusing is sometimes accomplished by moving

requires a fairly complex mechanism that

the entire lens closer to or further away from

translates zoom ring rotation into precise group

the image sensor plane, although some lenses

movement along the optical axis of the lens. Zoom

employ a "floating construction" in which groups

mechanisms must be precisely manufactured

of elements move independently in order to

to exacting tolerances so that all elements and

maintain optimum optical performance at all

groups stay in perfect alignment throughout the

shooting distances.

zoom range.

Read your lenses

There is a lot of pertinent information

printed or engraved on the outside of

lenses that can help you understand their

characteristics and how to best use them.

Here are a few examples.

Focal length

This is the most basic, most important

characteristic of any lens. Focal length plays

a primary role in determining what types of

subjects and compositions the lens is suitable

for (see page 10 for more details).

AF/MF switch

This switch lets you switch between

autofocus and manual focus modes.

Lens configuration example: 7 groups/9 elements

Lens element

Lens group

Mount

Aperture

Lens barrel

Aspherical lens (see page 16 for more details)

ED glass (see page 16 for more details)

How lens elements and groups move in a zoom lens

Wide

Medium

Telephoto

Distance scale

The distance scale indicates the

approximate distance from the

camera's image plane to the object

that the camera is focused on.

Autofocus drive type

Lenses marked "SAM" or "SSM" feature

built-in motors that drive the lens's

focusing mechanism. Lenses that

don't have internal motors are driven

by a motor in the camera body

(see page 17 for more details).

Maximum aperture

This number represents the maximum

aperture, or "f-number," of the lens

and tells you how "bright" the lens is

(see page 9 for more details).

Lens format

Sony lenses marked "DT" (Digital

Technology) have been specifically

designed for use on APS-C format

A-mount cameras (see page 8 for

more details).