What is a lens?

Definition

A lens is a piece of of a transparent substance, with one, or more usually both sides curved

such that light passing through it will bend in a specific way. A photographic lens almost

invariably comprises a number of these to achieve a better image. These are known as compound

lenses.

Cameras without lenses

It is, of course, possible to construct a camera without lenses.
These so-called 'pinhole' cameras utilise a very small hole in place of a lens. However, image

sharpness can only be increased to an acceptable level with an incredible reduction in pinhole size,

and so only a very small amount of light passes through. This renders a pinhole camera very

impractical for normal photographic use, since either very fast film* or

exceptionally long exposure times.

This is not to say that a pinhole camera is without its uses; some modern spy cameras utilise

the incredibly small size of a pinhole to hide a cameras of utterly invisible proportions inside

screws and in other unlikely places. However, the image quality improvements from using a camera

with a lens has led to most cameras, including the Minox spy

camera range, to be made with lenses.

Cameras with lenses

The simplest camera lens is one which bulges on both sides, and is known as a biconvex lens.

This concentrates light onto a point behind it, and the point at which light coming directly toward

the front of the lens would theoretically converge is known as the lens's 'focal

length'*.

However, a simple biconvex, even-density glass, spherical-surface lens is prone to a number of

types of optical distortion which render its image, while significantly better than that of a

pinhole, very blurry indeed.

Thus, compound lenses, some of which contain 'aspherical' elements, some containing optical

glass that varies in density, and most containing five or more elements* were devised to create images of an acceptable quality.

The technical stuff

Focal length

The focal length of a lens, as has been mentioned, is the distance behind it that a wide beam

of light coming straight at it would converge. This measurement has a more practical application in

photogrpahy, however, in that it defines how magnified an object appears on the film.

A 'standard' lens displays the image at the same magnification as the human eye would. It is

defined as having a focal length equivalent to the diagonal of the film format being used. For

example, 35mm film, which is 24×36mm in size, would be about 43mm. However, for some reason, the

'standard' lens of nearly every photographer is 50mm, due to an error or decision somewhere in the

format's history.

This means that different formats have different 'standard' lenses, and indeed their entire lens range will be shifted up or down in focal length by a factor depending on how much buigger or smaller the film is than 35mm. This is the reason that lens focal length appears to change when used with CCDs rather than conventional film; the area of the CCD, and therefore its diagonal, is significantly smaller than that of a 35mm frame, and so it only shows a small central section of the image. The change in focal length is relatively simple to find out; take 43, divide it by the length of the CCD's diagonal, and presto, you have the conversion factor. Multiply this factor by the focal length of the lens to get its 'effective' focal length.

Lenses of a longer focal length make the image appear magnified. Lenses of a shorter focal

length allow you to fit more into an image frame.

Most lenses are split into groups according to their focal length, and indeed that is how,

later in the article, it has proved most practical to categorise them.

Aperture

As has been mentioned before, the amount of light received by the film is crucial in

photography. The aperture is the hole through which light enters the camera body, and the word most

often refers to the lens's f/number, which is an indication of how much light will reach the film.

The f/number is equivalent to the size of the hole divided by the focal length of the

lens*. Thus, a 50mm lens with a 50mm aperture would be classified as f/1.

Larger apertures have smaller numbers, as a 50mm lens with a 25mm aperture would be just f/2.

However, the amount of light let through is dictated by the inverse square law, meaning that an f/2

lens lets through 4 times* less light than an f/1 lens. Apertures in common

usage are found on this scale, each number being the square root of two times bigger (so it lets in

half the light) of the last number:

f/1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 45, 64...

To make all lenses have large maximum apertures is, however, impractical in real terms. The

size of the optics required to manufacture an f/1.2 50mm lens, let alone anything bigger, makes

these lenses prohibitively expensive, very heavy, of lower optical quailty, and not much better at

light-gathering than their f/1.4 or 1.7 cousins.

Real-life lenses

So, here's a quick guide to most of the lenses you can buy, what they do and how practical they

are.

The focal lengths stated for this section are for the 35mm format, since it is by far the most

widely used.

Fisheye lenses

Fisheye lenses have the most bizarre name of any lens! Some sources satate that thay are named because they bulge outwards like a fish's eye, while others maintain that it is because thier image is similar to the view of the world on land as seen by a fish underwater.

Their field of view encompasses 180º, and no attempt is made to compensate for the distortion caused

by trying to squeeze such a large angle onto a flat piece of film.

As such, fisheye lenses make for very distinctive photographs, which look as though they were

being viewed on the inside of a curved surface.

Less powerful fisheyes create a full-frame image, while those with very short focal lengths

create a circle in the middle fo the frame. Special fisheye lenses can reach up to 270º, and

perhaps beyond, but these are useful only for scientific purposes, or very wacky special effects...

Fisheyes are normally in the range of 6mm to 20mm for 35mm cameras, and are prohibitively

expensive. However, if you really must take a photograph of a 180º panorama in one shot, or if you

really want some particularly bizarre special effect, a fisheye lens is one of the very few things

capable of the job!

Ultra wide-angle lenses

Ultra wide-angle lenses seek to encompass huge fields of view with the very minimum of

distortion. This is quite an optical feat, and invariably there is some difference between

rendition of objects at the edge and centre of the film despite the best attempts of the optical

engineers.

Ultra-wides have the property of enhanced perspective. This is because it is very easy to get

every object in a photograph in focus due to the huge depth-of-field* which they afford, and

since closer objects appear to dwarf the background.

Ultra-wides often have the same focal length as longer fisheyes, around 16-24mm, and the longer

ones are at least within the range of an amateur's budget. The maximum apertures of these

ultra-wides does vary, but it's usually not more than f/2.8 since the difficulty in constructing

these lenses would make a lens with larger elements very expensive indeed. Many photographers think

that a 24mm is a very nice lens to use, for some reason, probably since it's just that little bit

wider than the more widely-available 28mm...

Wide angles

Wide-angle lenses are a range of lenses which allow more into the frame than a standard lens,

but are not so extreme as an ultra-wide or fisheye. They are usually used for landscape

photography, but they are suitable for pretty well any application.

They share the huge depth-of-field inherent with wide lenses, though not to quite the same

extent, and can also utilise similar perspective tricks.

They cover the range from 28-40mm, and usually come with a choice of maximum aperture in the

most common lengths. A 35mm f/2 lens is quite expensive, however, when compared to a 35mm f/2.8, or

even a 28mm f/2.8.

Standard lenses

These lenses approximate perpective and proportion as seen by the human eye. They are

optically simpler to produce than any other lens, though often they have large maximum apertures and

this means they grow in complexity. They are cheap, suitable for use in low light, and up until

recently, every photographer had one.

Standard lenses range from 40mm to 55mm depending on the manufacturer and model of your camera,

though why exactly these values were chosen when 43mm in closest to the true value is something of a

mystery! The biggest advantage of a standard lens is usually the maximum aperture, since they are

optically quite simple to produce, and so f/1.4 is quite a common lens. However, lenses up to f/1,

or more usually f/1.2 have been produced, and many amateurs prefer the slightly cheaper f/1.7 or f/2

lenses. However, if you got a lens with your camera, this would invariably have been it - until the

popularisation of near-standard zooms.

Telephotos

Telephotos are the lenses that magnify distant objects an appear to bring your subject closer

to you. They are generally said to have less depth-of-field, and to compress perspective, and this

is a relatively useful myth in that it approximates the truth as a photographer would see it. Of

course, in mathematical terms both of these statements are total myths; apparent reduced

perspective is simply caused because you're usually magnifying distant objects*, causing

the apparent reduction. Similarly, were you to get sufficiently close to make a subject appear the

same size in the frame with a shorter lens, the background would be equally out-of-focus.

These two qualities, however, lead to the telephoto's primary application; portraiture. To

take a full-face picture of someone with a wide lens results is severe distortion since their nose

is so much closer to the camera than their other facial features. The increase camera-subject

distance with a telephoto reduces this effect to the point of insignificance. The other helpful

feature is the apparent reduced depth-of-field; an unsightly background can be rendered

out-of-focus with ease, and so not distract from the subject.

The usual quoted range for telephotos is 55-200mm. These range from those capable of only

slight magnification (85mm, and so on), through the 'portrait lenses' of 100-135mm, and right up to

those capable fo three or four times the magnification which the human eye is accustomed to

(150-200mm). Maximum apertures vary, with the possibility of obtaining an 85mm f/2 not being too

remote, but shrinking increasingly toward the end of the range. A 200mm f/4 would be considered

'normal', though for extra cost, most of these lenses can be obtained with larger maximum apertures.

Macro lenses

A macro lens is designed to take photographs of very small subjects at magnifications which

would be inconceivable with 'normal' lenses. They achieve this by allowing the lens to move further

from the film plane, allowing objects to be focussed on which are significantly closer to the

camera. This extra movement makes the lens harder to construct, though.

They are convinient, compact and easy to use, but simply don't provide the magnification which

can be found from bellows or extension tubes.

A macro lens is not strictly confined to any specific focal length, but they are most commonly

somewhere between 50 and 200mm. Most have a slightly smaller maximum aperture than their telephoto

counterparts, and are somewhat more expensive.

Super telephotos

If you really need to get pictures of a subject, but can't get very close, what you

need is a super telephoto lens. The minimum length for a lens on a safari, for example, is often

quoted as 300mm, and surveillance photography can require 1000mm lenses, which are quite capable of

resolving detail utterly invisible to the naked eye.

These lenses can be absolute beasts, weighing kilograms and being up to a metre in length.

They compress perspective to a serious extent, and have only a very small depth of field. The

slightest wobble of the camera can result in blurring the photograph beyond recognition. However,

while impractical, they are wonderful lenses and are so good at the job they do that many

photographers put up with or learn their way around these problems.

They range in length from 200mm to the very limits of practicality, which normally lie at about

1000mm. Maximum apertures are normally quite small, ranging from f/4 at the lower end of the range

to perhaps only f/8 (quite dark, in fact) at the other end of the scale. These lenses range from

reasonable in cost, to utterly bank-breaking, but, if you need one, they are an invaluable asset.

Catadioptric lenses

For those with a desire to take photographs of distant objects but no desire to break their

back or their bank through purchase of a super-telephoto, the solution lies in a catadioptric

lens*.

These use mirrors to bend the light such that it travels two, three, or perhaps even four times

the length of the lens barrel, thus effectively allowing you to cram a long lens into only a tiny

portion of the space required for a telephoto. They're lighter, they're cheaper, and they often

suffer from less optical distortion since mirrors are not so prone to that kind of thing as lenses.

However, before you rush out and buy one of these 'perfect solutions', there is one factor to

consider. They have a fixed, and usually quite small aperture. This is extrememly limiting in

photographic terms, despite what you may think, and means that exposure must be altered using dark

and light filters, if it is necessary. It also renders them useless excpet on bright days, when

sufficiently fast exposures to ensure that camera shake is not visible can be achieved. A tripod is

a partial solution, but there's still the risk of subject movement!

They can increase potential focal length to 2000mm, but a more common lens, a 500mm f/8, is

found in many photographers' kit bags.

Zoom lenses

In earlier eras, so-called 'prime'* lenses

outclassed zooms to such an extent that they were seen as fripperies. However, with significant

advances in optical technoloy, and consumer demand for all-singing, all-dancing lenses, the zoom has

jumped in to fit the bill.

A zoom lens has a variable focal length, which is easily adjusted lens movement. Any focal

length in the range can be employed. They have a number of advantages; subjects can be precisely

framed using focal lengths that wouldn't normally be available in prime lenses (like 127.33mm, if

you were taking a portrait at a certain distance, where perhaps only 120mm and 135mm primes are

available, a 70-200mm zoom would cover the whole ranfge steplessly), there is no need to move

yourself or the subject, they are generally lighter than an equivalent range of primes, and can be

used for a number of interesting effects using long exposures and zooming during them...

However, some dislike zooms because they are heavier than a single prime lens, have a smaller

maximum aperture or the quality is lower. While issues of quality have been largely resolved, the

other two points are fair ones; a particular gripe of purists is that som many cameras are sold

with 'standard zooms' with maximum apertures of perhaps f/2.8 or f/3.5, where a fixed-length f/1.4

lens would have been sold in the past - no prizes for guessing which is the most versatile in low

light.

If there's a type of lens listed here, there's almost invariably a zoom to cover it. Wide

20-35mm zoms, 35-70mm 'standard' zooms, 70-210mm zooms (a very popular lens), and even 135-600mm

ultra-telephoto zooms... However, buying one of these lenses does require you to add up to pros and

cons; if you want optimum quality, to be able to work in low light, or a particularly light lens of

a relatively specific focal length, the prime lenese are your best choice. Otherwise, join the many

who believe that the versatility a zoom provides cannot be matched by the benefits of 'olde-worlde'

fixed-focus lenses!

Some rules of thumb

Lenses of a longer focal length tend to make subjects appear larger at a given distance, have a smaller apparent depth-of-field, and compress perspective to a greater extent.

Longer focal-length lenses also require faster exposures to prevent blurring due to camera shake. A general rule for 35mm is that the lowest speed at which you can hand-hold a lens is 1/focal length - so a 500mm lens cannot be hand-held below 1/500 of a second. Blurring can be prevented using a tripod should a lower speed be used.

In conclusion...

There is a very wide variety of lenses on the market today. Some are ridiculously expensive,

others serve little practical purpose, but all have their niche. If you want to get a camera kit,

then the only advice I can give is choose the lenses which you will find most useful for what you

want to do - and also, don't take this article as gospel - it's all true, but there's no substitute

for looking at photographs to see what can be achieved with lenses of different focal lengths so you

can make your decision!

Further reading

Only a few relevant sites out there on h2g2 - us photographers are going to have to get

writing!! However, if this little lot has whetted your appetite, this little lot should help you on

your way...

A link to The Black and White Photography Process, an

interesting page with quite a bit about photography in general too.

Or, if you prefer, Telescopes, in case you are more into

the latter part of this and are a budding astronomer...

And Becoming a telescope nut, in case a catadioptric lens

simply isn't long enough for your tastes, here's somewhere where you can make your own scope!

And, of course, a Random Edited Entry link, coz

many of you are probably just browsing and curious, and every page ought to have one.