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UNDERSTANDING CAMERA LENSES

Understanding camera lenses can help add more creative control to digital photography. Choosing

the right lens for the task can become a complex trade-off between cost, size, weight, lens speed

and image quality. This tutorial aims to improve understanding by providing an introductory

overview of concepts relating to image quality, focal length, perspective, prime vs. zoom lenses

and aperture or f-number.

LENS ELEMENTS & IMAGE QUALITY

All but the simplest cameras contain lenses which are actually comprised of several "lens

elements." Each of these elements directs the path of light rays to recreate the image as

accurately as possible on the digital sensor. The goal is to minimize aberrations, while still

utilizing the fewest and least expensive elements.

Optical aberrations occur when points in the image do not translate back onto single points after

passing through the lens — causing image blurring, reduced contrast or misalignment of colors

(chromatic aberration). Lenses may also suffer from uneven, radially decreasing image brightness

(vignetting) or distortion. Move your mouse over each of the options below to see how these can

impact image quality in extreme cases:

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Original ImageLoss of Contrast Blurring

Chromatic Aberration Distortion

Vignetting Original

Any of the above problems is present to some degree with any lens. In the rest of this tutorial,

when a lens is r e f e r red t o as hav ing lower op t ica l qua l i t y t h an another lens , t h is is

man i f es ted as some combina t ion o f t he above ar t i f ac t s . Some of these lens artifacts may

not be as objectionable as others, depending on the subject matter.

INFLUENCE OF LENS FOCAL LENGTH

The focal length of a lens determines its angle of view, and thus also how much the subject will

be magnified for a given photographic position. Wide angle lenses have short focal lengths, while

telephoto lenses have longer corresponding focal lengths.

Note: For a more quantitative and technical discussion of the above topic, please see the

tutorial on camera lens quality: MTF, resolution & contrast .

Note: The location where light rays cross is not necessarily equal to the focal length,

as shown above, but is instead roughly proportional to this distance.

 

Required Focal Length Calculator

Subject Distance

Subject Size

Camera Type

meters meters

APS-C

36

2

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Many will say that focal length also determines the perspective of an image, but strictly speaking,

perspective only changes with one's location relative to their subject. If one tries to fill the frame

with the same subjects using both a wide angle and telephoto lens, then perspective does indeed

change, because one is forced to move closer or further from their subject. For these scenarios

only, the wide angle lens exaggerates or stretches perspective, whereas the telephoto lens

compresses or flattens perspective.

P erspec t ive con t ro l can be a power fu l compos i t iona l t oo l in ph otography , and o f t en

determ ines one 's cho ice in f oca l leng th (when one can photograph from any position). Move

your mouse over the above image to view an exaggerated perspective due to a wider angle lens.

Note how the subjects within the frame remain nearly identical — therefore requiring a closer

position for the wider angle lens. The relative sizes of objects change such that the distant

doorway becomes smaller relative to the nearby lamps.

The following table provides an overview of what focal lengths are required to be considered a

wide angle or telephoto lens, in addition to their typical uses. Please note that focal lengths listed

are just rough ranges, and actual uses may vary considerably; many use telephoto lenses in

distant landscapes to compress perspective, for example.

Lens Foca l Length * T er m i no lo gy Ty p ic al Ph ot o gr ap h y

Less than 21 mm Extreme Wide Angle Arch itec ture

21-35 mm Wide Angle Landscape

35-70 mm Normal Street & Documentary

70-135 mm Medium Telephoto Portraiture

135-300+ mm Telephoto Sports, Bird & Wildlife

Other factors may also be influenced by lens focal length. Telephoto lenses are more susceptible

to camera shake since small hand movements become magnified, similar to the shakiness

experience while trying to look through binoculars. Wide angle lenses are generally more resistant

to flare, in part because the designers assume that the sun is more likely to be within the frame.

Required Focal Length:

Note: Calculator assumes that camera is oriented such that the maximum

subject dimension given by "subject size" is in the camera's longest dimension.

Calculator not intended for use in extreme macro photography.

*Note: Lens focal lengths are for 35 mm equivalent cameras. If you have a compact or digital SLR camera, then

you likely have a different sensor size. To adjust the above numbers for your camera, please use the focal length

converter in the tutorial on digital camera sensor sizes .

Calculate 400.5 mm

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A final consideration is that medium and telephoto lenses generally yield better optical quality for

similar price ranges.

FOCAL LENGTH & HANDHELD PHOTOS

The focal length of a lens may also have a significant impact on how easy it is to achieve a sharp

handheld photograph. Longer f oca l leng ths requ i re shor t e r exposure t imes t o m in im ize

b lu r r ing caused by shaky hands . Think of this as if one were trying to hold a laser pointer

steady; when shining this pointer at a nearby object its bright spot ordinarily jumps around less

than for objects further away.

This is primarily because slight rotational vibrations are magnified greatly with distance, whereas

if only up and down or side to side vibrations were present, the laser's bright spot would not

change with distance.

A common rule of thumb for estimating how fast the exposure needs to be for a given focal

length is the one over f oca l leng th ru le . This states that for a 35 mm camera, the exposure

time needs to be at least as fast as one over the focal length in seconds. In other words, when

using a 200 mm focal length on a 35 mm camera, the exposure time needs to be at least

1/200 seconds — otherwise blurring may be hard to avoid. See the tutorial on reducing camera

shake with hand-held photos for more on this topic.

Keep in mind that this rule is just for rough guidance; some may be able to hand hold a shot for

much longer or shorter times. For users of digital cameras with cropped sensors, one needs toconvert into a 35 mm equivalent focal length.

ZOOM LENSES vs. PRIME LENSES

A zoom lens is one where the photographer can vary the focal length within a pre-defined range,

whereas this cannot be changed with a "prime" or fixed focal length lens. The primary advantage

of a zoom lens is that it is easier to achieve a variety of compositions or perspectives (since lens

changes are not necessary). This advantage is often critical for dynamic subject matter, such as in

photojournalism and children's photography.

Keep in mind that us ing a zoom lens does no t necessar i l y mean t ha t one no longer h as t o

change t he i r pos i t ion ; zooms just increase flexibility. In the example below, the original

position is shown along with two alternatives using a zoom lens. If a prime lens were used, then

a change of composition would not have been possible without cropping the image (if a tighter

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composition were desirable). Similar to the example in the previous section, the change of 

perspective was achieved by zooming out and getting closer to the subject. Alternatively, to

achieve the opposite perspective effect, one could have zoomed in and moved further from the

subject.

Two Options Available with a Zoom Lens:

Change of Composition Change of Perspective

Why would one intentionally restrict their options by using a prime lens?Prime lenses existed long

before zoom lenses were available, and still offer many advantages over their more modern

counterparts. When zoom lenses first arrived on the market, one often had to be willing to

sacrifice a significant amount of optical quality. However, more recent high-end zoom lensesgenerally do not produce noticeably lower image quality, unless scrutinized by the trained eye (or

in a very large print).

The primary advantages of prime lenses are in cost, weight and speed. A n inexpens ive p r ime

lens can genera l ly p rov ide as good (o r be t t e r ) image qua l i t y as a h igh -end zoom lens .

Additionally, if only a small fraction of the focal length range is necessary for a zoom lens, then a

prime lens with a similar focal length will be significantly smaller and lighter. Finally, the best

prime lenses almost always offer better light -gathering ability (larger maximum aperture) than

the fastest zoom lenses — often critical for low-light sports/theater photography, and when a

shallow depth of field is necessary.

For compact digital cameras, lenses listed with a 3X, 4X, etc. zoom designation refer to the ratiobetween the longest and shortest focal lengths. Therefore, a larger zoom designation does not

necessarily mean that the image can be magnified any more (since that zoom may just have a

wider angle of view when fully zoomed out). Additionally, digital zoom is not the same as optical

zoom, as the former only enlarges the image through interpolation . Read the fine-print to ensure

you are not misled.

INFLUENCE OF LENS APERTURE OR F-NUMBER

The aperture range of a lens refers to the amount that the lens can open up or close down to let

in more or less light, respectively. Apertures are listed in terms of f-numbers, which quantitatively

describe relative light-gathering area (depicted below).

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Note that larger aperture openings are defined to have lower f-numbers (often very confusing).

These two terms are often mistakenly interchanged; the rest of this tutorial refers to lenses in

terms of their aperture size. Lenses w i t h la rger aper t u res a re a lso descr ibed as be ing

" f as te r , " because for a given ISO speed, the shutter speed can be made faster for the same

exposure. Additionally, a smaller aperture means that objects can be in focus over a wider range

of distance, a concept also termed the depth of field.

f -# Corresponding Impact on Other Properties:

L igh t -Gather ing A rea(A per t u re S ize)

R eq u i r e d Sh u t t e r S pe e d D e pt h o f F i e l d

Higher Smaller Slower Wider

Lower Larger Faster Narrower

When one is considering purchasing a lens, specifications ordinarily list the maximum (and maybe

minimum) available apertures. Lenses with a greater range of aperture settings provide greater

artistic flexibility, in terms of both exposure options and depth of field. The maximum aperture is

perhaps the most important lens aperture specification, which is often listed on the box along with

focal length(s).

An f-number of X may also be displayed as 1:X (instead of f/X), as shown below for the Canon

70-200 f/2.8 lens (whose box is also shown above and lists f/2.8).

Portrait and indoor sports/theater photography often requires lenses with very large maximum

apertures, in order to be capable of a narrower depth of field or a faster shutter speed,

respectively. The narrow depth of field in a portrait helps isolate the subject from their

background. For digital SLR cameras, l enses w i t h la rger max im um aper t u res p rov ides ign i f i can t ly b r igh te r v iew f inder images — possibly critical for night and low-light

photography. These also often give f as te r and more accura te au to- f ocus ing in low-light.

Manua l f ocus ing is a lso eas ie r because the image in the viewfinder has a narrower depth of 

Note: Aperture opening (iris) is rarely a perfect circle,

due to the presence of 5-8 blade-like lens diaphragms.

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field (thus making it more visible when objects come into or out of focus).

Typ ica l Max imumA per tu res

Re lat ive L igh t -Gather ing A b i l i t y

Typical Lens Types

f/1.0 32XFastest Available Prime Lenses

(for Consumer Use)

f/1.4 16XFast Prime Lenses

f/2.0 8X

f/2.8 4XFastest Zoom Lenses

(for Constant Aperture)

f/4.0 2X Light Weight Zoom Lenses or ExtremeTelephoto Primesf/5.6 1X

Minimum apertures for lenses are generally nowhere near as important as maximum apertures.

This is primarily because the minimum apertures are rarely used due to photo blurring from lens

diffraction, and because these may require prohibitively long exposure times. For cases where

extreme depth of field is desired, then smaller minimum aperture (larger maximum f-number)

lenses allow for a wider depth of field.

Finally, some zoom lenses on digital SLR and compact digital cameras often list a range of 

maximum aperture, because this may depend on how far one has zoomed in or out. These

aperture ranges therefore refer only to the range of maximum aperture, not overall range. A

range of f/2.0-3.0 would mean that the maximum available aperture gradually changes from

f/2.0 (fully zoomed out) to f/3.0 (at full zoom). The primary benefit of having a zoom lens with a

constant maximum aperture is that exposure settings are more predictable, regardless of focal

length.

Also note that just because the maximum aperture of a lens may not be used, this does not

necessarily mean that this lens is not necessary. Lenses t yp ical ly have f ewer aber ra t ions

w h e n t h e y p e r f o r m t h e e x p o s u r e s t o p pe d d o w n o n e o r t w o f - s t o p s f r o m t h e i r m a x i m u m

aper tu re (such as using a setting of f/4.0 on a lens with a maximum aperture of f/2.0). This

*may* therefore mean that if one wanted the best quality f/2.8 photograph, a f/2.0 or f/1.4 lens

may yield higher quality than a lens with a maximum aperture of f/2.8.

Other considerations include cost, size and weight. Lenses with larger maximum apertures are

typically much heavier, larger and more expensive. Size/weight may be critical for wildlife, hiking

and travel photography because all of these often utilize heavier lenses, or require carrying

equipment for extended periods of time.

FURTHER READING

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For more on camera lenses, also visit the following tutorials:

Using Wide Angle Lenses

Using Telephoto Lenses

Macro Lenses: Magnification, Depth of Field & Effective F-Stop

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