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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 aims to direct the path of light rays such that they 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 of 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.  Try moving your mouse over each of the options below to see how these can impact image quality for extreme cases.

Original Image	Loss 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 referred to as having lower optical quality than another lens, this is manifested as some combination of the above artifacts.  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 small focal lengths, while telephoto lenses have larger corresponding focal lengths.

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.  Therefore longer focal lengths still result in narrower angles of view, as depicted.

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 achieve the same subjects filling the frame with 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.

Perspective control can be a powerful compositional tool in photography, and often determines one's choice in focal length (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 a 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 Focal Length*	Terminology	Typical Photography
Less than 21 mm	Extreme Wide Angle	Architecture
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

*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.

Other factors may also be influenced by lens focal length.  Telephoto lenses are more susceptible to camera shake since small hand movements become magnified within the image, similar to the shakiness experience while trying to look through binoculars with a large zoom.  Wide angle lenses are generally more resistant to flare, partially because the designers assume that the sun is more likely to be within the frame for a wider angle of view.  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 focal lengths require shorter exposure times to minimize burring 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 focal length rule.  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.  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 than this rule estimates.  For users of digital cameras with cropped sensors, one needs to convert 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 using a zoom lens does necessarily mean that one no longer has to change their position; 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 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 gotten further from the subject.

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 modern high-end zoom lenses generally 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.  An inexpensive prime lens can generally provide as good (or better) image quality as a high-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 ratio between 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).

Note: Above comparison is qualitative; aperture opening (iris) is rarely a perfect circle,
due to the presence of 5-8 blade-like lens diaphragms.

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 with larger apertures are also described as being "faster," 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.

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 faster shutter speeds or narrower depth of fields, respectively.  The narrow depth of field in a portrait helps isolate the subject from their background.  For digital SLR cameras, lenses with larger maximum apertures provide significantly brighter viewfinder images-- possibly critical for night and low-light photography.  These also often give faster and more accurate auto-focusing in low-light.  Manual focusing is also easier because the image in the viewfinder has a narrower depth of field (thus making it more visible when objects come into or out of focus).

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 typically have fewer aberrations when they perform the exposure stopped down one or two f-stops from their maximum aperture (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.