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The most pertinent aspect of focal length is the field of view that it provides. A lens with a wide focal length, such as a 20mm lens, sees a very wide field of view. We call this a wide angle lens, and it is popular for taking photos of large subjects, like a building, a mountain or a beautiful view. Conversely, a lens with a long focal length, like 100mm, sees only a very narrow field of view. However, this has the effect of expanding what is in that field of view, so that you can capture subjects that are further away from you, like a far away animal or an athlete in a sporting match. If you were to stand in the same spot and use a wide angle lens vs a telephoto lens, the resulting field of view would be much different.

When shooting portrait photography in particular, many photographers like an 85mm focal length. We often think that we want pictures that look just like we appear in real life...but that's not actually true. Using an 85mm lens offers just a bit of image compression, which helps a subject's face appear more balanced and even. This avoids the problem caused by wider focal lengths that distorts facial features. Longer focal length lenses also provide more shallow depth of field, which is generally desirable in portrait photography.

Short focal distances can also be great when you're trying to capture a big group of people without having to stand 500m away! It's a better choice than trying to back farther and farther up to get the full image!

Ultimately, choosing your focal length is a matter of the type of image that you are taking.  Most photographers will carry multiple lenses across a range of focal lengths so they can change the style of photos that they take. Many new photographers start with a kit lens on their camera, which is generally in the range of 18-55mm. When it comes time to buy a new lens, think about what focal length you want. Are you often trying to fit everything  into the frame and capture a very wide image? Then consider a wide angle lens. Are you always trying to get just a bit more reach? Look at a telephoto lens. Do you want more shallow depth of field? You probably want a short telephoto lens that also has a fast aperture.

Focal length has a tremendous effect on the appearance of your photography. Changing your focal length changes how your subject looks and, quite literally, what you're focusing on.

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Top: an image taken at 24mm. Bottom: an image taken from the exact same location but with a 120mm lens. The focal length affects the field of view.

Focus distance

Holoplexing, a technique devised by KOSI in which two gratings are placed together in the same structure to cover multiple spectral ranges at one time, is useful for imaging on charge-coupled-device (CCD) cameras for broadband applications. Holographic transmission gratings are also used in Raman spectroscopy and for pulse compression in ultrafast lasers.

You certainly hear a lot about focal lengths as you begin to learn more about photography, but you may not be entirely sure what the term means. Focal length is a measurement; specifically, it's the measurement between the lens itself and the image sensor when the subject of your photograph is in focus. This measurement is shown in millimetres. The higher the measurement, the longer the focal length.

Portraiture favours semi-telephoto lenses (typically between 85mm and 120mm) because they flatten the face favourably and provide increased shallow depth of field

2020419 — A camera sensor and its size determines image size, depth of field, resolution, low-light performance, a camera's physical size, and more.

The blurry background in this image indicates a very shallow depth of field. Longer focal lengths result in a shallower depth of field at comparable apertures than wider focal lengths

Many lenses have adjustable focal lengths (zoom lenses), but some have fixed focal lengths (prime lenses). Newer photographers sometimes favour zoom lenses, especially as they start to learn about the many ways that focal lengths can change picture composition. However don't underestimate the utility of a high-quality prime lens!

Focal length

Kaiser Optical Systems Inc. (KOSI; Ann Arbor, MI), has developed an alternative to the classical or surface-relief holographic grating--the volume transmission holographic grating (see photo at top of this page; also Laser Focus World, Oct. 1995, p. 95). The grating is created in the traditional manner by recording interference patterns generated by two mutually coherent laser beams. After the pattern is defined in the photosensitive material, coated on glass, and the film developed, a top layer of glass is added, creating a totally transparent grating assembly. Light strikes the grating on one side and diffracts out through the other.

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Grating applicationsLight incident on a diffraction grating is dispersed away from the grating surface at an angle dependent on its wavelength, allowing a grating to be used to select a narrow spectral band from a much wider band. This ability of a grating is particularly useful for laser tuning, especially in the visible region of the spectrum. Two primary configurations for selecting a narrow wavelength are Littrow and Littman. In the Littrow configuration, the wavelength of interest diffracts at exactly the same angle as the light incident on the grating. Littrow tuning is done either with fine-pitch first-order gratings (typically 1800 or 2400 grooves/mm, either ruled or holographic) or a coarser grating used in higher orders. The alternative approach is to use the grating in a fixed grazing incidence mode together with a rotating reflecting mirror. Pairs of diffraction gratings can also be used to compress or stretch a laser pulse. When a spectrally broad laser pulse is incident on a diffraction grating, the various wavelengths that make up the pulse will diffract from the grating at angles determined by those wavelengths. If the pulse is chirped so that the frequency changes linearly during the length of the pulse, then diffraction will spread the pulse out across the second grating. When the light diffracts from the second grating, which is oriented parallel to the first grating, the different parts of the pulse will diffract at angles that yield a pulse whose parts are synchronized. This increases the peak power while the total energy remains the same. Pulse compression uses two gratings with the same groove frequency and efficiencies peaked for the polarization and wavelength of the laser. If the gratings are arranged in a nonparallel arrangement, a pulse can be stretched. Pulse stretching uses two identical gratings, allowing lower peak power to be transmitted through the laser system and increasing the amount of stored energy that can be extracted. Since the invention of the replication technique, diffraction gratings have replaced prisms in many commercial spectrometers. A prism will bend short wavelengths more than longer ones (see Laser Focus World, Jan. 1997, p. 101). Prisms that transmit visible light absorb most UV and infrared wavelengths, whereas reflection gratings can be suitably coated for high reflectivity in wide spectral regions. Gratings are considered superior to prisms in many applications. Seeking to combine the best of both, Richardson Grating Laboratory has fabricated a "grism," a part-grating, part-prism optical element useful in spectrometers that require in-line presentation of the spectrum, as in astronomy. The light diffracted by the grating is bent back in line by the refracting effect of the prism. The dispersion of the grism is not linear, because the dispersive effects of the prism and grating are superimposed.New fabrication techniquesKaiser Optical Systems Inc. (KOSI; Ann Arbor, MI), has developed an alternative to the classical or surface-relief holographic grating--the volume transmission holographic grating (see photo at top of this page; also Laser Focus World, Oct. 1995, p. 95). The grating is created in the traditional manner by recording interference patterns generated by two mutually coherent laser beams. After the pattern is defined in the photosensitive material, coated on glass, and the film developed, a top layer of glass is added, creating a totally transparent grating assembly. Light strikes the grating on one side and diffracts out through the other.An advantage of a transmission volume grating is its relative insensitivity to angle, says James Arns of KOSI. A Bragg-type structure follows the classical grating equation concerning image position but with the added ability to adjust the intensity profile over a range of wavelengths. To describe the capability, Arns compares a Venetian blind to lines painted on a window. When the blind is positioned with the slats horizontal, it diffracts light in the same way as the painted lines or a surface-relief grating. When the slats are angled, the element of depth is added to how the light is diffracted. Because of this added dimension, the grating efficiency can be adjusted over the wavelength bandwidth to favor one side or the other. Also, the low sensitivity to incidence angle means the grating can be angularly tuned without influencing the image position."It also has a high efficiency," says Arns. "Depending on the configuration, the grating can produce 90% efficiency in the first order. If the thickness or the frequency of the grating is high enough, higher orders that otherwise might be propagated are extinguished." Another advantage, says Arns, is that the element can be handled and cleaned in the same fashion as a high-quality cemented lens because the grating is sandwiched between two layers of glass. Also, because the Bragg-type grating is a transmission device, optical elements and instruments can be brought close to it, resulting in a compact design.Holoplexing, a technique devised by KOSI in which two gratings are placed together in the same structure to cover multiple spectral ranges at one time, is useful for imaging on charge-coupled-device (CCD) cameras for broadband applications. Holographic transmission gratings are also used in Raman spectroscopy and for pulse compression in ultrafast lasers.Holographic gratings can also be made from computer-generated interference patterns. The patterns are written onto a chrome mask using an electron-beam machine. The patterns on the mask are then etched into a material, such as fused silica, using photolithographic masking and etching techniques. "Computer-generated gratings have really just reached maturity within the last two years," says Michael Feldman, of Digital Optics Corp. (Charlotte, NC). "They are very flexible and easy to mass-produce." Their versatility offers many advantages. "Ruled and holographic gratings are limited to relatively simple structures by the fabrication methods that are used," says W. Hudson Welch, also of Digital Optics. "The flexibility provided by computer-generated gratings allows the creation of essentially arbitrary grating patterns."Fiber gratingsFiber Bragg gratings, another recent development in grating applications, are made within a fiberoptic cable. Fiber gratings are fabricated by exposing the core of a single-mode fiber, 8 to 10 µm thick, to a periodic pattern of intense ultraviolet light. This pattern is created when a 248- or 193-nm laser passes through a special diffractive phase mask. When a fiber is placed in the intense UV light pattern of the mask, a permanent modulation of the index of refraction is generated in the fiber core. This photo-generated index modulation acts as a grating. Light traveling along the fiber core impinges on the grating, and each area of different refractive index scatters a small portion of the beam. If the wavelength of the signal is twice the distance between the periodic refractive elements (typically <1 µm), then the signals scattered back down the fiber core will add constructively to give a large reflection. The wavelength at which the reflection occurs is the Bragg wavelength. A Bragg grating can operate at precise wavelengths that can be accurately preset and maintained, says Keith Brundin at 3M Specialty Optical Fibers (West Haven, CT).There are also long-period fiber gratings that have index modulations with periods of hundreds of microns (see Laser Focus World, June 1996, p. 293). Instead of producing a reflected signal, these gratings create a phase-matching, or Bragg, condition that couples a forward-traveling signal into forward-traveling cladding modes. The signals coupled into the cladding are absorbed by the coating, creating a loss. Long-period gratings thus act as wavelength-selective absorption filters and are used in wavelength-division-multiplexing networks and in gain-shaping filters for rare-earth-doped fiber amplifiers. Fiber Bragg gratings have been commercially available only since 1995. They are becoming increasingly popular in telecommunications and the laser industry for such applications as external reflectors for stabilizing semiconductor lasers (see Fig. 4) and single- frequency fiber lasers.

Holographic gratings can also be made from computer-generated interference patterns. The patterns are written onto a chrome mask using an electron-beam machine. The patterns on the mask are then etched into a material, such as fused silica, using photolithographic masking and etching techniques. "Computer-generated gratings have really just reached maturity within the last two years," says Michael Feldman, of Digital Optics Corp. (Charlotte, NC). "They are very flexible and easy to mass-produce."

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This is a fundamental aspect of focal length and will probably be the main reason why you might choose one focal length over another.

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Joseph Fraunhofer first used diffraction gratings in 1819 to observe the spectrum of the sun. Earliest devices were multiple-slit assemblies, consisting of a grid of fine wire or thread wound about and extending between two parallel screws, which served as spacers. A wavefront that passed through the system was confronted by alternate opaque and transparent regions, so that it underwent a modulation in amplitude.

"The grooves are similar to the indentations made by a plow in soil," says John Hoose of Richardson Grating Laboratory (Rochester, NY), except that they are much closer together. Anywhere from one to 10,000 fine parallel lines per millimeter can be engraved. Light waves diffracted from these lines interfere, and all wavelengths but one are canceled in any particular direction through destructive interference. The depth of the groove changes the wavelength of the light wave being diffracted.

The concept of diffraction gratings is simple, yet elegant. For more than one hundred years, they have been used in dispersive optical systems. Applications for gratings are expanding as the fabrication technology grows. Fields as diverse as telecommunications, astronomy, microlithography, lasers, and metal analysis are driving these changes.

In addition to affecting the field of view, focal length also affects the perspective of your images. A longer focal length will make the distance between the subject and the background appear smaller - it compresses the background. A wider focal length will stretch out and exaggerate the distance between subject and background, kind of like a funhouse mirror. Portrait photographers often prefer a long focal length (often 85mm) because compressing is more flattering to the human face. Wide angle lenses tend to make people's noses exaggeratedly large, which is often not a desired look.

Since the invention of the replication technique, diffraction gratings have replaced prisms in many commercial spectrometers. A prism will bend short wavelengths more than longer ones (see Laser Focus World, Jan. 1997, p. 101). Prisms that transmit visible light absorb most UV and infrared wavelengths, whereas reflection gratings can be suitably coated for high reflectivity in wide spectral regions. Gratings are considered superior to prisms in many applications. Seeking to combine the best of both, Richardson Grating Laboratory has fabricated a "grism," a part-grating, part-prism optical element useful in spectrometers that require in-line presentation of the spectrum, as in astronomy. The light diffracted by the grating is bent back in line by the refracting effect of the prism. The dispersion of the grism is not linear, because the dispersive effects of the prism and grating are superimposed.

Wide angle lenses are typically not ideal for portraits or any kind of closeup of humans as it unflatteringly distorts the face.

Camera Size comparison with lens

When you want to capture an image close to how it's viewed by the human eye, you want to use a focal length between 35mm and 50mm. This is typically called a "normal" focal length. This will balance the distance between your subject and the background in much the same way that we see through our binocular vision. Pictures will feel very realistic and natural. This is great for snapshots or other images that you want to feel very natural.

Light incident on a diffraction grating is dispersed away from the grating surface at an angle dependent on its wavelength, allowing a grating to be used to select a narrow spectral band from a much wider band. This ability of a grating is particularly useful for laser tuning, especially in the visible region of the spectrum. Two primary configurations for selecting a narrow wavelength are Littrow and Littman. In the Littrow configuration, the wavelength of interest diffracts at exactly the same angle as the light incident on the grating. Littrow tuning is done either with fine-pitch first-order gratings (typically 1800 or 2400 grooves/mm, either ruled or holographic) or a coarser grating used in higher orders. The alternative approach is to use the grating in a fixed grazing incidence mode together with a rotating reflecting mirror.

While aperture is the main setting that affects depth of field, focal length also plays an important point. A long focal length provides a shallower depth of field while a wide focal length provides a deeper depth of field. For that reason, using an aperture of f/1.8 on a wide angle lens will provide a deeper depth of field than an aperture of f/1.8 on an 85mm lens. This is another reason why portrait photographers often use longer lenses, as this provides shallower depth of field than a wider angle lens.

Light traveling along the fiber core impinges on the grating, and each area of different refractive index scatters a small portion of the beam. If the wavelength of the signal is twice the distance between the periodic refractive elements (typically <1 µm), then the signals scattered back down the fiber core will add constructively to give a large reflection. The wavelength at which the reflection occurs is the Bragg wavelength. A Bragg grating can operate at precise wavelengths that can be accurately preset and maintained, says Keith Brundin at 3M Specialty Optical Fibers (West Haven, CT).

In 1882, Henry A. Rowland invented the process of ruling, or scratching parallel notches into metal deposited onto the surface of a flat, clear glass plate—a method that produced gratings of exceptionally high quality. Modern ruled gratings can be either reflective or transmissive and are fabricated with a single diamond point that burnishes grooves on flat or concave surfaces.

focallength中文

What we've presented here are just guidelines. Exciting new types of photography have developed because of people trying to do things differently, creating innovative effects. Using focal length and combining it with different subjects and backgrounds can have astonishing results.

It's definitely a good idea to experiment and find out what focal length settings you like best in different photography scenarios, but if you just want to get started before you start playing, here are some common types of photography and the most common focal lengths.

A telephoto lens - one that has a focal length of up to 300mm - is used in very specific situations. Wildlife photography and sports photography are often best captured using this focal length. It helps the photographer seem much closer to the action than they can physically get, but it also compresses the background towards the subject. This makes the subject stand out in sharp relief while also making sure that the background itself seems closer than it otherwise might.

Subjects like sports and wildlife, where there is typically a large distance between the photographer and the subject, favour long focal lengths

Focal lengthcamera

When you want to take in as much of the scene as you can, a focal length of 35mm or less is going to be your go-to. This will allow you to capture a very wide field of view. This is great for landscape photography and can create some incredible images with a huge feeling of distance in the image.

If you're not sure what type of lens, camera, or other photography equipment might suit you best, get in touch with digiDirect. We offer a fantastic selection of cameras, lenses, and other accessories that will get you started on your journey towards the best photography has to offer. Whether you're looking to take a better snapshot with your family or get started as a formal photographer, we have what you need. Contact us today to learn more.

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The author wishes to thank John Hoose of Richardson Grating Laboratory (Rochester, NY) for his help in preparing this article.

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Normal lenses tend to approximate what we see with our eyes, without wildly differing perspectives. This makes them popular for street photography and general use

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Focaldistance vsfocal length

Pairs of diffraction gratings can also be used to compress or stretch a laser pulse. When a spectrally broad laser pulse is incident on a diffraction grating, the various wavelengths that make up the pulse will diffract from the grating at angles determined by those wavelengths. If the pulse is chirped so that the frequency changes linearly during the length of the pulse, then diffraction will spread the pulse out across the second grating. When the light diffracts from the second grating, which is oriented parallel to the first grating, the different parts of the pulse will diffract at angles that yield a pulse whose parts are synchronized. This increases the peak power while the total energy remains the same. Pulse compression uses two gratings with the same groove frequency and efficiencies peaked for the polarization and wavelength of the laser.

When you're first learning about photography, there are many different terms that get tossed around. Aperture, sensor, focus, colour temperature, and more. All of these different terms and measurements affect the ways in which your camera captures an image. Adjusting any of these settings, from colour temperature to focal length, will create a different final result in your picture. Key amongst these is focal length, which is the defining characteristic of any lens. Choosing the right focal length can be integral to the image or in some situations more a matter of artistic preference, but when you're just getting started, knowing what's possible with your equipment can make a big difference.

Portrait photography (left) and landscape photography (right) are two disciplines that often use distinctive focal lengths

There are also long-period fiber gratings that have index modulations with periods of hundreds of microns (see Laser Focus World, June 1996, p. 293). Instead of producing a reflected signal, these gratings create a phase-matching, or Bragg, condition that couples a forward-traveling signal into forward-traveling cladding modes. The signals coupled into the cladding are absorbed by the coating, creating a loss. Long-period gratings thus act as wavelength-selective absorption filters and are used in wavelength-division-multiplexing networks and in gain-shaping filters for rare-earth-doped fiber amplifiers.

Two images with a similar composition but different focal length. The image on the top, with a wide 24mm lens, exaggerates distance in the background. The bottom image, taken with a 120mm lens, compresses the background.

An advantage of a transmission volume grating is its relative insensitivity to angle, says James Arns of KOSI. A Bragg-type structure follows the classical grating equation concerning image position but with the added ability to adjust the intensity profile over a range of wavelengths. To describe the capability, Arns compares a Venetian blind to lines painted on a window. When the blind is positioned with the slats horizontal, it diffracts light in the same way as the painted lines or a surface-relief grating. When the slats are angled, the element of depth is added to how the light is diffracted. Because of this added dimension, the grating efficiency can be adjusted over the wavelength bandwidth to favor one side or the other. Also, the low sensitivity to incidence angle means the grating can be angularly tuned without influencing the image position.

Their versatility offers many advantages. "Ruled and holographic gratings are limited to relatively simple structures by the fabrication methods that are used," says W. Hudson Welch, also of Digital Optics. "The flexibility provided by computer-generated gratings allows the creation of essentially arbitrary grating patterns."

As you start learning about focal length, you will probably also learn more about depth of field. The aperture of your camera has a big effect on depth of field, but focal length also has a role to play. Depth of field refers to how much of your image is in focus. A shallow depth of field means only a small part of the image is in focus (usually your subject) while much of the background is out of focus. This is common in portrait and artistic photography. A deep depth of field means most of your photo is in focus, regardless of the distance from the camera. Landscape photography often uses deep depth of field. Learn more about depth-of-field here.

Diffraction gratings are fundamental optical elements that have a precise pattern of grooves superimposed on them. These minute, periodic structures diffract, or disperse, incident light in such a way that the individual wavelengths making up the incident light can be differentiated. Gratings are indispensable in helping physicists determine the structure of atoms or helping astronomers calculate the chemical composition of stars and the rotation of galaxies. Applications are expanding; one of the fastest growing areas for gratings—laser pulse compression—didn’t even exist until a few years ago.

"It also has a high efficiency," says Arns. "Depending on the configuration, the grating can produce 90% efficiency in the first order. If the thickness or the frequency of the grating is high enough, higher orders that otherwise might be propagated are extinguished." Another advantage, says Arns, is that the element can be handled and cleaned in the same fashion as a high-quality cemented lens because the grating is sandwiched between two layers of glass. Also, because the Bragg-type grating is a transmission device, optical elements and instruments can be brought close to it, resulting in a compact design.

Focal length doesn't actually have any impact on shooting in low light. For that, you'll want to understand the Exposure Triangle. However, dropping your aperture to increase the light captured also results in increasing shallow depth of field. If you want to shoot low light images with a deep depth of field, this will be better on a wider angle lens as you can shoot at the same aperture (therefore capturing the same amount of light) while the wider focal length will result in a deeper depth of field than a longer focal length would.

If the gratings are arranged in a nonparallel arrangement, a pulse can be stretched. Pulse stretching uses two identical gratings, allowing lower peak power to be transmitted through the laser system and increasing the amount of stored energy that can be extracted.

Here we're going to dig into focal lengths and talk about how they play into creating the kind of photograph that you want.

Commercial surface-relief gratings are produced using an epoxy casting replication process developed in the mid-1900s. The process involves pouring a liquid into a mold, allowing the liquid to harden, and then removing the hardened material from the mold without damaging either. The replication process yields a grating that is an optically identical copy of the original. The two basic types of grating masters are ruled and interference.

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Das 60 mm f/2.8 Makro-Objektiv von Dörr eignet sich ideal für Natur- und Tierfotografie, grossartige Detailaufnahmen oder auch als Portraitobjektiv.

Fiber Bragg gratings, another recent development in grating applications, are made within a fiberoptic cable. Fiber gratings are fabricated by exposing the core of a single-mode fiber, 8 to 10 µm thick, to a periodic pattern of intense ultraviolet light. This pattern is created when a 248- or 193-nm laser passes through a special diffractive phase mask. When a fiber is placed in the intense UV light pattern of the mask, a permanent modulation of the index of refraction is generated in the fiber core. This photo-generated index modulation acts as a grating.