Figure 1: If a broadband-wavelength source, like a lamp or LED, is placed one focal length (f ) away from a lens, light from each point on the source is individually collimated. The full divergence angle (θ ~ d / f ) of the output beam depends on the focal length and source diameter (d ).

Please note that lens compression is not related to the lens, but to the distance from the subject (Ex. You can achieve the same field of view and perspective shooting an element with a short focal length like 20 mm or cropping the same area from a 50 mm as long as both images are taken from the same distance).

The simplest focal length definition is a description of the distance between the center of a lens and the image sensor when the lens is focused at infinity.

Focal length touches upon many elements of the photography basics; composition, aperture, depth of field, and other aspects all shift when taking focal length into account! Each twist of the zoom ring or swap of a prime lens is a shift in the interplay of focal length and your creative vision.

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in which () is the wavelength. When the input beam's waist and Rayleigh range are smaller, the collimated output beam has a lower divergence. (See Figures 2 and 3 for an example.) The divergence along the entire collimated beam can be reduced by shrinking the input source beam's waist diameter.

Figure 2: A 632.8 nm laser source, which has a 5 µm diameter waist (Wo) and a 31 µm Rayleigh range (zR), is collimated by a 50 mm focal length (f ) lens.

By twisting the barrel of a zoom lens, you can adjust the focal length and field of view of your lens, which affects depth of field, distortion, and all other aspects of your image.

Figure 4: The input laser light better resembles a point source when it has a smaller beam waist (Wo), and therefore a shorter input Rayleigh range (zR). The collimated output beam has a Rayleigh range (zR') that depends on Wo , focal length, and wavelength. Increasing the focal length, or decreasing the wavelength, increases zR' and decreases the overall output beam divergence. The values plotted above were calculated for a 632.8 nm wavelength and a focal length of 50 mm. For comparison, values for a wavelength of 1550 nm and a focal length of 25 mm have also been calculated.

Focal length determines many of the characteristics of the photos you can take as well as the physical dimensions of the lens.

Collimated Lamp or LED LightAn ideal point source emits light uniformly in all directions, and a positive spherical lens placed one focal length (f ) away collimates the light it collects into a beam with zero divergence. This ideal model can be adapted for use with a broadband-wavelength source, like a lamp or LED. Physical sources like these resemble groupings of multiple point sources. An adapted model for these sources takes into account the diameter (d ) of the physical light source, as well as the distance (i.e. the focal length) between the light source and the lens. The ratio of these values provides an estimate of the full divergence angle (θ ),

Zoom lenses are what anyone who has ever picked up a camera in recent times is familiar with. In fact, they are so common that I often find non-photographers get baffled when I hand them a camera with a prime lens attached!

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If the source waist is one focal length away from the lens, the collimated beam's waist is located one focal length away from the lens' opposite side. The beam's divergence increases with distance from the waist, and generalizations about the beam divergence can be made for regions close to and far from the waist.

Unlike LEDs and lamps, laser sources do not emit light uniformly in all directions, although the laser light's divergence can be large. If the light comes from a single-wavelength laser, there's a good chance its intensity cross section resembles a Gaussian function. The collimated beam's behavior can then be described using Gaussian beam equations, and the equations describing beam divergence would need to include the wavelength.

Hello Dan! I’m Mariam from Mauritius. I wanted to thank you for this deep explanation on focal length! I just bought my first camera and your article helped me so much!

Fixed lens camera focal lengths can also be multiplied by the crop factor of the sensor in order to compare it across formats. This is generally only important if you’re trying to replicate a look across focal lengths; i.e. you know you want a 135mm full-frame portrait style.

Focal length differences are especially stark with wide-angle lenses. Each mm of width makes a visible difference, unlike telephoto lenses where it takes several mm to be easily noticeable.

Super telephoto lenses have very different focal lengths, ranging from 300mm and beyond. When considering focal length and field of view, crop factor is incredibly important because you can gain significant amounts of reach using a crop sensor like Micro 4/3rds.

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In practical terms, angle of view and field of view are used interchangeably in photography to indicate the way our cameras “see” the scene, and using a short focal length or a long focal length will change drastically the field of view or the amount of the scene that is photographed.

The angle of view in photography is the area of the scene that is captured by the camera sensor. This area is described in degrees of coverage in front of the camera.

Hi Anne, it totally depends on the sensor size of the camera. For example, if you’re using a Micro 4/3 sensor camera with the 60mm f2.8 lens, since the crop factor is 2x, it means that you’re cropping the image the double, so technically it’s like shooting at 120mm focal length. Hope it’s clear. 🙂

Focal length and depth of field are different properties of both a lens and sensor but are somewhat related. Depth of field is how much of a scene is in sharp focus. How narrow or wide the depth of field is, is an interaction between focal length, sensor size, subject distance, and aperture value.

Laser Point SourcesThe point source model can also be adapted for laser light, but in this case the source is defined as the input beam waist. The source size is the waist diameter (2Wo ). Typical sources include a focal spot, an optical fiber's end face, and the facet of a single-mode laser diode.

Understanding your camera’s zoom ability helps you know what sort of pictures you can reasonably expect to take. Just like with an interchangeable lens camera (ILC), the camera will have a focal length range that tells you about the properties of the lens.

Hopefully, this article on explaining focal length has clarified some of the stickier aspects of the topic. Especially where they affect wide, normal, and telephoto fields of view.

Focal distance is the distance between the subject you are focusing to the camera sensor. Lenses that can work at close focal distances have higher magnification (or reproduction ratios) relative to lenses that need you to stay far from the subject.

Telephoto lenses are those beyond 50mm. These lenses are also physically longer than wide and normal lenses. The field of view is smaller but you gain significant reach.

A Lamp or LED as a Good-Enough Point SourceA physical source adequately resembles an ideal point source when the divergence of the collimated output beam meets the application's requirements. If the divergence is too large, one option is to increase the focal length of the collimating lens, so the source can be moved farther away. However, this can have the negative effect of reducing the amount of light collected by the lens and output in the collimated beam.

Can you help me understand why a lens advertised as 60 mm f2.8 macro is described as 120mm and equivalent to 35mm focal length? I am completely bewildered.

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Focal length is one of the main considerations when buying and selecting a lens. And understanding how focal length works is essential to capturing the photos you want. Otherwise, you’ll be choosing lenses with random angles of view that only serve to confuse you with choices.

of the collimated beam. The better the physical source resembles a single ideal point source, the lower the divergence angle of the collimated beam (Figure 1). The resemblance is closer when the physical source is very small, far away from the lens, or both.

When you look at the focal length of the lens in question and you see a number below 35mm, you know you’re looking at a short focal length.

Macro lenses have the highest magnification due to their unique construction, which reduces the focal distance they operate within and allow you to focus in very close subjects.

Focal length can feel a little abstract when looking at one lens versus another. One of the easiest ways to understand focal length is to look at images that use the lenses in question! To wrap up, here are some focal length examples for you to consider:

Wide-angle lenses or short focal lengths offer a great opportunity for shooting landscapes, cityscapes, Milky Way photography, and Northern Lights photography.

Far from the output beam waist (>>zR'), the beam divergence is comparatively larger. The divergence in this region is reduced when the output beam waist diameter (and Rayleigh range) are increased. Here, the beam has an approximately constant full divergence angle,

Camera zoom lenses allows to change the angle of view without moving and are more versatile than prime lenses. The downside is that maximum apertures are not usually as fast as the fastest prime lenses. For example, an f/1.4 zoom would be ridiculously expensive and massive.

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Remember that if you tend to specialize in any type of photography, it’s crucial to understand how focal length affects your image so you can have a better idea of what sort of lenses you should be shopping with, as well as the impact they have on a subject and background. You can download my PDF photography guide to get more information about this.

For example, if I use a short focal length in portrait photography, parts of the body that are closer to me can be unflatteringly emphasized. On the other hand, in genres like landscape photography, it’s more common to see images taken with a wide-angle view, not only to capture the entire scene in a single image but to emphasize elements in the foreground.

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While we do go into some depth here and there are some confusing elements surrounding the topic, I’m confident that by the end of this article, you’ll have a solid foundation on what is focal length and how it relates to digital photography.

These lenses take on an expansive field of view that’s wider than what we normally pay attention to. An image taken with a 15mm lens will seem abnormally expansive, taking, for example, an entire landscape with ease.

Focal length also relates to field of view (also called angle of view) because changing the focal length changes the field of view – I’ll explain more about how the field of view and focal length of a lens interact with each other in greater detail below.

As you can see, the focal length we choose affects the final image. Also, the field of view and lens distortions fundamentally affect the type of photography you do.

Some applications specify a distance <

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In the relation between focal length and depth of field, when all other values are equal, short focal lengths (or wide angles of view) have deeper depth of field relative to long focal lengths (narrow fields of view). Thus, the longer the focal length, the farther the hyperfocal distance will be.

Standard lenses or medium focal lengths are suitable for shooting many different genres like portrait, street photography, landscape, etc.

So what are the mm in lenses? The key is to understand that focal length is calculated by measuring the distance from the optical center of the lens to the image sensor, and this distance is measured in mm. The longer the focal length, the physically longer the lens will be. Lenses with a wider view will have a shorter focal length and are physically shorter by comparison.

Collimated Laser LightThe laser source's size helps determine the properties of the collimated beam. Size is typically referenced to the beam waist radius (Wo ) or diameter (2Wo ), often through its Rayleigh range (zR),

I emphasize that we get the field of view because again, lens focal length is inherent to a lens. A 25mm Micro 4/3rds lens has a 50mm full-frame field of view. However, it remains a 24mm lens with the distortion properties of a 24mm lens; it’s not magically a 50mm lens.

However, using a prime lens means that you have to physically move in order to create a given field of view. Which kind of lens is better is an age-old question and really comes down to your own personal preferences!

Lens focal length is a surprisingly nuanced topic! A simple physical measurement leads to so many considerations that go into how an image is composed. From the angle of view to depth of field, no aspect of a photograph goes untouched.

These super long focal lengths are usually more expensive but they allow to capture subjects from a very far distance like in sports and wildlife photography, and to shoot other genres like deep astrophotography.

A lens focal length that’s greater than 50mm is considered a long focal length. This view is narrower than the normal view we’re accustomed to when paying bare attention. An image taken with a 100mm telephoto lens will have a much smaller section of coverage than a normal or wide-angle field of view.

There are great apps and websites that allow you to calculate depth of field for a given focal length, aperture, subject distance, and sensor.

Figure 3: The input parameters in Figure 2 create a collimated output beam with a waist located a distance one focal length (50 mm) from the lens. Note that this figure's scale is larger than the scale in Figure 2. Close to the beam waist (<>zR'), the beam's divergence can be described by a full divergence angle (). A wavelength of 632.8 nm was assumed.

Apart from the definition of focal length in photography as the measurement in millimeters from the lens to the image sensor, focal length has a direct impact on the angle of view, which is a static property of the lens in question but is impacted by the sensor crop factor.

One key to understanding focal length is recognizing how the look of the image changes using lenses with different focal lengths. Using a wide-angle lens, foreground elements are emphasized while background elements are pushed even further away, looking smaller than they really are.

Ideally, collimated light would have a constant diameter from the lens out to infinity, but no physical collimated beam maintains exactly the same diameter as it propagates. The collimated beam's divergence, which is the rate at which the beam's diameter changes, depends on the properties of both the light source and the collimator. Due to this, the collimated beam from a broadband-wavelength source, such as a lamp or light emitting diode (LED), behaves differently than the collimated beam from a narrowband-wavelength source, like a laser.

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Near the waist, the output beam best resembles an ideally collimated beam. In this region, the beam divergence is lowest, and the beam diameter stays close to the output beam waist diameter (2W'o ). But, since the beam diameter increases with distance from the waist, the extent over which the beam is considered collimated is limited. The application determines the limit, which is typically referenced to the output beam's Rayleigh range (zR'),

Standard focal lengths range from 35mm to 50mm depending on the type of camera sensor. The field of view provided by standard focal lengths approximates the field of view of the human eye.

Normal or standard lenses have a focal length between 35mm and 50mm in full-frame terms. When comparing across formats, they have a field of view that’s also equivalent to a normal lens. These lenses best duplicate how we see the world.

By taking the crop factor of a specific sensor and multiplying it by the field of view, we get the field of view as if it were viewed in the 35mm standard.

It’s important to consider that focal length is a static property of a lens that’s true regardless of crop factor; for example, a 16mm APS-C lens is a 16mm lens even if it has a 24mm equivalent field of view in a full-frame camera. We’ll see + in-depth info about this below.

Focal length in photography comes up far more often than focal distance, which is an entirely different property of a lens. Focal distance is related to focal length but is not dependent on it.

There are similarities between point source model adaptations for lasers and broadband sources. For example, laser sources should also be placed one focal length away from the lens to minimize the divergence of the collimated beam. In addition, the smaller the laser source (input beam waist diameter), the lower the collimated beam's divergence.

As we step into normal focal lengths, which are closer to our human vision (like 35mm), this effect is subtle to invisible. However, once we hit telephoto angles of view, the background appears to be closer to the subject. This effect increases as your lens mm does.

A prime lens can’t be adjusted and its focal length and field of view are fixed. The trade-off for flexibility in focal length is usually a wider aperture and better image quality because the prime lens can be optimized for its angle of view. Usually but not always; some top-quality zoom lenses nowadays are absolutely comparable to prime lenses in terms of quality for a given focal length.

Understanding focal length in photography can feel a bit overwhelming when trying to account for all of the ways it changes the properties of an image. Below, I’ve outlined the focal length differences across the major fields of view.

Photographs taken with short focal lengths show distortion in certain elements; those closer to the foreground are magnified whereas all the elements in the background will show a diminished perspective.

How focal length works is by describing each lens in terms of millimeters (lens mm). This description is a hard-physical reality of the lens in question, no matter the brand, format, or aperture.

Photos taken with a long focal length look more “compressed” as compared to shorter focal lengths, and allow you to capture subjects from a farther distance without losing image quality.

Focal length comparisons are incomplete without the infamous crop factor discussion. Nowadays, we have APS-C, Micro 4/3rds, medium format, and full-frame, all of which use a lens focal length description centered around 35mm film gear. While this made sense when the digital revolution began, it’s often simply confusing to parse nowadays.

This concept can be complicated when the crop factor and the field of view come into play since we began to ask what is the equivalence of the focal length of a lens using it in a camera with a different sensor format.

In Figure 1, light emitted from each point on the source fills the lens' clear aperture. Rays traced from different points on the source show the output beam's diameter is smallest at the lens, and the beam appears to diverge from the clear aperture of the lens. If light from each point on the source filled only a fraction of the lens' clear aperture, the beam waist would be displaced from the lens.

Focal length is the physical measurement of distance between the lens and the imaging sensor when the subject is in focus.