Focal Length and Lenses

Table of contents:

Focal-Length and lenses in Photography


In its simplest definition, focal length is the distance between optical center of a lens and the sensor/film, when focus is set to infinity. It is generally expressed in terms of millimeter(mm).

Table of contents:

Focal length is a factor of lens, in fact, the principal factor of a lens. When we want to buy a lens, we first decide what focal-length should the lens have. 16mm? 50mm or 300mm prime? Or a zoom 16-35mm? Or 18-300mm?

Focal length of a zoom lens is changed physically by rotating the zoom-ring on the lens. When we zoom in, focal length gets longer (100mm, 250mm, 300mm ... ), and we get a magnified view of the scenario/subject. The more we zoom in, nearer the subject approaches. When we zoom out, we get a shorter or wider focal length, such as 24mm, 16mm, 14mm ... etc. The more we zoom out, farther the subject moves, and lower the magnification we get.

Focal Length and Field of View

Focal length of a lens does not change, when used with differently sized sensors. A 70mm lens is always 70mm, regardless of the size of the sensor is used.

In combination with the sensor-size, focal length determines what we call the field of view; how much of the scenario can be seen by the camera, through the lens. A short or wide focal length can produce a large field of view, and a long focal length produces a narrow field of view. Again, with long focal lengths we can get highly magnified image; in case of short focal length opposite happens. Please check the following:

Focal Length & Field of View Lens Wide Narrow Focal Length

Observe how different focal lengths change the field of view of a scenario. Field of view also changes when different sensor-sizes are used.

Focal Lengths Wide to Long

When a very wide focal length is used with a close foreground, the lens tends to exaggerate the foreground element(s), and farther elements tend to be smaller in size. On the other hand, a very long focal lenth tends to compress the background elements nearer to each other, which is known as Lens Compression. For a visual representation of the same, check this page.

Following are the examples shot in the field. The same scenario was captured using a tripod with different focal lengths and multiple results are displayed one by one.

different focal-lengths marked

Focal Length - 24mm Focal Length - 35mm Focal Length - 50mm Focal Length - 70mm Focal Length - 85mm Focal Length - 100mm Focal Length - 140mm Focal Length - 200mm Focal Length - 250mm Focal Length - 300mm

When a lens has very short focal length, say 14mm... 16mm... 18mm..., we get a very wide field of view; so a large portion of the scenario is captured by the sensor (with a very low magnification). Those lenses are called wide angle or ultra wide angle lenses, and generally used for landscape, architecture and astro photography.

Wide angle focal length


f/17, 71 sec., iso 100, FL 15mm eqv., Nikon d7000, Sigma 10-20mm f/4-5.6, Sonbeel, Assam, India 2014

The above image is an example of wide-angle photography shot at 15mm focal length.

On the other hand, by using a very long focal length, say 200mm ... 300mm.. etc., a very narrow field of view is produced; so very small portion of the scenario is photographed (with a magnified view). Lenses having very long focal lengths are also called telephoto/super telephoto lenses, and are suitable for wildlife, birding, sports etc. Following is an example of a 300mm shot:

Long focal length

badarpur bird in flight focal-length at 300mm.

f/5.6, 1⁄640 sec., iso 500, FL 300mm eqv., Nikon d810, Lens Nikon 70-300mm f/4-5.6

Using a long focal-length requires a faster shutter-speed to avoid recording of any camera-shake. Generally, if you shoot a 300mm shot, set your shutter-speed 1⁄300 sec. or faster.

Focal length of a lens is very closely related with depth-of-field of a photograph. Medium telephoto lenses, such as 85mm, 105mm, 135mm, 200mm... etc. produce shallow/narrow depth-of-field, and are suitable for portrait photography. Following is a 127mm(eqv.) shot which blurs out the background almost completely with the help of the used focal length and aperture.

Medium telephoto focal length

tulip and bokeh focal-length 85mm

f/2, 1⁄640 sec., iso 100, FL 127mm eqv., Nikon d7000, Lens Nikon 85mm f/1.8 Srinagar, Jammu & Kashmir, India 2018

Note: Aperture is also very closely related with depth-of-field. The page aperture and depth-of-field is a must read to understand the contribution of aperture towards the depth-of-field of a photograph.

Normal focal length

normal focal-length at 52mm

f/6.3, 1⁄800 sec., iso 320, FL 52mm eqv., Nikon d810, Lens Tamron 24-70mm f/2.8, Varanasi, UP, India 2015

A 35mm to 50mm focal length is said to be standard/normal focal length. Such a focal-length can record images similar to what human-eyes can see. This range is also a good match for street photography.

Focal-length and aperture are the two principal characteristics of a lens. Before buying a lens, we must ask ourselves: which one, prime lens? or zoom lens? What should be the focal-length? what will be the maximum aperture? constant or variable aperture?

The other properties of a lens are selected then, such as : auto-focus or manual, VR (IS/OS) or non-VR, filter-size, number of aperture-blades, size, weight, price etc.

Prime and zoom lenses

A lens is a piece of glass (or a group of glass) that converges light to the focal point to form an image on the film or digital sensor.

It is said that if camera is the brain, lens is the eye. A lens sits in front of the camera sensor or film; it controls the quantity of light allowed to the sensor through aperture. A lens may be fixed to the camera; so that it cannot be changed or detached. Cameras like Canon g7x, Sony Rx-100 and Nikon P1000 etc. have fixed lenses.

a 70-300mm focal-length lens

A Nikkor 70-300mm zoom lens

Most of the DSLRs and mirrorless cameras have the facility to change its lens. The same camera can be mounted with different lenses as per our requirement. Those are called ILC (Interchangeable Lens Cameras). Those cameras can be mounted with large variety of lenses.

There are two types of lens, prime lens and zoom lens. A prime lens has a fixed focal-length. So, there is no option for zoom-in or zoom-out. With such a lens, we have to move our camera forward or backward to get zoom-in or zoom-out effect. Even we may have to move ourselves forward or backward by our feet (called zoom-by-feet). They are less complex in the internal construction, small, light and less costly. By the way, telephoto and fast primes are not cheap.

On the other hand, a zoom lens has different focal lengths. An 18-55 lens has a widest focal-length of 18mm, and longest of 55mm. Any focal-length inbetween may be used for shooting. Say, at 18mm, we shoot a landscape scene; we zoom in to range 30 to 35mm, and take a street shot. Again, by zooming in to 55mm, we take a portrait shot. With an 18-200mm zoom lens, we dont have to frequently interchange lenses, as compared to a prime lens.

Here is an animation of the behaviour of a 35mm 1.4 prime lens with different apertures. Check that the focal-length (sensor to lens distance) is fixed, because it is a prime lens. For simplicity, the little movements of the lens(group) due to focusing are not shown here.

A 35mm f/1.4 PRIME lens sensor lens 35mm

Note: The internal structure of a lens is not so simple as shown above.

On the body of a Nikon 85mm f/1.8 prime lens, the following text is written :

nikon 85mm focal-length lens

85mm refers to the fixed focal length of 85 millimeter. The 1 : 1.8 part indicates that the lens has a largest aperture of f/1.8. This lens allows to shoot with all the apertures in the range of f/1.8 to f/16 at 85mm. Any 85mm lens with largest aperture of f/1.4 or even f/1.8 is most suitable for portrait photography. When a lens has very large aperture, it is also called a fast lens. The smallest aperture-value(f/16 in this lens) is generally not mentioned on any lens-body. The "AF-S" part indicates that the lens has its own auto focus capabilities, and the "G" part indicates that this lens has no aperture ring.

In case of a zoom lens, things get a bit complicated. The following text is found on the body of a Nikon 18-55 lens :

nikon 18-55mm focal-length lens

The "DX" part indicates that this lens is meant for APSC sensor or crop-sensor cameras. Those sensors have a dimension of 23.5 x 15.5 mm. By 18-55mm we understand that the lens has variable focal lenths. By rotating the zoom-ring, we can zoom in and shoot at 55mm (longest FL for this lens, generally used for portraits), and by zooming out we can shoot at 18mm(widest FL, generally used for landscapes, cityscapes, astro photography). The entire range of 18mm to 55mm can be used to shoot by zooming in and out.

The "1:3.5-5.6" indicates that this lens has largest aperture of f/3.5. That means you can never shoot at f/3, f/2.8, f/1.8 apertures with this lens. The limitation does not end here. The maximum aperture of f/3.5 is available at 18mm only. When you zoom in to 55mm, you will be allowed a maximum aperture of f/5.6, not f/3.5.

Here is the behaviour of the aperture of an 18-55mm 3.5 to 5.6 DX lens at different focal lengths. Set focal length to 18mm, and aperture to f/3.5. But when you zoom in to 24mm, the aperture will automatically stopped down to f/4.

An 18-55mm f/3.5-f/5.6 DX Zoom lens sensor aperture 18mm Largest aperture at is

Note: The internal structure of a zoom lens is even more complex than prime lenses.

Same is applied on minimum aperture. At 18mm, you will be allowed a minimum aperture of f/22; and at 55mm, you will get a minimum aperture of f/36.

Zoom lenses may have variable apertures or a constant aperture. The above-mentioned 18-55mm lens has variable apertures. A 24-85mm f/3.5-4.5 lens also has variable apertures. At 24mm the largest aperture f/3.5 can be used; but at longer 85mm, largest aperture is f/4.5.

But in case of a 24-70mm f/2.8 lens, we get a constant largest aperture of f/2.8 throughout the zoom range. Constant aperture lenses are larger and heavier and can allow more light. They are costly too.

Choosing an appropriate lens is a vital decision. Lenses determine the actual image quality of a photograph. New cameras are always emerging into the market, but a costly lens produces good image quality and may serve for a long period of time.

learnasyoulike.com Learn Visually

Want to leave a message for me?