What is focal length? Explanation of basic knowledge about angle of view and focal length

Here is some basic knowledge about focal length with interchangeable lenses that you will need to know when shooting with an SLR or mirrorless SLR camera. When shooting with an interchangeable lens camera, you can take beautiful and impressive photos by working with the angle of view, focus point and degree of bokeh. Particularly when choosing a lens, it is important to understand the characteristics of focal length as the angle of view changes depending on focal length. Let’s learn the basic knowledge and make use of it to take the photos as you imagine.

When light enters a lens, the point where the light gathers is called the focal point. While light travels in a straight ray, when it passes through a lens it refracts and is concentrated at a single point so called focal point. When a light passes through a lens and at this focal point it forms an image, when an image sensor is placed at this position, the image can be captured.

A focal length is a distance from the center of the lens (principal point) to the image sensor, and this is an important value that characterizes a lens.

Generally, on a camera lens there is an indication of focal length. For a product name such as “28-75mm F/2.8, ” this “xx mm” indicates the focal length.

There are two types of lenses: prime lenses which have a single focal length, and zoom lenses that have a variable focal length. If the focal length is described with a range, such as “xx-xx mm,” it is a zoom lens. For a prime lens, only a single figure is indicated, such as “xx mm.”

Example of a zoom lens: 28-75mm F/2.8 Di III VXD G2 (Model A063)
Example of a prime lens: 20mm F/2.8 Di III OSD M1:2 (Model F050)

The important point about focal length is that the angle of view changes with its actual number. The angle of view is the angle of area captured by the camera through the lens, and the larger the angel of view, the wider the area is captured.

Lens types can be roughly divided into three categories: wide-angle, standard, and telephoto. A standard lens can capture images at an angle of view close to that of the human field of view. Generally, lenses with a wider angle of view are called wide-angle lenses, and lenses with a narrower angle of view are called telephoto lenses.

As a general rule of thumb for focal lengths, focal length smaller than 35mm are generally classified into wide-angle lens, lenses around 50mm are called standard lens, and lenses larger than 80mm are telephoto lens.

Check out the differences in impression and composition caused by changing the angle of view. Here is an example of a portrait taken with the TAMRON 35-150mm F/2-2.8 Di III VXD (Model A058) at a different focal length.

At the wide-angle end (35 mm), the beautiful impression of the figure standing stands out with the background. At the telephoto end (150 mm), on the other hand, the expression on the person's face is compelling and the background bokeh accentuates this image. In this way, by changing the focal length of a zoom lens, various angles of view can be created, and different impressions can be captured.

Another characteristic of focal length is depth of field. Depth of field refers to the range in which the image looks it is in focus, and a deep depth of field means that the image looks in focus through wide range, such as from a subject to background. And if the depth of field is shallow, only the area around the subject will be in focus and the background will have a smooth bokeh.
The shorter the focal length of the lens and the larger the aperture (F-number), the deeper the depth of field. This is why wide-angle lenses are suitable for landscape photography, as the short focal length increases the depth of field and allows the photographer to capture details in the background clearly.

On the other hand, as telephoto lenses have a long focal length and shallow depth of field, the bokeh in the background of a subject is more pronounced. By utilizing these bokeh effects, you can capture portraits and other images where the subject stands out against the background.

As explained above, focal length is a very important figure, but when choosing a lens you also need to be mindful of the sensor size on your camera body.

With digital SLR cameras, light reaching the image sensor is stored as an electrical charge, and an image is created based on the color information obtained through the color filter.

These image sensors come in mainly two types: 35mm full-frame, and APS-C. The size of a 35mm full-frame sensor is 36mm x 24mm. The larger the sensor size allows a wider area to be captured with greater sensitivity and its rich gradation. On the other hand, the size of an APS-C sensor is a step smaller at 23.5mm x 15.6mm* and captures narrower area.
*APS-C image sensor sizes vary by camera.

Therefore, you will need a lens that is compatible with the sensor size of the camera body. Camera lenses have a specific sensor size that they are compatible with, so choose a lens that is compatible with the camera you have. When APS-C camera is used, you will need to do some calculation to figure out the real coverage.

The angle of view of each focal length depends on the sensor size. Generally, focal lengths and angles of view are based on 35mm full-frame size, so when selecting an APS-C compatible lens, it is easier to understand the angle of view when converted to full-frame size. Such conversion is known as '35mm equivalent'.

In general, the focal length of APS-C lenses can be converted to the focal length of lenses for full-frame cameras by multiplying the focal length by 1.5 (for Sony, Nikon and 1.6 for Canon). The relationship between angle of view and focal length can be mapped as shown in the table below, which you can use as a reference when selecting a lens for APS-C.

In addition to the focal length, the F-number (aperture value) is another important figure that expresses the characteristics of the lens: the smaller the F-number, the more light-gathering power it will have, and will broaden your possibility.

The F-number is calculated as 'focal length ÷ lens effective diameter'. The smaller the 'maximum aperture F value' when the aperture is at its widest, the more sensitive the lens is. When choosing a lens, it is better to mind the F-number as well as the focal length.

Related article：What is F-number (aperture)? A comprehensive explanation including example settings and the relationship between aperture, shutter speed and ISO

Here are TAMRON's recommended lenses by lens type. TAMRON zoom lenses cover a wide range of focal lengths with a single lens yet are of a size that makes them easy to carry. On top of this, they are characterized by their high descriptive performance and close-up shooting ability. We hope you will find the lens that suits you best.

Wide-angle zoom lenses cover the range from ultra wide-angle to standard range. Ultra wide-angle lenses allow you to get closer to buildings and vehicles and still capture a wide range of subjects, resulting in more dynamic shots. It also allows for unique portrait and pet photography, making the most of the perspective. Lightweight and compact TAMRON lenses are particularly useful when shooting landscapes, as the time and distance spent carrying them around can be long.

A standard zoom lens covers wide-angle to telephoto focal lengths around 50mm. In addition to snap shots and portraits, these lenses prove their worth in a wide variety of shooting scenarios such as landscapes at the wide end and shots of animals and buildings at the telephoto end.

Telephoto zoom lenses enable photographers bring the subject closer, making them ideal for events such as sports events, as well as for birding and wildlife. They are also useful in natural landscapes photographed in conjunction with trekking, hiking and mountaineering.
They are also recommended for impressive portrait photography with naturally blurred backgrounds and compression effects.

As ultra-telephoto zoom lenses allow you to enlarge the faraway subjects, they are best suited for shooting subjects you cannot get close to, such as wildlife, sporting events held at large venues, aircraft and trains. By combining the compression effects and bokeh that telephoto zooms are known for, you can also compose more appealing images.

The focal length of a lens is defined as the distance from the center of the lens (principal point) to the image sensor of the camera. Focal length is an important figure that determines the angle of view. A lens with a short focal length is a wide-angle lens, a lens with a long focal length is a telephoto lens and a standard lens is somewhere in between. When choosing a lens, pay attention to the focal length and select a lens that suits the subject or scene you want to photograph.