Focal length can be a strange concept when your first start out in photography. Those seemingly arbitrary numbers labeled on a lens, usually in millimeters. However, once you learn the principles behind focal length, the information can teach you on a number of levels from how wide-angle or zoomed in your image will look to shutter speed and what the effects of each focal lengths are on an image.
What Is Focal Length?
From a technical standpoint, focal length is the distance from the point where light converges in a lens to the image sensor or film plane. This affects how wide-angle or zoomed the subject looks.
The smaller the number in millimeters means a shorter the focal length, and larger the number in millimeters means a longer the focal length, For example, at 24mm, an image looks wide-angle and less zoomed in, while at 200mm, the image looks more zoomed in. Wide-angle = smaller numbers, zoomed in = bigger numbers.
As the numbers change, the angle of view changes. That is why this focal length is always quoted with a lens. For example, an 8mm lens or a fisheye lens can have a nearly 180-degree angle of view, while in a 400mm lens, the viewing angle is much more narrow.
Effects of Focal Length
As with other camera settings, focal length has an impact on your image and other settings. Generally, you don’t want your shutter speed to be faster than your focal length. This goes out the window with modern image stabilization systems which can achieve really low shutter speeds, but it’s still a good rule to follow.
Focal length also determines the look of your image. Wide-angle lenses can give an almost panoramic view, but close-up to a subject and they can make things look bulbous. At the other end of the spectrum, long, telephoto lenses don’t just have a narrow angle of view, but they also flatten out an image. This is most important for portraits as you can see in the image at the top of page.
After some practice, you’ll start to know what each focal length is good for. 85mm is an ideal focal length for portraits, while 24mm can be good for landscapes, and 200mm and above for sports events.
Crop factor really throws a spanner in the works for focal length. Take the common 50mm lens. On a full frame camera (usually high-end cameras) the focal length is 50mm as the sensor crop factor is 1:1. No worries there, as it does what it says on the tin.
On a crop sensor camera, the sensor sizes are smaller which means the crop factor can be anything from 1.5x – 2x. This means that the 50mm lens on a 1.6x crop factor camera body has a focal length of 80mm, making images look more zoomed in.
This aspect is important as each focal length gives a different look to an image, which means if you own a crop sensor camera body, you will always have to factor in the end result focal length.
Prime vs. Zoom Lens
In the two camps of camera lenses, there are prime lenses and zoom lenses. Primes have a fixed focal length while zoom lenses have variable focal lengths. Zoom lenses are more versatile as there are lots of focal lengths to choose from, generalizing wide-angle to telephoto. Prime lenses are usually smaller, weigh less, and can offer a bigger aperture. In the past, primes were renowned for being better quality than zooms, but that gap is getting ever more narrow.
On a full frame camera, this is 10-24mm. Wide-angle means lots of coverage which means this focal length range is good for landscapes, interiors, and group shots. You can also use slow shutter speeds, so they are arguably the best in low light conditions.
On a full frame camera, standard lenses are found in the 50-60mm range. Standard lenses are named so as they are roughly the same angle of view as the human eye sees. Because they are a happy medium between wide-angle and telephoto, they can be used for lots of applications from portraits to landscapes. They usually also have a large aperture, which means they are good in low light and can produce a very pleasing background blur, or bokeh.
Many photographers have a 50mm prime lens in their kit, even if it’s a fallback lens for a regular view of the world.
On a full frame camera, telephoto lenses fall in the 70-200mm range. At these focal lengths, images will look zoomed in with close crops, and the may look flatter. These focal lengths are good for portraits or shooting at a distance.
Super Telephoto Lenses
On a full frame camera, lenses with a focal length of 300-600mm are super telephoto lenses. These are best for very distant shooting and also have the narrowest view. Used a lot for sports and wildlife photography when you need to keep your distance. Also very pricey lenses.
Just to complicate matters, a variety of focal lengths can be used for close-up or macro photography. If you want to capture the smallest things in the world at good quality, you need a lens that has a macro mode and shoots at a 1:1 reproduction, basically life-size. A lens will specifically say if it is capable of macro photography and if it is capable of reproducing a 1:1 ratio.
When learning about focal lengths, practice is key. Experiments with the smallest millimeter, wide-angle lenses to the largest millimeter telephoto lenses. Review how focal lengths impact an image at each number and you will quickly learn what each one is good for.