Understanding focal length

Understanding focal length

Last updated on August 8th, 2018 at 03:01 pm

In practical terms, the focal length of a lens determines its magnification power, so a longer focal length will magnify the subject more compared with a shorter focal length at the same distance from the subject.

Focal length also governs the angle-of- view of the lens, in other words how much of the scene will be recorded. The two values are inversely proportional, so as focal length increases the angle-of-view narrows and vice versa.

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Choose the right lens for your camera focal length

A lens with a focal length comparable to the diagonal length of the sensor is often known as a standard or normal lens. This is because its angle-of-view is similar to that of the human eye.

On a full-frame (24x36mm), the diagonal of the frame is 43mm, hence a standard lens for this format usually has a focal length of 50mm. Due to the smaller size of the sensors used in APS-C format cameras, a focal length of 35mm is considered standard.

For any given format a lens with a focal length shorter than the ‘standard’ focal length is usually known as a wide-angle lens, while a lens with a greater focal length is said to be a long-focus, or long focal length lens.

These days the term telephoto is often used to describe all long-focus lenses. A telephoto lens is a particular type of long-focus lens that incorporates a special group of lens elements to extend the light path allowing a much short overall design.

A close-up of an elephant eye with an equivalent 200-800mm telephoto lens (Panasonic Leica 100-400mm)

A close-up of an elephant eye with an equivalent 200-800mm telephoto lens (Panasonic Leica 100-400mm)

Keep it sharp

The combination of shutter speed and focal length will limit your ability to attain a sharp picture when shooting with a handheld camera.

A useful guide to the slowest ‘safe’ shutter speed to use in this situation is to take the reciprocal of the focal length of the lens with a full-frame camera, for example, with a 50mm lens set the shutter speed to 1/60sec or faster; with an APS-C format camera multiply the focal length by 1.5x first and then take the reciprocal.

However, one way to break this rule and shoot at slower shutter speeds is to use IS (image stabilisation) if your lenses have the feature.

Image stabilisation works by moving optics within the lens to counteract any movement when it’s detected. Many lens manufacturers suggest that you can shoot at around 3 stops below the recommended focal length of the lens when image stabilisation is activated.

This is true, but you still have to have an extremely steady hand to avoid camera shake, so it’s still best to use a tripod, or monopod, when you can.

Read more tips for a sharp landscape. It will definitely help to improve any future photos

A tripod was definitely needed for this photo, at f/9 and shutter speed of 0.6sec! Alternative solutions are either a lower aperture value, down to F/2.8 and more, or an higher ISO, however it would have been a compromise on the background details, not as sharp as with f/9 and ISO 100

Understanding focal length

Understanding focal length

Prime lenses

By definition, prime lenses have no zoom but rather a fixed focal length, so in many ways they’re less versatile and less convenient than zoom lenses.

Primes tend to be more compact and lightweight compared to zooms, and are often cheaper, but most significantly they usually have a larger maximum aperture (f/1.2 to f/2.8) for getting blurrier backgrounds.

You can buy a prime lens from around AUD100 and you’ll find it will probably offer you better image quality than your kit lens.

If you’re shooting on an APS-C camera, a 35-50mm prime is perfect for portraits, and a 50-85mm is perfect on full-frame cameras.

To get the most out of your prime lens, shoot with a large aperture to keep the background out-of-focus. This is especially useful when shooting portraits, as the shallow depth-of-field helps your subject stand out.

The 50mm lens is still one of my favourite for landscape. It is also a good training lens. Being prime you always need to move and find the right spot, no space for zooming.

The Nikkor 50mm 1:1.8 becomes super handy in the low light environments as the local markets

The Nikkor 50mm 1:1.8 becomes super handy in the low light environments as the local markets

Sometime it is hard to find the right location, as in this photo of Southern Cross Station in Melbourne, Australia, but hey that’s part of the photography fun

Focal length @ 20mm (equivalent 40mm) using a Panasonic Lumix G 20mm F/1.7

Focal length @ 20mm (equivalent 40mm) using a Panasonic Lumix G 20mm F/1.7

Wide-angle lenses

One of the most effective ways to inject some extra creativity into your shots is to use a wide-angle lens. They’re perhaps most commonly used to capture eye-catching landscapes.

If you own an APS-C camera and only have a kit lens, you can zoom out to 18mm, which is the equivalent of around 24mm on a full-frame camera. This will enable you to include some of the foreground that’s near to the camera, giving your landscape images a feeling of depth and helping to anchor the eye.

If you want to produce really wide-angle shots, you’ll need to invest in a lens with a shorter focal length. Ideally this will be as wide as 10mm. This extra width allows you to get lots of foreground interest in your images.

Extreme wide-angle lenses are not only used for landscapes. They’re popular with extreme sports photographers as the huge angle-of-view captures all the action, and they’re great for quirky portraits of pets, children and even bands.

They are also a must for shooting in enclosed spaces like inside buildings, where photographers cannot get far enough back to get everything they want in the frame.

I quite like the wide-angle lenses in my city photography trips. It allows me to include such a wider space than I could otherwise and it gives a great sense of action.

Melbourne-Photo-Blog-12 Apostles_ Australia_ Great Ocean Road_ Sunset_ Victoria-DSC_6490

In the photo above the wide angle lens (Sigma 12-24mm at 12mm) was used to capture the beautiful beach at the 12 apostles


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Jacky The Techy
Photography is taking me everywhere in the world. I love the technical side of it, how to create my photos minimizing the post-production side of it. More time on the field and less time on the computer.
Ten years of experience and counting. Is it enough? Never! I learn new things everyday.

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