CAMERA CLUB: Looking at life through the lens

The camera lens focal length is measured in millimetres. For practical purposes, it indicates how much the lens magnifies or reduces a subject. It also illustrates how narrow or wide its field of view compared with other lenses.

By Ivor Rackham
Thursday, 23rd April 2020, 12:00 am
A telephoto lens reduced perspective, bringing distant backgrounds closer to the subject.
A telephoto lens reduced perspective, bringing distant backgrounds closer to the subject.

A standard focal length lens does not magnify an image; if you look through the camera’s viewfinder, objects look the same size as in real life. A standard lens on a 35mm (full frame) camera is 50mm. However, most digital cameras on the market have sensors smaller than that which crop the image being projected by the lens, thus narrowing the angle of view, and effectively magnifying the image. These are called crop sensor cameras.

Nikon, Pentax, and Sony APS-C sensors crop 1.5 times, so a 33.3mm lens is standard (50 / /1.6 =33.3). The smaller Canon sensors crop 1.6 times, therefore a standard lens is 31.25mm. Meanwhile Olympus and other Micro Four Thirds cameras crop by 2 times so a 25mm lens is standard. Standard lenses are great for full body shots and large pet portraits.

Any lens length less than standard is a wide-angle. Sometimes called short lenses, their shorter focal length reduces the size of the subject. Great for landscapes, they emphasize foreground interest and their wide-angle view with plenty of depth of field increase perspective. This makes them not so good for portraits. Take a close-up selfie with a wide-angle lens and see what I mean. That technique produces cute pictures of kittens and puppies with huge noses, but for us humans the look is not so flattering.

Sign up to our daily newsletter

Sometimes also called long lenses, those with focal lengths longer than standard are usually called telephoto lenses. The prefix tele is Greek, meaning far. A 100mm lens’s angle of view is half that of a 50mm lens, and consequently subject is magnified by a factor of two

compared to a 50mm. A 300mm lens magnifies 6 times (300 / 50 = 6).

Why the photographic industry didn’t standardise the nomenclature so long lenses are called narrow angle instead, is a mystery. Alternatively, short lenses could have been called “kontaphoto”, konta being Greek for near. Alas, it wasn’t to be.

With their shallow depth of field and reduced perspective, telephotos are used when you want to isolate the subject. Lenses double the length of standard are great for head and shoulders portraits. Wildlife and sports photographers use even longer lenses to fill the frame with the subject but must overcome the subject’s distance.

Prime lenses have one fixed focal length while zoom lenses cover a range. Image quality of primes outperform zooms, but they are less versatile. Zooms have a ‘sweet spot’, a focal length and aperture combination that has better sharpness than at other settings. Although noticeable in cheaper lenses, with professional grade zooms the difference is minimal. Super zooms cover a long range of focal lengths. They will not perform as well as zooms with a shorter range, especially at their extreme settings.

The f/ number of a lens is the focal length divided by the aperture. The smaller the f/number, the bigger the aperture and the more light it gathers, so the faster the lens. With a long lens, a relatively large aperture is required to achieve a lower f/ number. A 300mm f/2

lens needs an aperture of 150 mm. That would be both large and expensive to build.

Therefore, cheaper zoom lenses have higher f/ numbers, which results in slower shutter values.

Macro lenses are close-up lenses, great for filling the frame with small objects.

Interesting results are achieved using lenses for unintended purposes. Give it a go!