Photography News Issue 41

Photography News | Issue 41 |

Technique 21

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Maximumaperture This tells you howwide the aperture will open. On many zooms, the maximum aperture will change across the focal length. Focus distancewindow Here you’ll be able to read the focus distance in feet and metres, between infinity and the lens’s closest focusing distance.

Focal length If the lens is a zoom, several focal lengths will be marked between the two extremes. If it’s a prime, the lens is restricted to a single focal length and you’ll need to move your feet.

Focus ring The focus ring is used to manually focus the lens, and also, in the right AF mode, to make minor corrections to autofocus.

Lens hood Using a lens hood cuts the risk of lens flare, which reduces contrast and leaves artefacts on the image. It can also protect the front element from harm.

Front element The business end is best protected by a suitable protection or UV filter so that it doesn’t get scratched or knocked. Some lenses also feature water and oil-repellent coatings to make cleaning easier.

Zoomring If the lens is a zoom rather than a prime, the zoom ring is used to set the focal length from the widest to the longest settings the lens allows. These correlate to the field of view.

Controls Switches will include

Filter thread The filter or accessory thread size will vary frommodel to model. You need to know this if you’re planning to use filters or a filter holder on the lens.

Lensmount Metal mounts show better build quality and longevity. If you’re likely to be using the lens outdoors a lot, look for weather sealing, too.

features like picking the AF mode, limiting the focusing range and activating image stabilisation, as well as the type of stabilisation used.

Tripod collar For greater stability when shooting, the tripod collar lets you mount your glass on a monopod or tripod.

Cropped or full-frame? Focal length ismuddied a littlewhen sensor size is taken into account. Lens names are standardised to include the focal lengths they cover, and these suggest the field of view, but if you’re using a camera with a sensor that’s smaller than full- frame (roughly 36x24mm), you’ll need to apply a crop factor. A typical APS-C sensor, as found in many DSLRs has a crop factor of 1.5x. So, a 70-200mm lens becomes effectively an 105-300mm. Similarly, a 50mm can become a useful portrait lens, hitting 75mm. Other popular crop factors are 1.6x (Canon) and 2x (Micro Four Thirds, ie. Olympus). Steady as she goes The majority of telephotos come with image stabilisation, which is a huge benefit. Longer focal lengths exaggerate camera shake and while you can improve results by using a tripod or monopod, image stabilisation is the perfect aid when shooting handheld. Lens makers have unique names for image stabilisation – Sigma calls its OS or Optical Stablizer. So check the lens has it and how many stops of shutter speed it compensates for. A 4EV benefit is common and in theory, this means shooting at 1/15sec is eqiuivalent to shooting at 1/500sec. Of course there are many variables that impact on this.

Primes on the other hand tend to offer greater image quality (though of course this isn’t always the case) because of their simplified construction, which often allows wider apertures to be used. Fast or slow? On some, usually more expensive zooms, you can use the same aperture at all focal lengths. Where the maximum aperture is f/2.8 or wider, these are called ‘fast’ lenses. On variable-aperture zooms, as you extend the focal length, the widest apertures will become unavailable. Zooms with a constant aperture, like f/2.8 or f/4 will be larger and heavier than those with a variable aperture, but you get more control over depth-of-field, and more efficient focusing. This applies even when shooting with small apertures because more light is coming through the lens. Of course, that’s not to say that you can’t get great shots with a variable zoom though. Whether you need a zoom with a constant maximum aperture depends on the depth-of-field you want to work with, and what kind of light you’ll be working in. The wider the aperture, the easier you’ll find it to blur backgrounds and use faster shutter speeds. Faster speeds mean it’s easier to freezemovement, which is important for sports, wildlife or working in low light.

The focus distance is important and dictates how large you can reproduce the subject in the frame compared to a lens with the same focal length, so if closeups are vital to you look to the lenses with the shortest distance. Focusing speed Focusing speed is improved with the latest focusing motors. Sigma, for example, use HSM or Hyper Sonic Motors so you get fast AF that is also effectvely silent. Speed is especially important if you’re shooting moving subjects. Designs with internal focusing and zooming are an advantage, too. In these, the lens size is constant or the front element doesn’t rotate. You might skip over the lens’s filter size, but checking it could save you money; if it’s the same as filters you own they can be reused on your new lens. Higher quality, fast lenses will tend to have large front elements, though, so it’s difficult to avoid splashing out as you’ll be looking at 77mm or 82mm filters. Lenses first Finally, always invest in the best quality lens you can afford: you’re more likely to change your camera body over time than lenses, so that’s where your money should be prioritised, and, if well treated, will serve you well for many years. lens’s minimum

Your choice There’s loads of spec to look for when choosing a lens, but things like image stabilisation (here, on a Sigma 150-600mm lens it’s OS or Optical Stabilizer), can be vital for successful telephoto shots.

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