Back to basics: lenses in photography
A lens is your camera’s ‘window on the world’ and is arguably the most important item in your kit bag when it comes to producing the highest quality images.
Post author: Chris Gatcum
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Buy the best lens you can afford?
There are countless pieces of homespun advice that get handed down from veteran photographers to novice snappers. While some of this guidance can be decidedly spurious, there are also some fantastic gems that get passed across the generations. Foremost among these is: ‘your lens is more important than your camera’, ‘buy the best lenses you can afford’ or any one of the similar sentiments that emphasise the importance of your optics.
The reason this advice is so sound is because the lens on the front of your camera is perhaps the most fundamental part of the imaging chain. You can have the best camera body in the world, but if you stick a low-quality lens on it the camera will only ever function at the level of the lens, whereas a high-quality lens will allow you to get the best out of any camera. But what exactly should you be looking for in a lens?
Construction
Glass
It goes without saying that the glass that is used to construct the elements in a lens plays a huge part in image quality. To shows what their lenses are capable of, manufacturers produce MTF (Modulation Transfer Function) charts for each lens they make. These are graphs that show the contrast and resolution of a lens at both the centre and edges of the frame, and at various aperture settings. Or, to put it simply, how ‘good’ the lens is.
An MTF chart not only tells you if the performance starts to tail off at a certain aperture setting, enabling you to avoid these settings whenever possible, but as MTF charts are created independently of any specific camera, they also provide a universal means of comparison across different lenses and different brands. In this way you can use an MTF chart to compare a lens from your camera manufacturer to lenses from an independent lens maker.
Lenses are constructed from multiple glass elements each carefully shaped and with slightly different characteristics that are designed to optimise the quality of the image.
Stabilisation
Image stabilisation is designed to reduce (or remove) the risk of camera shake and blurred images, particularly when you shoot handheld. This can be a great help in low-light shooting situations, as it means you might not need to set a wide aperture or high ISO, or think about using a tripod or flash.
There are two types of image stabilisation: sensor-based and lens-based. Sensor-based stabilisation is built into the camera, which has the advantage of working with any lens you use. While this is convenient, it isn’t found in every camera body, so changing your camera can mean you lose your image stabilisation.
By comparison, lens-based image stabilisation is built into the lens, so it gives you the same benefits regardless of the camera you attach the lens to. This can be useful if you use more than one camera body and switch your lenses between them, or if you upgrade your camera. Moreover, lens-based stabilisation can be tailored to the specific lens that it’s in, so the anti-shake technology can be optimised for the focal length. In theory this gives lens-based stabilisation a slight edge in terms of its effectiveness, although in use there is little if anything to differentiate between the two stabilisation technologies.
Different manufacturers use different names for their stabilisation technologies (e.g. “image stabilisation” or “vibration reduction”), but they are all designed to do the same thing: help prevent camera shake so your photographs are sharp.
Weatherproofing
Weatherproofed lenses use a variety of O-rings and seals in their construction to better protect them from the ingress of water, dust and other debris. Not all manufacturers produce lenses that are weatherproofed (and those that do tend offer quite a limited range of focal lengths), but if you do a lot of outdoor photography – particularly around coastal areas or in wet or dusty conditions – a weatherproofed lens can help you avoid expensive repair bills.
A weatherproofed lens provides an additional layer of protection in adverse conditions, although you may find there’s a limited range of lenses on offer for your particular camera.
Aperture
Fast lenses: The wider the maximum aperture of a lens, the ‘faster’ it is said to be, so a zoom lens with a maximum aperture of f/2.8 is ‘faster’ than a zoom that only opens up to f/4 or f/5.6. Fast lenses have two main advantages: they let you shoot in lower light conditions without having to use a slower shutter speed or higher ISO, and they can deliver a more limited depth of field for a ‘shallow focus’ look.
A fast lens is ideal for shooting in low-light situations, such as indoors, and for achieving a really shallow depth of field that focuses the viewer’s attention on a specific part of the image.
Constant maximum aperture: With most entry level zoom lenses, the maximum aperture gets smaller as you zoom in, so you might be able to set the aperture to f/4 at the widest focal length setting, but only set f/5.6 (or smaller) at the longest focal length. However, premium lenses typically have a maximum aperture setting that remains constant throughout the focal length range, which gives you the same low-light shooting capabilities and shallow depth of field options regardless of the zoom setting.
The maximum aperture of a lens is usually shown on the lens barrel. This consumer level 18-105mm zoom has a variable maximum aperture ranging from f/3.5 at the wide end of the zoom to f/5.6 at the longest focal length; a pro-spec lens covering a similar focal length range would most likely have a constant maximum aperture of f/2.8.
Aperture blades: The number of aperture blades used to create the opening (or ‘iris’) in the lens might not be a major consideration for most people, but it can have a significant impact on the look of your images. Specifically, the number of aperture blades determines the shape of the iris, with more blades producing a rounder iris. In turn, this determines the shape of any out-of-focus points in an image, which is often referred to as ‘bokeh’: more aperture blades produce rounder and smoother bokeh.
The shape of the aperture in a lens is determined by the number of blades that create it. More blades mean a more circular aperture, which determines the shape of out-of-focus highlights (or ‘bokeh’) in an image.
Focus
Focus motor
For autofocus to function, it needs a motor that physically moves elements within the lens to bring things into focus. This motor can either be inside the camera body or in the lens, with lens-based motors falling into one of two categories: those that are fast and silent, and those that are not. Generally speaking, fast, silent motors are most prevalent in lenses aimed at professional users and/or lenses that tend to be used in speed-sensitive settings (i.e. long telephoto lenses that lend themselves to photographing fast-moving sports and wildlife subjects).
Internal focusing
A lens that focuses internally retains its overall length, regardless of the distance it is focused at. Conversely, a lens that doesn’t focus internally will ‘telescope’ as you focus, growing to maybe twice its length as you focus on a particularly near or far subject.
With a lot of subjects this isn’t necessarily a problem, but it isn’t ideal if you like to shoot macro and close-up. For a start, if you’re already close to a flighty subject, such as a butterfly or dragonfly, your rapidly extending lens could be enough to make it take off. The extension can also cause exposure issues, as it effectively reduces the amount of light passing through the lens, requiring a wider aperture, slower shutter speed or higher ISO setting to compensate. An internal focus (IF) lens avoids both of these issues.
A long lens that ‘telescopes’ as it focuses physically changes the handling characteristics of the lens, potentially making it more unwieldy and increasing the risk of cameras shake. This is not a problem with an IF lens, as the lens retains its length, regardless of focus distance.
Manual focus
It might not be a major consideration for a lot of photographers a lot of the time, but focusing manually can prove beneficial at times. For example, in low-light or low-contrast conditions your camera might struggle to get a lock using autofocus, or it might not be quick enough to focus on a fast-moving subject, meaning you may want to pre-focus manually instead. Manual focus can prove useful for close-up and off-centre subjects as well.
Most autofocus lenses will allow you to switch to manual focus, but it’s how they focus manually that’s important. Some lenses have a very narrow manual focus ring, with a loose feel and minimal ‘throw’ (the distance the focus ring turns to take you from the nearest to furthest focus distance), none of which is conducive to focusing manually. Conversely, lenses that are better suited to manual focus will have a deeper focusing ring with a little bit of resistance (not too much, though!) and a longer throw, which makes it easier to focus accurately.
It’s easy for a camera’s autofocus system to get confused by a scene like this, due to the layers in the shot. But switch to manual focus and that confusion ends – you can quickly and accurately determine the focus point.
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