Buying a DSLR camera with one or more ‘kit’ lenses is an affordable way to start out your photographic adventure. However, the physical and optical designs of most of these lenses involve certain compromises to reduce weight and bulk and maintain affordability. Consequently, once the initial learning period is over, most serious photographers look at adding new lenses to their kit and/or replacing the kit lenses.
The quality of the camera’s lens is as important as the size and resolution of the sensor. It’s also important to match the lens to both the camera body and the types of pictures you want to take. Several factors should be taken into account when selecting lenses, the first being whether to purchase a dedicated digital lens or a lens designed for a 35mm camera.
One great advantage of owning a DSLR camera is the huge range of lenses available to expand your photographic capabilities.
Understanding Lens Jargon
All lenses are specified by focal length in millimetres and maximum aperture. The aperture of a lens is the opening that allows light to pass through the lens to the image sensor. The size of the aperture is controlled by an iris diaphragm (which works in much the same way as the pupil in your eye). The ratio between the diameter of this aperture and the focal length of the lens is given in an f/number, with the maximum aperture representing the widest the iris can be opened for a particular focal length.
Aperture settings are adjustable in many cameras, with most lenses offering at least some of the following settings: f/1.4, f/2.0, f/2.8, f/4, f/5.6, f/8, f/11, f/16 and f/22. The smallest number represents the largest lens aperture. Lenses with a maximum aperture of between f/1.4 and f/2.8 allow much more light to pass through them than lenses whose maximum aperture is between f/4 and f/5.6. Photographers refer to the latter as being ‘slower’. Because an f/1.4 lens admits four times more light than an f/2.8 lens, it is four times ‘faster’.
The aperture diaphragm in a lens opens and closes (‘stops down’) to control the amount of light reaching the image sensor. The aperture is stopped down in the top illustration, while the lower illustration shows the aperture wide open.
Manufacturers command a premium price for fast lenses because they cost more to make. They are also larger and heavier than slower lenses. The main advantages of faster lenses are greater flexibility for low-light shooting and more control over depth-of-field (how much of the subject is sharply focused).
Fast lenses allow photographers to focus on a narrow plane in the subject, leaving both the background and foreground unsharp. With slower lenses, the plane of focus is wider, making it more difficult to isolate the subject.
Zoom lenses are specified with the focal length range followed by the maximum aperture range. A 28-105mm f/4-5.6 lens, for example, has its widest field of view and maximum aperture at 28mm and f/4 and zooms to a focal length of 105mm with a maximum aperture of f/5.6. (It is common for the maximum apertures of zoom lenses to become smaller as focal length increases.)
Fast telephoto lenses make it easier to shoot with a narrow depth-of-field to de-focus potentially distracting backgrounds. Taken with an 85mm lens at f/2. (Photograph supplied by Canon.)
Some zoom lenses can maintain a constant maximum aperture throughout their zoom range. A 70-200mm f/2.8 lens will allow photographers to shoot with a maximum aperture of f/2.8 all the way up to the full 200mm zoom extension. Lenses in this category are always bigger, bulkier and more expensive than those with similar focal length ranges but varying maximum apertures. However they are usually better performers.
35mm vs Digital Lenses
Dedicated digital (or ‘digitally-integrated’) lenses are designed for imaging onto smaller sensors and can’t be used with 35mm camera bodies – or DSLRs with ‘full-frame’ (i.e. 36 x 24mm) sensors. They can usually be identified by the letter ‘D’ or ‘Di’, although Canon uses ‘S’ (as in EF-S) as its identifier.
A good digital lens should provide marginally better performance on a DSLR body with an ‘APS-C’ sized sensor than an equivalent 35mm lens – although, in some cases, it’s difficult to see much difference in actual photographs. For a ‘full-frame’ DSLR, regular 35mm lenses are ideal. Take account of the lens crop factor/LMF (see Chapter 2) when selecting lenses for cameras with ‘APS -C’ sensors. Most manufacturers include the equivalent 35mm focal length in specifications for dedicated digital lenses but you’ll need to calculate the LMF for 35mm lenses that are used on these cameras. For Canon cameras, the LMF is 1.6x; for all Nikon, Pentax, Samsung and Sony DSLRs it’s 1.5x, while Olympus and Panasonic DSLRs have an LMF of 2x (but can’t use 35mm lenses).
Crop factor/multipliers are a mixed blessing. Long lenses will zoom further but wide angle lenses will lose part of their ability to capture a full scene.
Prime or Zoom Lenses
Which is better: a ‘prime’ lens that covers only one focal length or a zoom lens that ranges across many? Prime lenses are usually two or more f-stops faster than zooms. Their design tends to be simpler and the lens can be optimised to deliver outstanding quality at its designated focal length, usually across a wide range of aperture settings.
If you want top image quality, a prime lens is more likely to provide it than a zoom. For this reason, professional photographers are more likely to choose prime lenses – despite their relatively high price tags. Much of the cost of a prime lens is due to the high-quality glass used in the lens elements, the high precision of manufacture and the wide maximum apertures many prime lenses provide.
This professional-quality 400mm prime lens has a maximum aperture of f/2.8, which is very fast for its focal length. The large glass elements required add bulk and weight to the overall construction.
However, you will need a range of prime lenses to cover the focal lengths covered by even a conservative zoom – and this means a heavier camera bag to carry and higher up-front cost.
Zoom lenses are popular for their convenience and cost-effectiveness but are usually slower than prime lenses and also much more difficult to design. Many compromises must be made to cover even a moderate zoom range and zoom lenses often deliver lower image sharpness and contrast than prime lenses. Colour reproduction can also suffer. The longer the zoom range, the more compromises are required and the greater the loss of image resolution, sharpness and contrast.
Many photographers will focus their lens choices on buying one high-quality prime lens that will cover most of their shooting requirements and adding short zoom lenses to expand their shooting capabilities on either side (wide or ‘tele’) of the focal length of the prime lens. Shorter range zooms have fewer faults than long zooms.
For travellers, where size and weight must be minimised, a good twin lens choice is a wide standard lens (20mm or 24mm) plus a moderately wide zoom (28-105mm or 28 -200mm for example). Many travellers prefer the convenience of a single, extended range zoom lens covering focal lengths from 28mm to 250mm or 300mm, despite knowing that the longer the zoom range, the more imaging performance will be compromised.
Special Purpose Lenses
Tilt/shift lenses have been designed to allow photographers to correct distortions related to perspective. When you photograph a tall building, the top stories appear to taper in when they are photographed with a normal or wide angle lens. In most tilt/shift lenses the range of movement is limited to +/- 8 degrees of tilt and +/- 11 degrees of shift, which is adequate for the majority of situations.
Tilt/shift lenses have adjustments that allow photographers to chenge the relationship of focal plane to the subject in order to prevent rectilinear distortions.
With a tilt/shift lens, you set the camera so its focal plane is parallel to the nearest wall of the building. Using the tilt adjustment on the lens, the front of the lens is tilted upwards to take in the top of the building, while the camera stays in position. You can keep the shape of the building rectangular and minimise the tapering at the top of the building.
The picture at the top was taken with a normal lens. Note the converging vertical lines in the building. The picture above shows the effect that can be obtained with a tilt/shift lens, which allows the vertical lines to appear vertical.
The horizontal shift control on a tilt/shift lens can be used to shoot panoramas by dividing the scene into several shots. When the shots are stitched together, the distortion at the edges of sequential shots is minimised and stitching is faster, easier and more effective. Shifting can also be useful for preventing the photographer’s reflection from appearing in pictures with large glass windows or mirrors.
Fish-eye lenses have ultra-wide angles of view and produce photographs with interesting distortions. They also allow very close focusing. A typical 15mm fish eye lens covers 180 degrees, which means that almost everything that is in front of the camera can be recorded, including the sky above, the ground below and surrounding objects to left and right.
While the very centre of the field is relatively undistorted, everything around it is distorted to some degree. Straight lines bulge out towards the edges of the frame, creating a strong sense of perspective. Some fish-eye lenses provide a circular field of view by cropping the corners of shots (which are otherwise heavily distorted). Others cover the full frame and capitalise on the corner distortion.
An example of a shot taken with a 15mm fisheye lens at f/5.6. (Photographed by Eugene Tan, www.aquabumps.com.)
Teleconverter and Extender Lenses
These lenses fit onto the back of a normal lens and extend its focal length by a specific factor, usually 1.5x or 2x. They’re an affordable way to achieve a longer focal length with a tele lens or convert a standard 50mm into a portrait lens. However, they reduce the effective maximum aperture of the lens by the same factor as the teleconverter. For example, fitting a 1.5x teleconverter to a 200mm f/2.8 lens will extend its focal length to 300mm but reduce its maximum aperture to f/4.
On the plus side, the close focusing distance of the original lens is retained, although image magnification is increased. This can be convenient for macro photography. Teleconverters are most suitable for focal lengths of 50mm and longer and the slight loss of performance they incur can be useful for portraiture as it can give a softer look.
In recent years, lens manufacturers have introduced some handy new technologies, some of which are noted by additions to the name of the lens. It’s useful to understand what these features are and how they work.
Image Stabilisation is commonly indicated by the letters ‘IS’, ‘VR’ (for Vibration Reduction) or ‘OS’ for (Optical Stabilisation).
Most systems are based on two built-in gyro sensors, which detect movement in the horizontal and vertical planes and counteract it by shifting a group of internal elements with respect to the optical axis of the lens. Many lenses have two IS settings, one for photographing stationary subjects and the other for moving subjects.
Image stabilisation is particularly useful for close-ups, when even slight camera shake can produce blurring. The stabilisation system in some IS lenses must be switched off when you mount the camera on a tripod; other lenses detect tripod mounting automatically. (Note: image stabilisation is also being built into some DSLR bodies, eliminating the need for stabilised lenses. In-the-lens systems are slightly more effective.)
The illustrations above show what a difference an image stabiliser can make, particularly with a telephoto lens. (The top image is from an unstabilised lens.)
Lens-incorporated Motors help to achieve fast, accurate and quiet autofocusing. In Canon’s lenses the presence of this technology is denoted by ‘USM’ in the lens name. Nikon’s technology is known as SilentWave Motor but no indication of its presence is given in the lens name. Sigma uses Hyper-Sonic Motor (‘HSM’) as its designation.
Internal Focusing, denoted by ‘IF’ indicates that the lens is focused by moving internal elements. This means the lens stays the same length and its barrel does not rotate while focusing occurs, allowing angle-critical accessories like polarisers and graduated filters to be used. Some lenses use Rear Focusing (‘RF’), which moves only the rear lens group, to achieve the same objectives.
Apochromatic (‘APO’) lenses use special, low-dispersion glass to minimise chromatic aberration (the inability of a lens to focus short and long wavelengths – i.e. blue and red light – at the same point). Such lenses can be designated ‘LD’ (Low dispersion), ‘AD’ (Anomalous dispersion), ‘ED’ (Extra-low dispersion) or ‘UD’ (Ultra-low dispersion). These types of glass are particularly popular in telephoto lenses.
Other special glass components can often be identified in lens names. The most common include Aspherical (‘Asph’ or ‘ASL’) elements that cause all light rays passing through the lens to converge at a single point, eliminating spherical aberration. These elements are used to reduce the number of components in the lens system and make it more compact, while also delivering improved optical performance.
Most modern lenses come with external and internal coatings to prevent light from being reflected back from the lens/air boundaries due to differences in refractive indices. These reflections can cause flare and ghosting when backlit subjects are photographed. Effective coatings reduce such reflections to a minimum without affecting colour reproduction.
Coatings on the internal and external surfaces of the glass elements in lenses reduce reflections that can cause flare and ghosting.
Most lenses are supplied with clip-on hoods that shade the front element of the lens and minimise the risk of flare and ghosting. Each hood is matched to a specific lens design. Hoods for telephoto lenses are usually cylindrical, while wide angle lenses have ‘petal-shaped’ hoods. Using a hood ensures that the pictures you take are not compromised by a loss of contrast.
Filter options for DSLR cameras are many and varied as all lenses are threaded to accept screw-on filters. Clip-on filter holders can be used with most lenses, too. The most popular filters for digital photography include polarisers, graduates, soft focus filters and neutral density filters. Skylight and UV filters are unnecessary as all DSLR sensors are protected by a UV-blocking filter.
Polarisers minimise the impact of reflected light rays and are used for emphasising the blue of the sky, subduing reflections from water or glass and improving colour saturation. They are simple to use as they only need to be rotated until the desired effect is seen.
The photograph on the left was taken without a polarising filter, while the one on the right was taken with a polariser. Note how the polariser subdues the reflections from the water and allows more detail to be recorded. It also adds intensity to the colours of the sea and sky.
Graduated filters darken part of the picture – usually the sky. Available in several neutral density levels or in coloured form they are often used for effect but can also help to turn a fairly ordinary shot into one with more impact. (Note: the Gradient filter in editing software can be used to achieve similar effects post-capture.)
Soft focus filters are used mostly in portraiture, where they subdue wrinkles and blemishes in the subject’s skin. Stronger filters can produce ethereal-looking landscape shots.
Neutral density filters reduce the amount of light entering the lens without changing its colour. They are used by photographers who want differential focusing or blurring due to long exposure times.
The top picture was taken without a graduated neutral density filter. The picture above was taken with one. Note the difference in tone of the sky.
Different types of lenses can impart a particular ‘flavour’ to pictures, making them a useful creative tool. Wide-angle lenses tend to ‘spread’ the subject and ultra-wide-angle lenses can introduce some interesting distortions, as shown in the illustration below.
Telephoto lenses, on the other hand, compress perspective and make objects look closer together than they actually are. They also make it easier to shoot with out-of-focus backgrounds, as shown in the illustration below.
An ultra-wide shot taken with a 10mm EF-S lens at f/18.
A telephoto shot taken with a 200mm lens at f/5.6.
Image Stabilisation Options
Camera buyers have two choices when purchasing a camera with image stabilisation. They can look for cameras that use optically-stabilised lenses or choose cameras with the stabilisation built into the camera body. Lens-based stabilisation systems are more effective when it comes to compensating for camera movements, allowing photographers to shoot with shutter speeds three to four f-stops slower than they could with unstabilised lenses.
They are better equipped to counteract camera movements at longer focal lengths. The image you see in the viewfinder is also more stable.
However, when the stabilisation system is in the camera body it works by shifting the image sensor (‘CCD-shift’ technology). Consequently, it will be usable with any lens that is fitted to the camera. CCD-shift stabilisation systems typically provide only two to 3.5 f-stops of exposure advantage.
Most systems have separately-controlled pitch and yaw detectors, which pick up horizontal and vertical movement. Many cameras and lenses provide two IS modes. Mode 1 uses both sets of detectors, while Mode 2, which is used for panning shots, switches off the horizontal detectors.
The diagram above shows the stabiliser components in an optically-stabilised lens.
The following websites provide additional information on the topics covered in this article.
en.wikipedia.org/wiki/Photographic_filter for an excellent tutorial on photographic filters.
web.canon.jp/imaging/lens/index.html for an animated demonstration of the benefits of image stabilisation and an outline of in-lens stabilisation technology.
Canon. Advanced Simplicity. Visit canon.com.au for more details.