All About lenses
A camera body is pretty well useless without a lens, so how do these essential components of photography actually work? In this first article we cover all the key ingredients of a photographic lens’s optical design.
The earliest lenses were simply a single element of high-quality glass ground and polished into a spherical shape so the curved surfaces bent the light rays which passed through them in order to focus an image. More sophisticated designs added more elements – both concave and convex in shape – to improve the optical quality across the image frame, and allow for focusing. Over time, optical designs have become a lot more complex, involving larger numbers of elements arranged in groups which can be moved independently to facilitate focusing and zooming.
The key specification of a camera lens is its focal length which is expressed in millimetres (although you’ll occasionally see vintage lenses with the focal length marked in centimetres). The focal length – or range, in the case of zooms – forms the lens’s model number and is always marked somewhere on its exterior, either on the barrel or around the front, along with other key specifications such as the maximum aperture.
The focal length of a lens is actually the distance from its optical centre – also called the rear nodal point – to the focusing point in the camera which, obviously, coincides with the focal plane in the camera (also called the plane of sharp focus).
The focal length of a lens controls the image size (or magnification) and the angle-of-
view. Thus a short focal length – which means the light rays have to be bent at more acute angles to focus them at a point – has more wide-angle characteristics. A shorter focal length lens therefore produces a closer or smaller image.
In a long focal length lens, the light rays are bent at much shallower angles and so converge over a much greater distance, giving telephoto characteristics. A longer focal length lens therefore produces a larger (more magnified) image. Telephoto lenses are physically longer than wide-angle types, and this is true of both zooms and primes. A prime lens has a fixed focal length such as 24mm, 35mm, 50mm, 85mm, 200mm, etc. A zoom has a variable focal length such as 24-70mm, 70200mm, 100-400mm, etc.
All these focal lengths relate to the 35mm format, based on the frame’s diagonal measurement. If the sensor size or film format is different, a magnification factor comes into play and determines an effective focal length. This is particularly relevant today because many digital cameras have sensors which are smaller in size than a 35mm frame so there’s an increase to focal length as it’s marked on the lens… as the image size will be bigger on the smaller format. The Micro Four Thirds (M43) sensor format has a 1.97x magnification factor compared to 35mm while the ‘APS-C’ size sensors have a 1.5x magnification factor (except for Canon cameras where it’s 1.6x). Thus an M43 lens with, say, a focal range of 12-35mm is effectively a 24-70mm (i.e. 12x2.0 and 35x2.0) and an APS-C lens with a focal range of 16-60mm is effectively a 24-90mm (i.e. 12x1.5 and 60x1.5). Likewise, if you’re using a 35mm format lens on a camera with a smaller sensor (either directly or via mount adaptor), the effective focal length will also increase. For example, a 24-70mm zoom on an M43 camera becomes effectively a 48-140mm and, on an ‘APS-C’ camera, becomes a 36-105mm. This may all seem a bit confusing at first, but it’s related to the size of the imaging area (specifically the diagonal length), the standard or normal lens focal length for this size, and its angle-of-view. There are various mathematical formulae involved which you don’t need to know. All you need to know is that lens focal lengths in photography are always expressed for the 35mm format (film or digital sensor) so if the imaging area is smaller, the effective focal length will increase and, if its larger (as in digital medium format) it will decrease.
Another important point to make here is that the focal length of a lens doesn’t directly control perspective. The reason the two are so often erroneously linked is that perspective is related to the distance of the lens from the subject and, in turn, this distance can be a factor of the focal length (i.e. you may choose to move closer to a subject when shooting with a wider angle lens). However, if a subject was photographed from the same distance with both a telephoto lens and a wide-angle lens, the perspective would be the same. True, the wide-angle lens will make the subject seem further away, but if you enlarged the centre of this image so it matched the telephoto’s field-of-view, the two pictures would look exactly the same. Changing the subject distance changes the perspective… i.e. the spatial relationship between objects in the foreground and background of an image. Move in closer when using a wide-angle lens, and the distance between the foreground and background is exaggerated. Move further away from the subject when shooting with a telephoto lens and the distance between the foreground and background is compressed, an effect known as foreshortening.
The earliest zoom – or variable focal length – lens designs date back to the mid-1940s, but it wasn’t really until the mid-1970s that modern optical design and manufacturing techniques allowed for more compact and affordable zooms which delivered an acceptable level of optical performance. Today, zoom lenses are more popular than primes (lenses with a fixed focal length) and range from ultra-wide-angle to extreme telephoto.
As noted earlier, all lenses incorporate a number of elements – made from either optical glass or optical-grade resins – which bend (or refract) light to a point of focus. Zoom lenses incorporate an extra set of elements to vary the focal length, and this used to mean big and bulky designs with quite a number of aberrations which degraded the image quality. Essentially, every extra element in a lens’s optical construction has the potential to create extra problems which require some form of correction. This used to be done by adding more elements, but obviously this could also lead to more problems, so it’s easy to see how zoom lens designers faced real challenges. A breakthrough came with the perfecting of glass moulding techniques which has enabled lens elements to be created, via computer-aided design, to provide ‘built-in’ correction. These are known as aspherical elements which indicates the surfaces are shaped (i.e. not spherical) in order to correct for various aberrations. Aspherical elements are also often made from optical-quality resins or, in some instances, use a spherical glass core over which is coated a resin to create the aspherical surfaces. These are known as hybrid aspherical elements.
Aspherical lens elements allow complex zoom designs to now be achieved with a high degree of correction for various aberrations, excellent optical performance, compact and lightweight constructions and comparatively low prices.
There are wide-angle zooms (17-35mm, for example), medium range zooms (28-105mm) and telephoto zooms (100-300mm, 100-400mm, etc.). However, there are also zooms with much longer focal ranges such as 28300mm which spans wide-angle to telephoto in one lens.
The downside to zooms is that they are generally slower than
a prime lens. Lens speed refers to the maximum aperture or the maximum amount of light the lens will let in to reach the imaging sensor. Because of the additional elements and the nature of a zoom’s optical design, more light is lost internally and there are limits to the diameter of the diaphragm (which controls the aperture size). So, for example, a 24-70mm zoom may have a maximum aperture of f2.8 at 24mm and f4.0 at 70mm whereas a 50mm prime lens may have a maximum aperture of f1.4 or even f1.2 (remember that the smaller f-numbers indicate a larger aperture opening).
As a general rule faster lenses are more expensive because of the measures taken to increase their light transmitting efficiency. A 24-70mm zoom with a constant maximum aperture of f2.8 may well cost twice as much as one with the f2.8-4.0 maximum aperture range just quoted.
Traditionally, a wide-angle lens is one with a focal length shorter than the 50mm so-called ‘standard’ focal length (all the focal lengths quoted here relate to the 35mm format). However, in reality, it’s any lens with a focal length between 20mm and 35mm.
As the focal length decreases, the angle-of-view increases and the subject size becomes smaller (relative to the whole frame area being recorded). Lenses with a focal length shorter than 20mm – i.e. 14mm, 15mm or 19mm – are described as ‘ultra-wides’. Beyond this, you move into the area of the fish-eye lens which has an angle- of-view of 180 degrees and gives a distinctive ‘bulbous’ look. Ultrawide angle lenses (and fish-eyes) need to be used with care as the degree of distortion they produce doesn’t work with all subjects.
The more ‘general purpose’ wide-angle focal lengths are 21mm, 24mm or 28mm, with the first two providing a good balance between an expansive angle-of-view (for landscapes, street scenes, interiors, etc.) and practicality in pictorial terms.
Wide-angle lenses have inherently increased depth-of-field which means that when you focus on a particular point more of the scene in front of, and behind, this point will also be sharply rendered. Ultra-wide lenses actually have such long depth-of-field you don’t really have to worry about focusing at all. Depth-of-field is also varied according to the selected aperture, with smaller apertures (i.e. f16 or f22) giving much greater depth-offield than wider apertures (i.e. f2.8 or f2.0).
Again, the definition of a telephoto lens is one with a focal length greater (or longer) than 50mm, but the telephoto range is generally accepted to begin at around 85mm which would be classified as a ‘short telephoto’ lens. Beyond 300mm you move into the ‘supertelephoto’ range which includes 400mm, 500mm and 600mm lenses. Anything longer than 600mm is generally for specialised applications such as wildlife or some sports.
As a rule, image brightness decreases as the focal length increases so many telephoto lenses or telezooms are described as being ‘slow’ because they may only have a maximum aperture of f5.6 or smaller. Image brightness can be increased by increasing the diameter of the aperture, but this results in a bulkier and more expensive lens. For example, a 300mm f2.8 lens may cost as much as $10,000, but a 300mm f5.6 lens could be as cheap as $500. Professional sports photographers need ‘fast’ telephoto lenses, but as they’re earning money from their pictures it’s easier to justify the investment.
“Aspherical lens elements allow complex zoom designs to now be achieved with a high degree of correction for various aberrations.”
than 1/(focal length) – so, for example, with a 300mm lens you shouldn’t select a shutter speed slower than 1/300 second or, with a 500mm lens, a speed slower than 1/500 second. Of course, if you use a tripod, then you can select slower shutter speeds, but always make sure the tripod is securely set-up so there’s no possible source of vibration. A good idea is to use the self-timer to trigger the shutter (or a remote control) so you can avoid actually pressing the button which could cause vibrations.
An increasing number of telephoto lenses incorporate optical image stabiliser mechanisms which are designed to correct for a certain amount of camera shake movement and allow for hand-held shooting at slower shutter speeds. Alternatively, many mirrorless camera bodies have sensor-shift image stabilisation which also provides correction for camera shake. Some systems combine both in-camera and optical image stabilisation to provide enhanced compensation for camera shake.
Telephoto lenses have an inherently shallow depth-of-field which means focusing can be critical if you need particular parts of a scene to be reproduced sharply.
The term ‘macro’ appears quite frequently on modern zoom lenses, but is actually often used incorrectly. The true definition of a macro lens is one that focuses sufficiently closely to the subject to give a lifesize reproduction (i.e. the object, or part of the object, is the same size in the image as it is in real life). Most so-called macro zooms are actually better described as ‘close focusing’ and rarely give a reproduction ratio greater than 1:4 (i.e. quarter lifesize).
In a true macro lens, the optical design is fully corrected for the aberrations which occur when focusing at such short distances.
Macro lenses are generally prime designs with focal lengths of 50mm, 100mm or 200mm (in the 35mm format). Closer focusing capabilities can also be achieved with ‘normal’ lenses by using either extension tubes which fit between the camera body and the lens, or close-up ‘filters’ which screw to the front of the lens.
Both are available with various magnification powers.
Perspective Control (PC) Lenses
Perspective control lenses – also called tilt-shift lenses because of the way they work – are fairly specialised tools, primarily designed for architectural work, but with applications elsewhere, including landscape photography.
The design principle is the same as that of the traditional large format film view camera which comprised a lens standard and a film standard connected by a set of bellows. The flexibility of the bellows enabled the lens and film to be tilted, swung or shifted independently of each other, enabling much greater control over both perspective and sharpness than is possible with a rigid-bodied camera.
However, the same effects can be achieved with a rigid-body camera fitted with a perspective control (PC) or tilt-shift lens. These lenses incorporate a mechanism which allows the lens’s optical axis – the image plane – to be moved in relation to the focal plane (i.e. either tilted or shifted). A vertical shift adjustment allows for the correction of convergence which otherwise makes tall buildings appear as if they’re toppling over. Most PC lenses allow for shifts to be made in either the vertical or horizontal planes.
The tilt adjustment allows the plane of sharpness to be adjusted (literally tilted from the normal perpendicular) which will give infinite depth-of-field without the need to select smaller apertures. If you think of the plane of sharpness as a sheet of paper, held vertically, then only the thickness of the paper represents what would be sharply rendered in the image. If you then lay the sheet of paper down, which is the effect of the tilt adjustment, then everything covered by it, from front to rear, would now be sharply rendered… which, in photographic terms, represents greater depth-of-field. Again, a tilt can be applied in either the vertical or horizontal plane. A tilt adjustment that’s applied in the horizontal pane is called a swing and enables sharpness control in this plane.
In the next article, we’ll look at some of the features of modern lens design including optical image stabilisation, autofocusing drives, special elements and anti-reflection multi-coatings.
The optical construction of a photographic lens comprises elements set in groups. Shown here is the inner workings of Nikon’s AF-S 800mm f5.6E FL ED supertelephoto with the special elements shown in colour. All the elements work together to focus the light rays at the camera’s focal plane while also correcting for various aberrations. Flourites ED Glass Elements
Lens ‘model numbers’ are made up of the focal length (or focal range for zooms) and maximum aperture (or aperture range for zooms). In modern lenses, the extra sets of initials refer to various features such as optical image stabilization.
Here’s Nikon’s 800mm on the outside. The focal length of a lens is the distance from its optical centre to the focusing point in the camera. So long focal length lenses – such as this supertelephoto – are going to be physically long too.
A mid-range zoom such as this 70200mm f2.8 is popular because it covers a useful range of focal lengths from short to mid telephoto with the speed of a constant f2.8 maximum aperture.
There’s even more size savings to be had with Micro Four Thirds telephoto lenses such as the Leica DG Vario-Elmar 100-400mm 4.0-6.3. It’s the equivalent of a 100-800mm, but is compact enough to be used hand-held (assisted by optical image stabilisation).
The appeal of mirrorless cameras with a smaller-than-35mm sensor is that it allows for more compact lenses. Olympus’s PRO series 12100mm for its Micro Four Thirds camera bodies is equivalent to a 24200mm, but in a lens that’s half the size of a full-35mm format version.
Short focal length lenses (which are also physically much shorter) deliver a wider angle-of-view which is why they’re known as wide-angle lenses. The ultimate wide-angle is the fisheye which can deliver an angle-ofview of up to 180 degrees.
Long telephoto lenses are inherently big and heavy (due to the longer focal length) which makes them more difficult to handle. Additionally, the higher magnifications increase the risk of camera shake which causes blurring of the image… as any tiny The Nikkor PC 19mm tilt-shift lens shown with a lateral shift applied (right) to correct for perspective in the horizontal plane; and a right swing (left) which will increase the zone of sharpness in the horizontal plane. Both these adjustments can also be applied in the vertical plane.
movements during an exposure are also greatly magnified. This is why you see sports photographers always using tripods or monopods to support the camera and lens. If you want to hand-hold the lens, the general rule is that the shutter speed should not be slower
Perspective control (PC) lenses allow for the optical axis to be shifted or tilted in relation to the image plane in order to give much greater control over, respectively, perspective and sharpness.
Lens focal lengths are expressed in relation to the 35mm format so there’s a magnification factor at play with lenses designed for smaller or larger size sensors. The Fujinon GF 23mm f4.0 for Fujifilm’s GFX mirrorless digital medium format camera is designed for the 44x33 mm sensor so its effective focal length is 18mm due to the relative difference in the image sizes.