Nature in Focus – Lenses

The availability of high quality digital cameras, coupled with the computer processing options that follow, provide many outlets for the pictures produced.  This has enticed many nature enthusiasts to start recording their own experiences and views of birds, animals and plants.  While it is easy to take photographs a little extra knowledge can improve the final product enormously.  In this series of articles we will start with the choice of equipment and then move on to some of the topics such as exposure, depth of focus etc. that mystify the beginner finally dealing with handling of the image in the computer.  An opportunity will be provided for readers to submit questions which will be answered in subsequent issues.


The choice of lenses for both DSLR cameras and the compact cameras with interchangeable lens has now become enormous for example Canon and Nikon each offer about 70 different lenses for their DSLR cameras.  Add this to the lenses made by other manufacturers that fit Canon and Nikon cameras and the choice becomes bewildering.

Lenses can be categorised into a number of groups, the first division is between zoom and prime lenses.  A prime lens is one which has a fixed focal length while, as the name suggests, a zoom lens can be adjusted to give a range of focal lengths.  Most DSLR packages will be offered with a zoom lens often covering the focal length range from 18-(55)-70 mm.  This type of lens is ideal for taking shots of scenery and the longer focal length serves very well for portrait photography.  Also available are zoom telephoto lenses which appear to be an ideal choice for bird photography, these can be purchased with various focal length ranges eg. 70-200 mm, 200-400 mm, 70-300 mm.  One thing that becomes immediately apparent as purchase is contemplated is that there is a wide variation in price between these options.  The reason for this is that some of the lenses have a fixed maximum aperture over the entire zoom range while others have a reducing value for the maximum aperture as the focal length is increased.  For the latter lenses at the longer focal lengths the lens becomes ‘slower’ ie. less light is available for the photograph and therefore the shutter speed has to be decreased (longer exposure time).  This can create problems in low light conditions with moving subjects.  Not surprisingly the zoom telephoto lenses with a constant maximum aperture are considerably more expensive that those with variable maximum aperture.  The cheaper zoom telephoto lenses may have the following specification 70-300 mm focal length f4.5-5.6, while a fixed maximum aperture lens may be 200-400 mm focal length f4.  As we shall see in later articles the maximum aperture of a lens is an important issue when you are photographing in low light conditions and in general a lens with a large maximum aperture is an advantage.  Prime short focal length lenses eg 50 mm can be purchased with f values of 1.4 or 1.8, however as the focal length of the lens increases the penalties for having a fast lens with a large aperture are increased size and weight and more importantly cost.

Wide-angle – These are short focal length lenses, either prime or zoom, which cover the range from about 35 mm down to 10.5 mm, and produce an image with a wide angle of collection.  The very short focal length lenses are termed ‘fisheye’ lenses since the front lens is very convex and they produce a highly distorted image.  Lenses in this category are not really those that would find a regular place in the equipment bag of a wildlife photographer.
Macro (Micro)  – All lenses have a minimum focus distance eg. for a 50 mm lens this may be 0.5 m while for a 600 mm telephoto lens it may be 5.0 m.  If you wish to take a close-up picture of say a flower or an insect the obvious tactic is to move closer to fill more of the viewfinder with the subject.  The limit to how close you can get is the minimum focal distance.  The lens manufacturers have therefore produced specific lenses for close-up work variously called Macro or Micro lenses, a 60 mm macro lens may have a minimum focus distance of 0.2 m, considerable less than the standard lens.  Macro lenses are made in various focal lengths ranging from 40-200 mm some with image stabilisation and some without.  We will deal with image stabilisation shortly. The choice of macro lens is an interesting one since if you opt for a 40 mm lens you will need to get very close to the subject, often possible with flowers more difficult with butterflies.  My own choice is a 105 mm lens which allows useful shots to be taken when a plant is inaccessible or the subject is a nervous insect.  The disadvantage of the longer focal length lens is camera shake, which can be overcome either by using a tripod, or a bean bag or an image stabilised lens.
Telephoto  – As we have seen telephoto lenses may be zoom or prime and cover the range from about 200-800 mm.  Long focal length prime telephoto lenses are enormously expensive, very heavy and not easy to use, unfortunately the quality of image from a prime telephoto is significantly better than from the zoom telephotos since the zooms have to make compromises in lens design.  One misconception about big telephoto lenses is that they allow pictures to be taken of subjects at great distance, while this can be true conditions often do not make this feasible.  Dust or moisture in the air between the camera and the subject degrade the image while heat haze is a totally destructive phenomenon.  The real value of a 500 or 600 mm lens is, over relatively short distances (up to say 50 m), to fill the viewfinder with the subject to be photographed (see illustration).  If you intend to travel by air to new photographic locations you will soon encounter the airline hand baggage weight limitation problem and if you wish to take flight shots of birds these big lenses are very difficult to use due not only to the weight but also due to the angular velocity of the bird through the field of view.  From personal experience a good compromise is to use a 300 mm f2.8 lens, which can be easily hand-held for flight shots and is superb for low light environments.  Unfortunately these lenses are very expensive.

An apparently attactive option for telephoto work is to use a teleconverter.  These are optical devices fitted between the camera and the lens and which provide a magnification increase of between 1.4-2.0 x and since they are relatively cheap appear to be the ideal solution.  There are, however, a few words of warning all teleconverters degrade image quality to some extent, teleconverters slow the lens by one to two aperture stops and some combinations of teleconverter and lens work better than others.  If you can accept the limitations of teleconverters they do sometimes allow pictures to be taken that would otherwise be impossible, but I would never rely on them.


Female Black Redstart – Phoenicurus phoenicurus photographed using a 600 mm telephoto lens at a distance of about 20 m.

Image stabilisation is a technology developed to  eliminate image blur caused by small movements of the camera.  Different camera manufacturers have given their own system a specific name. It is only necessary for longer focal length lenses and cannot deal with large movements of the camera and has no effect on blur caused by the subject moving.

The sytems work through having a floating lens element which is moved by electromagnets to keep the image stationary on the the sensor.  The vibration of the camera is detected by angular velocity sensors.  Image stabilisation is marketed as allowing exposures at 2-4 stops slower.  What this means is that under a given set of light conditions, using image stabilisation, the lens aperture can be closed down 2-4 stops (exposure 4-16 times longer) and the stabilisation will compensate for the vibration resulting from the slower shutter speed.

When first introduced by Nikon in 1994 the critics were sceptical of the value of the technology but it has since become standard.
In the next article we will begin to examine how the various settings on the camera need to be manipulated  to obtain the best results.

Roger Marchant – ABS Member

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