Introduction to Lenses

This document looks at the camera lens (stills or video) and how it contributes to the quality of the photographic image. It describes the component parts of the lens and how they contribute to the final image. The document should also help the reader to select a specific lens for a task.

Introduction

In the highly technical world of modern photography it is very easy to forget that an image can be taken with nothing more than an old metal box with a small pinhole in one side and some light sensitive paper. You can read more about pinhole photography on our blog.

Hidden behind the mind boggling array of buttons and dials, flashing LCDs and other energy sapping features in today's digital cameras we can still find the core elements found in the most basic of pinhole cameras: the sensitive medium (sensor), a method of gathering light (lens) and a light proof box (camera).

These components have all changed dramatically since the earliest cameras - however, it is the camera lens which makes the greatest contribution to image quality and which has evolved the most.

What is a lens?

The simplest cameras are capable of focussing light from the subject onto a light sensitive surface with nothing more than a small pinhole. Pinholes do present some quality and operational issues - for greater image clarity and ease of use a simple convex glass lens can be placed at the front of the camera to direct the light. By moving the lens closer or further from the film/sensor the subject can be brought into focus. The single convex lenses used in the earliest cameras were prone to distortions - this can be reduced, though not eliminated by combining lenses and transparent materials to create compound lenses.

Cameras are used under a wide range of lighting conditions. To control the amount of light reaching the sensitive surface the camera is also fitted with a variable diameter aperture and a shutter. While most shutters are housed in the camera body (focal plane shutter) they can also be found in the lenses of some cameras (leaf shutter).

Get to know your lens

To get the most from your camera and lens combination it is worth familiarising yourself with the lens and how its features can contribute to the final image. Most lenses used for stills photography and some used for video have some controls around the side of the lens. Lenses will normally have markings on the front or around the side that provide valuable information for the filmmaker or photographer.

Annotated image of typical zoom lens used in stills camera

Sensor size and image circles

While the typical photograph or video clip appears in a rectangular frame, most lenses produce circular images. The camera captures a rectangular chunk from the centre of the larger image circle. The size of the chunk depends on the size of the camera's sensor.

The term 'full frame' is commonly used to describe any stills camera which has a sensor the same size as a standard 35mm negative, or 36mm x 24mm. Cameras with smaller sensors are often referred to as having cropped sensors. The smaller format takes a smaller chunk of the image circle and therefore delivers a smaller field of view.

For users of cropped frame cameras the lenses made for full frame models are larger, often more expensive and produce image areas much larger than their cameras can use. Manufacturers therefore produce lenses specifically for the smaller sensors - with a smaller image circle they are more compact and therefore comparatively cheaper, though if used on a full frame camera the smaller image circle often results in vignetting.

Illustration of image circle for full frame lens

Illustration showing full frame sensor's capture area

Illustration showing cropped sensor's capture area from a full frame lens, this is the equivalent field of view of a longer focal length lens on a full frame sensor.

Photograph taken with a full frame camera with lens designed for a cropped sensor, note the vignetting at the corners.

The choice of capture format (full frame or cropped sensor) is often based on the type of subject the photographer wishes to record. The cropped sensor captures a smaller area of the image circle which is visually equivalent to a higher magnification lens.

Cropped sensor cameras are therefore popular with photographers who prefer to use longer focal lengths such as sports or wildlife photographers while full frame cameras are often used when wider fields of view are preferred.

Focal length

One of the first factors to consider when choosing a lens is its focal length. The focal length, measured in millimetres, is the distance from the optical centre of the lens to the focal point (sensor or film). For a full frame sensor (36mm x 24mm) the normal focal length is 50mm, and with a field of view of 46 degrees it corresponds approximately to that of the human eye. Focal lengths shorter than 50mm will offer greater fields of view and are referred to as wide angle. Longer focal lengths are referred to as telephoto.

Some lenses may introduce distortion to the image, this is normally more obvious with wide-angle and long telephoto lenses. A picture where detail at the centre of the frame is larger in size, while at the edges it is reduced, is referred to as having barrel distortion. A lens which enlarges detail at the edges but reduces its size at the centre of the frame is said to exhibit pincushion distortion. Photographers who wish to avoid distorting their subjects use good quality lenses in a range of around 35mm to 100mm on a full frame camera (these focal lengths will differ for cropped sensors).

Rectangular subject photographed with wide-angle lens which shows barrel distortion

Originally lenses' focal lengths were fixed and the user would have to switch lenses for different fields of view. Fixed focal length lenses are often referred to as prime lenses. In the 1930s lenses were developed that offered variable focal lengths while maintaining sharp focus, these were the first true zoom lenses and were developed mainly for the film industry.

By the early 1960s zoom lenses were also available for stills photography. In the early days the quality of the 'jack of all trades' zoom lens was noticeably inferior to the equivalent prime lenses, but optics have improved radically and now it may be hard to distinguish between the image quality of a good zoom and a prime lens.

The greater complexity of the zoom lens along with the high price of photographic glass means that the largest available aperture in a zoom lens is normally considerably smaller than in the equivalent prime lens. This can have an impact on certain types of photography where larger apertures are required for aesthetic or practical reasons.

Photograph taken with extreme wide angle 17mm lens on full frame camera

Photograph taken with 35mm semi-wide angle lens on full frame sensor

Photograph taken with short telephoto 80mm lens on full frame sensor

Photograph taken with 200mm long telephoto lens on full frame sensor

The fields of view shown in the images above are captured using a full frame sensor. A cropped sensor would record a smaller area of the image and give the impression of a longer focal length.

Lens mount

All digital SLRs and a small number of video cameras use interchangeable lenses. The main manufacturers offer an extensive range of lenses that can satisfy the majority of photographic applications. These lenses are attached to the camera by a lens mount. The mount itself is normally a bayonet style fitting and has mechanical and electronic couplings to communicate settings between camera and lens. The mount incorporates a locking button to secure the lens to the camera.

The camera manufacturers have all developed their own lens mounts which makes swapping lenses between camera brands difficult or even impossible. The lens mount must be secure to protect the lens or camera from damage and free from movement to ensure sharpness. Lens mounts are normally made from metal, though plastic mounts are becoming more common at the cheaper end of the price range. There is some concern that over time the metal of the camera mount will 'shave' plastic from the lens mount, the loose shavings may then cause problems inside the camera or lens.

In addition to the established camera makers there are a number of third party lens manufacturers who make competitively priced lenses for most camera mounts.

Focus ring

In 35mm format digital SLRs and video cameras the auto-focus lens is dominant. There are a handful of highly specialist lenses that are exclusively manual focus as well as older non auto-focus lenses that can still be used on some modern camera models.

While auto-focus has made life easier for most photographers, there are occasions when the system is confused or the photographer wants greater control over focus. Most stills and video lenses therefore also offer manual focus. The method used to take control of focus may be found on the camera, lens or both.

An increasing number of digital SLR cameras now offer video capture - however, at present the auto-focus is very slow when compared to stills photography and the built-in microphone may pick up the sound of the auto-focus motor. Manual focus may offer more pleasing results both in terms of sound and picture quality.

The closest focussing distance (distance between the front of the lens and the subject) of a lens may be an important value when selecting a lens for specialist applications such as macro photography.

Zoom ring

Video cameras may have either a rocker switch mounted on the camera which delivers smooth zooming through a shot, or a manual zoom similar to that used in stills photography. The zoom facility in lenses for stills photography is not motorised and does not need to be smooth.

The zoom control is normally a ring or collar around the lens which is rotated or moved up and down the length of the lens to change focal length. Zoom and manual focus controls may be combined in a single large ring. The position of the zoom should only move under pressure - if it moves due to gravity, such as when mounted on a copystand, the field of view will change. The shift of focal length due to gravity is known as 'zoom creep'.

Aperture

The variable diameter aperture is used to control the amount of light passing through the lens. Along with the shutter this is the main mechanical method used to control the amount of light striking the film or sensor.

The aperture size is expressed as an 'f-number' or 'f-stop' - the lower the value the larger the aperture size. The f-number is calculated by dividing the focal length by the aperture diameter (aperture = f/D) e.g. an aperture diameter of 25mm (D) used with a lens with a focal length of 100 mm (f) will have an aperture of f4.

Lenses often have an adjustable aperture ring (though this is rapidly being replaced by a dial on the camera) marked with the f-stop values. As we close the aperture each whole stop reduces the amount of light that passes through it by a half.

While the largest and smallest available aperture size will vary between lens models, a standard 50mm prime lens might offer a widest aperture of f1.4, with whole stop values of f1.8, f2, f2.8, f4, f5.6, f8, f11, f16 and f22.

Photographers that work in low light conditions or need to capture fast moving subjects often choose lenses with large maximum aperture sizes - these are somtimes known as 'fast' lenses. A fast lens requires considerably more glass in front of the aperture which makes them both larger and more expensive than the equivalent smaller aperture lens.

Like the shutter, the aperture can be used to alter the appearance of the image. By adjusting the aperture setting the user will alter the range of acceptable sharpness within the image. This is known as depth of field. A small aperture will increase the depth of field, while a larger aperture reduces the depth of field.

Aperture and Depth of Field from JISC Digital Media on Vimeo.

A shallow depth of field (larger aperture, lower f-number) is used to isolate a subject from the background, for example in portraiture. A larger depth of field (smaller aperture, higher f-number) can retain sharpness from a foreground element to the distant horizon and is commonly used in landscape photography.

Some lenses have a distance scale on the focussing ring, which aligns with the aperture setting to provide depth of field data. Some cameras also offer a depth of field preview option, this is normally a mechanical system that closes the aperture to the desired setting and the user can view the scene and check the depth of field prior to exposure.

Generally speaking the larger the sensor or film size the greater the potential to control depth of field. With full frame and cropped sensor digital SLRs and the right lenses it is possible to produce images with a very shallow depth of field, however in video and small format stills photography, sensors are much smaller and the depth of field tends to be greater. It is therefore essential that users of full frame or cropped sensors take great care over focussing if using wider apertures.

Photograph taken with f2.8 (large aperture) showing shallow depth of field

Photograph taken with f22 (small aperture) showing large depth of field

While the choice of aperture may be made for aesthetic reasons the aperture also plays an important role in controlling exposure. By opening or closing the aperture the shutter speed will have to be adjusted to compensate for the increase or decrease in the amount of light that passes through it.

Image stabilisation

Long shutter speeds in stills photography can result in camera shake and in jerky footage with video. To reduce this affect some lenses include image stabilisation systems which can detect camera movement and neutralise it by shifting lens elements. This allows photographers to work with longer shutter speeds and filmmakers to get smoother footage.

While the system can help to reduce the affect of camera movement it cannot correct for subject movement. In stills photography image stabilisation controls are normally found (when available) on the side of the lens. Image stabilisation systems used in stills cameras can produce some noise and should be switched off when filming with video enabled stills cameras. Generally speaking the best solution to camera shake in still photography or jerky footage in video is to use a stable camera support such as a tripod. See our advice on Stands and Supports for Still and Moving Images.

Lens FAQs

Digital SLR cameras are normally sold with a short zoom lens covering a modest range from semi-wide to short telephoto. These lenses are designed for the 'average' subject and deliver adequate results - however, it is unlikely that the stock lens will satisfy all requirements.

Superzoom lenses offer a much greater focal length range from true wide angle through to long telephoto. These offer a little more flexibility but they normally also present a number of limitations. It is therefore common for a photographer to have a range of lenses, each with its own strengths and weaknesses from which to choose.

Extreme wide-angle lenses often introduce distortions and are therefore not recommended for applications where a natural perspective is required. Many zoom lenses claim to offer a macro facility, however the term is used very loosely - a true macro lens is considered to be one that can produce an image on the sensor close to the size of the original object. The FAQs below answer some common problems associated with camera lenses.

Is there a way of getting a wider field of view without buying a new lens?

Maybe. Make sure you are using the widest possible focal length (lowest number), step back if possible but check this doesn't introduce new distracting elements to the scene. Wide-angle lenses can be quite expensive but it may be possible to buy cheaper screw-on adapters which will produce a wider field of view.

The only other way of doing this is to digitally 'stitch' pictures together. Most of the more popular image editing applications as well as a few camera manufacturers offer stitching tools. To get the best results from a stitching application the photographer should follow some simple rules.

1. Turn the camera to portrait format
2. Set the camera to manual exposure and take an average light reading for the whole scene
3. Set the focus to manual and focus at a distance where a key subject appears
4. Try to rotate the camera around an imaginary axis between the back of the lens and the sensor
5. Take the first frame, rotate the camera but allow a generous overlap with the first frame
6. Repeat
7. Load the images into the image editing application and follow the instructions

Merging individual images to create the panoramic image below

Picture built out of 5 'stitched' photographs

How can I reduce lens flare?

Lenses are designed to record light reflected from the subject. If light from the lightsource strikes the lens directly it can produce lens flare. In its mildest form it may just result in reduced contrast or lack of detail over parts of the photograph, but in extreme cases the picture has a series of bright circular shapes and light beams across the image.

There is no simple way of removing this digitally so it is essential the photographer addresses the problem before the picture is taken. Most lenses can be fitted with a lens hood, this is a metal or plastic cylinder that attaches to the front of the lens and reduces the chance of light hitting the glass directly. A good lens hood should be able to protect the lens from direct light without appearing in the image.

Lens hoods for prime lenses are designed specifically for a single focal length and are normally efficient. Lens hoods for zoom lenses have to work across the full focal length range. The ideal lens hood for a telephoto lens is a deep cylinder while a wide-angle lens can only use a very shallow dish shape. If a deep cylinder hood is placed on a wide-angle lens the edge of the hood will appear at the corners of the picture. The depth of the hood used for zoom lenses is therefore dictated by the widest available focal length.

A piece of dark card or even a hand can be used as a makeshift lens hood or 'flag'. The hand or card is held to the light and brought towards the lens (but not into the shot) until the lens flare disappears. If the camera is on a stable support the photographer can check the front of the lens for flare. If a reflection of the light is seen on the lens glass then a hood or flag should be used.

Photograph with lens flare

Photograph using lens hood to remove lens flare

Lenses with lens hood attached to reduce lens flare. Note how the hood shape changes according to the focal length of the lens.

 

Do I need a special lens to take close up video or stills?

No. While there are specialist close-up or macro lenses available, it is possible to capture small objects using other cheaper accessories. Stills photographers can use extension tubes, which extend the distance between the back of the lens and the sensor. This effectively creates a larger image circle and the camera takes a smaller and therefore magnified section of the circle. Extension tubes require increased exposure time to compensate for the inverse square law and the larger image circle.

Cheap close-up filters can be placed on the front of both stills and video cameras to allow the camera to be used at closer working distances. Close-up lenses are sold at different strengths and used according to the required magnification. These lenses are prone to lens flare and so should be shaded.

An unusual yet effective solution is to attach a lens to the camera ‘back to front' using a special reversing ring. This method can produce high levels of magnification with a couple of drawbacks. The ring doesn't offer mechanical or electrical communication between camera and lens and so features such as auto-focus or an automatic aperture are unavailable. The front of the lens is sealed to prevent the ingress of harmful dust or moisture and the rear of the lens is normally protected by the camera body - however, if the lens is reversed the rear glass elements along with the electrical and mechanical contacts are exposed.

Lens attached to camera using a reversing ring

Can my camera be attached to a microscope?

Probably - it depends on the camera and microscope, but often the camera can be mounted onto the instrument using a combination of special lenses and adaptors. SLR camera users should be able to remove the lens and replace it with a special 'T mount' designed for their brand of camera. The T mount is attached in turn to the eyepiece of the microscope and the image is then brought into focus via the camera's viewfinder or LCD screen.

The weight of the camera, T mount and adapter can place considerable stress on the microscope, mount or camera and so it is advisable to use an additional camera support to spread the load. Without the lens's adjustable aperture, exposure has to be governed by the intensity of the light from the subject and the shutter speed. Neutral density (ND) filters can be used to reduce the amount of light before it hits the specimen.

Camera mounted (via T mount) on microscope. Photograph by Anita Thomhave Simonsen

How do I photograph tall or long objects?

A common problem when photographing tall objects from ground level is that of converging verticals. If the camera is tilted to view the top of the structure it might look as though it is falling backwards. This can be corrected artificially using an image editing application. Alternatively, keeping the camera parallel to the object, stepping back, or using a wider focal length may also resolve it.

Photograph taken with a tilted camera, note the converging verticals of the chimney and streetlamp

Photograph taken with camera parallel to streetlamp and chimney

If this is a common type of subject it might be worth considering a specialist 'tilt shift' lens which can correct for converging verticals or horizontals.

Lens care

It is the lens that directs the light towards the camera sensor and plays an essential role in image creation. Lenses normally incorporate a series of high quality glass elements which are coated to reduce lens flare and other optical problems, so great care should be taken to avoid any damage to the glass elements.

Lenses should be fitted with a lens cap and rear caps when not attached to the camera, and they should be stored in a dry dust-free environment. If a lens is exposed to moisture it should be carefully dried to reduce the risk of fungal attack, which once established is very hard to control.

Published in: Creating

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