The lens as a creative tool
A zoom lens fitted to broadcast camcorders is a complex, sophisticated piece of optical design. Its performance may be described in brochures in impenetrable techno-speak that many, at first glance, may feel has little or nothing to do with the ‘real’ business of making programmes. To take this view is a mistake, as above all else, the lens characteristics are the most important visual influence on the appearance and impact of an image.
The following pages describe focal length, lens angle, zoom ratio, aperture, depth of field. Altering any one of these zoom settings will have a significant influence on the perspective, the depiction of space and composition when setting up a shot. It is a fallacy to believe that the camera will truthfully record whatever is in shot whenever you put your eye to the viewfinder and press to record. If you are unaware of how the choice of focal length, etc., will affect the chosen image then this important creative decision will be left to chance and accident.
Prime lens or zoom?
Video cameras before the introduction of colour television were usually fitted with four prime lenses (a lens with a fixed focal length), individually mounted on a turret on the front of the camera. A specific focal length lens was chosen for a shot by rotating the turret until the appropriate lens was in front of the pick-up tube. It is still common film practice to select a prime lens to match the needs of the shot. Colour video cameras for technical reasons were almost universally fitted with a zoom lens. A zoom lens allows faster working methods and the option of zooming on shot to capture close-ups without moving the camera. Because of the ease and speed of changing the lens angle of a zoom it is easy to forget exactly what focal length is being used and, as we will discuss later, possibly the space or perspective of the shot will be compromised.
Image size
Because of the different CCD sizes (2/3 inch, 1/2 inch, etc.), the variation in flange-back distance (see page 49), the mechanical connection between lens and camera, and the variation in cable connections with the camera, it is often impossible to interchange lenses between different makes or models of cameras. The image formed by the lens on the face of the CCDs is called the image size of the lens. This must match the size of the camera sensor. Lenses designed for different sized formats (pick-up sensor dimension) may not be interchangeable. The image size produced by the lens may be much smaller than the pick-up sensor (see Image circle and image size opposite) and probably the back focus (flange back) will not have sufficient adjustment. A common lens on a broadcast video camera is a 14 × 8.5 f 1.7 zoom with a minimum object distance of 0.8 m or below. The following pages will identify the implication of this specification and its affects on the practical craft of camerawork.
Image sizes for 4:3 aspect ratio TV and film (not to scale)
The image formed by the lens is circular and is larger than the aspect ratio rectangle of the final television display. The corners of the rectangle of the CCD sensor picture area are positioned inside the optically corrected image circle. The lens can therefore only be used for the specific CCD sensor size for which it was designed.
When a camera converts a three-dimensional scene into a TV picture, it leaves an imprint of lens height, camera tilt, distance from subject and lens angle. We can detect these decisions in any image by examining the position of the horizon line and where it cuts similar sized figures. This will reveal camera height and tilt. Lens height and tilt will be revealed by any parallel converging lines in the image such as the edges of buildings or roads. The size relationship between foreground and background objects, particularly the human figure, will give clues to camera distance from objects and lens angle. Camera distance from subject will be revealed by the change in object size when moving towards or away from the lens (see Depiction of space, page 118).
For any specific lens angle and camera position there will be a unique set of the above parameters. The ‘perspective’ of the picture is created by the camera distance except, of course, where false perspective has been deliberately created.
Focal length
When parallel rays of light pass through a convex lens, they will converge to one point on the optical axis. This point is called the focal point of the lens. The focal length of the lens is indicated by the distance from the centre of the lens or the principal point of a compound lens (e.g. a zoom lens) to the focal point. The longer the focal length of a lens, the smaller its angle of view will be; and the shorter the focal length of a lens, the wider its angle of view.
Angle of view
The approximate horizontal angle of view of a fixed focal length lens can be calculated by using its focal length and the size of the pick-up sensors of the camera.
For a camera fitted with 2/3-inch CCDs the formula would be:
Zoom
Although there are prime lenses (fixed focal length) available for 2/3-inch cameras, the majority of cameras are fitted with a zoom lens which can alter its focal length and therefore the angle of view over a certain range. This is achieved by moving one part of the lens system (the variator) to change the size of the image and by automatically gearing another part of the lens system (the compensator) to simultaneously move and maintain focus. This alters the image size and therefore the effective focal length of the lens. To zoom into a subject, the lens must first be fully zoomed in on the subject and focused. Then zoom out to the wider angle. The zoom will now stay in focus for the whole range of its travel. If possible, always pre-focus before zooming in.
Focal length and angle of view for a 2/3” 14:1 zoom (4:3 aspect ratio)
| Focal length in mm | 8.5 | 10 | 20 | 50 | 75 | 100 | 150 | 0 | 300 | 400 |
| Horizontal angle of view in degrees | 54.7 | 46.5 | 24.8 | 10 | 6.7 | 5.04 | 3.3 | 2.5 | 1.7 | 1.2 |
Minimum object distance
The distance from the front of the lens to the nearest subject that can be kept in focus is called the minimum object distance (MOD). A 14 × 8.5 zoom would have a MOD of between 0.8 m and 0.65 m, whereas a larger zoom ratio lens (33:1) may have an MOD of over 2 m. Many zooms are fitted with a macro mechanism which allows objects closer than the lens MOD to be held in focus. The macro shifts several lens groups inside the lens to allow close focus, but this prevents the lens being used as a constant focus zoom.
The depth of field, how much of scene in shot is in acceptable focus, is a crucial element in shot composition and controlling how the viewer responds to the image. Cinemagraphic fashion has alternated between deep focus shots (Greg Toland’s work on Citizen Kane (1941)) to the use of long focal lenses with a very limited depth of field only allowing the principal subject in the frame to be sharp. The choice of focal length and f number control depth of field.
f number
The f number of a lens is a method of indicating how much light can pass through the lens. It is inversely proportional to the focal length of the lens and directly proportional to the diameter of the effective aperture of the lens. For a given focal length, the larger the aperture of the lens the smaller its f number and the brighter the image it produces. f numbers are arranged in a scale where each increment is multiplied by √2 (1.414). Each time the f number is increased by one stop (e.g. f 2.8 to f 4), the exposure is decreased by half:
| f1.4 | f2 | f2.8 | f4 | f5.6 | f8 | f11 | f16 | f22 |
The effective aperture of a zoom is not the actual diameter of the diaphragm, but the diameter of the portion of the diaphragm seen from in front of the lens. This is called the entrance pupil of the lens (see diagram opposite). When the lens is zoomed (i.e. the focal length is altered) the diameter of the lens which is proportional to focal length alters and also its entrance pupil. The effective aperture is small at the wide angle end of the zoom and larger at the narrowest angle. This may cause f number drop or ramping at the telephoto (longest focal length) end when the entrance pupil diameter equals the diameter of the focusing lens group and cannot become any larger. To eliminate f drop (ramping) completely the entrance pupil at the longest focal length of the zoom must be at least equal to the longest focal length divided by the largest f number. This increases the size, weight and the cost of the lens and therefore a certain amount of f drop is tolerated. The effect only becomes significant when working at low light levels on the narrow end of the zoom.
Depth of field
Changing the f number alters the depth of field – the portion of the field of view which appears sharply in focus. This zone extends in front and behind the subject on which the lens is focused and will increase as the f number increases (see diagram opposite). The greater the distance of the subject from the camera, the greater the depth of field. The depth of field is greater behind the subject than in front and is dependent on the focal length of the lens. f number and therefore depth of field can be adjusted by altering light level or by the use of neutral density filters.
The entrance pupil of a zoom lens changes in diameter as you zoom in. When the entrance pupil diameter equals the diameter of the lens focusing group it cannot become any larger and f-drop or ramping occurs.
Flange back
Flange back (commonly called back focus) is the distance from the flange surface of the lens mount to the image plane of the pick-up sensor. Each camera type has a specific flange-back distance (e.g. 48 mm in air) and any lens fitted to that camera must be designed with the equivalent flange back. There is usually a flange-back adjustment mechanism of the lens with which the flange back can be adjusted by about ±0.5 mm. It is important when changing lenses on a camera to check the flange-back position is correctly adjusted and to white balance the new lens.
Adjusting the back focus
1 Open lens to its widest aperture and adjust exposure as necessary by adding ND filters or adjusting shutter speed.
2 Select the widest lens angle.
3 Adjust for optimum focus with the flange-back control on the lens.
4 Zoom the lens in to its narrowest angle on a distant object from the camera and adjust the zoom focus for optimum sharpest.
5 Zoom out and repeat steps 2–4 of the above procedure until maximum sharpness is achieved at both ends of the zoom range.
6 Lock off the flange-back control taking care that its sharpest focus position has not been altered.
Variable lens angle
A zoom lens has a continuously variable lens angle and is therefore useful for adjusting the image size without moving the camera position. When operating with a monocular viewfinder on a portable camera, the zoom lens can be controlled manually, or from a rocker switch mounted on the lens, or by a thumb control remoted to the pan bar or on a pistol grip. A good servo zoom should allow a smooth imperceptible take-up of the movement of the zoom which can then be accelerated by the thumb control to match the requirements of the shot. In general, the zoom is used in three ways – to compose the shot, to readjust the composition on shot, to change the shot size in vision.
Readjustment on shot
The zoom lens angle is often used to trim or adjust the shot to improve the composition when the content of the shot changes. Someone joining a person ‘in shot’ is provided with space in the frame by zooming out. The reverse may happen when they leave shot – the camera zooms in to re-compose the original shot. Trimming the shot ‘in vision’ may be unavoidable in the coverage of spontaneous or unknown content but it quickly becomes an irritant if repeatedly used. Fidgeting with the framing by altering the zoom angle should be avoided during a take.
Zoom ratio
A zoom lens can vary its focal length. The ratio of the longest focal length it can achieve (the telephoto end) with the shortest focal length obtainable (its wide-angle end) is its zoom ratio. A broadcast zoom lens will state zoom ratio and the wide angle focal length in one figure. A popular zoom ratio is a 14 × 8.5. This describes a zoom with a 14:1 ratio starting at 8.5 mm focal length (angle of view = 54° 44′) with the longest focal length of 14 x 8.5 mm = 119 mm (angle of view = 4° 14′).
Lenses with ratios in excess of 50:1 can be obtained but the exact choice of ratio and the focal length at the wide end of the zoom will depend very much on what you want to do with the lens. Large zoom ratios are heavy, often require a great deal of power to operate the servo controls and have a reduced f number. A 14:1 8.5 mm zoom lens, for example, will give sufficient length at the narrow end to get good close-ups in sport or at the back of a conference hall, but still provide a reasonable wide angle for close interviewing work in crowds.
Extender
A zoom lens can be fitted with an internal extender lens system which allows the zoom to be used on a different set of focal lengths. A 2× extender on the 14 × 8.5 zoom mentioned above would transform the range from 8.5–119 mm to 17–238 mm but it will lose more than two stops of sensitivity.
A follow focus rig is attached to the focus ring on the lens to provide controlled manual movement of focus. It usually consists of support bars which are fastened to the base of the camera and a hand-wheel with marker scale. The hand-wheel on the lens can be replaced by a remote control Bowden cable.
The focus rig may have interchangeable pitch gears (0.5 mm, 0.6 mm and 0.8 mm) to match the lens model. Also the support bars must be designed or be adjustable to the particular combination of lens and camera in use to correctly position the rig to the lens focus ring.
Matte box
A matte box is an adjustable bellows supported on bars attached to the camera and is used in front of the lens as a flexible lens hood to eliminate flares and unwanted light and to hold and position front of lens filters. There is usually one large pivoting ray shield (French flag) and two or more filter holders, one of which is rotatable so that polarizing filters can be positioned.
There are 3 × 3 (3-inch or 75 mm square) or 4 × 4 (4-inch or 100 mm square) filters available for camcorder lenses but the choice will be governed by the focal length range of the lens and whether the filter holder comes into shot on the widest angle. Some lenses have a moving front element when focused. Check for clearance between filter and lens at all points of focus when attaching matte box.
Focusing is the act of adjusting the lens elements to achieve a sharp image at the focal plane. Objects either side of this focus zone may still look reasonably sharp depending on their distance from the lens, the lens aperture and lens angle. The area covering the objects that are in acceptable focus is called the depth of field.
The depth of field can be considerable if the widest angle of the zoom is selected and, whilst working with a small aperture, a subject is selected for focus at some distance from the lens. When zooming into this subject, the depth of field or zone of acceptable sharpness will decrease.
Follow focus
Television is often a ‘talking head’ medium and the eyes need to be in sharp focus. Sharpest focus can be checked ‘off-shot’ by rocking the focus zone behind and then in front of the eyes. As camera or subject moves there will be a loss of focus which needs to be corrected. The art of focusing is to know which way to focus and not to overshoot. Practise following focus as someone walks towards the lens. Adjust the peaking control on the viewfinder which emphasizes edges and is an aid to focusing and does not affect the recorded image.
Zoom lens and focus
A zoom lens is designed to keep the same focal plane throughout the whole of its range (provided the back focus has been correctly adjusted). Always pre-focus whenever possible on the tightest shot of the subject. This is the best way of checking focus because of the small depth of field and it also prepares for a zoom-in if required.
Pulling focus
Within a composition, visual attention is directed to the subject in sharpest focus. Attention can be transferred to another part of the frame by throwing focus onto that subject. Match the speed of the focus pull to the motivating action.
If the focus is on a foreground person facing camera with a defocused background figure and the foreground subject turns away from camera, focus can be instantly thrown back to the background. A slower focus pull would be more appropriate in music coverage, for example, moving off the hands of a foreground musician to a background instrumentalist. Avoid long focus pulls that provide nothing but an extended defocused picture before another subject comes into sharp focus unless this is motivated by the action (e.g. the subjective visual experience of someone recovering consciousness).
Differential focus
Differential focus is deliberately using a narrow depth of field to emphasize the principal subject in the frame in sharp focus which is contrasted with a heavily out of focus background.
■ Back focus: If, after focusing on a subject in close-up, you zoom out and discover the picture loses definition, it is likely that the back focus of the lens has not been properly set up or has been knocked out of adjustment. See page 52 on adjusting back focus.
■ Lens centring: If you need to pan or tilt the camera when zooming to a subject in the centre of the frame with the camera horizontal, it is likely that the lens is not optically centred to the CCDs. The optical axis of the lens must line up with the centre of the CCDs to ensure that the centre of the frame is identical when zooming from the widest to the narrowest angle without camera pan/tilt compensation.
■ Resolution: A television camera converts an image into an electrical signal that will eventually be transmitted. The resolving power required from a TV zoom lens will therefore be related to the maximum frequency of the bandwidth of the signal transmitted plus the specification of the imaging device and line structure. The finesse of detail transmitted will depend on the particular TV standard in use (e.g. 625 PAL; 525 NTSC).
■ Spatial frequency: Spatial frequency counts the number of black-white pairs of lines contained in 1 mm of the optical image and is a method of quantifying fine detail in a TV picture. Lack of resolution in a recorded image may be due to the limitation of the lens design, CCD design or the choice of recording format.
■ Modulating transfer function: Modulating transfer function (MTF) measures the range of spatial frequencies transmitted through the lens by focusing on a chart on which black-white lines become progressively more closely spaced until the contrast between black and white can no longer be detected. The modular transfer function is 100 per cent when the transition between white and black is exactly reproduced and zero when the image is uniformly grey and no contrast can be detected. The MTF curve plots reproducibility of contrast against spatial frequency. Different television systems (625, 16:9, 1105 progressive scan, etc.) require a lens design which matches the maximum resolution achievable with the bandwidth of the chosen system.
■ Fluorite and infinity focusing: Fluorite is used in the manufacture of some zoom lenses to correct chromatic aberrations at the telephoto end. As the refractive index of fluorite changes with temperature more than the refractive index of glass, there is provision when focusing on infinity at low temperatures (e.g. below 0°C) to allow the focus point to go beyond the infinity marking on the lens.
The transition between 4:3 aspect ratio television and the conversion to 16:9 has produced an interim generation of dual format cameras. Different techniques are employed to use the same CCD for both formats.
If a CCD block design is optimized for the 4:3 shape and is then switched to the 16:9 format, lines are discarded at the top and bottom of the frame in order to convert the image area to a 16:9 shape (see Figure a opposite). As 4:3 working occupies the same area of the CCD as a standard 4:3 camera there is no change in angle of view or resolution. When switched to 16:9, however, there is a reduction in resolution and a decrease in vertical angle of view.
If the CCD block is optimized for 16:9 format working (see Figure b) and is then switched to a 4:3 aspect ratio, the image area now occupies a smaller area than a standard 4:3 CCD image (see Figure a) and therefore has a reduced resolution and a reduction in horizontal lens angle.
Some camera manufacturers claim that it is not possible to satisfy the competing demands of both formats; one of the formats will be compromised in resolution or change in lens angle. Other camera manufacturers claim that they can offer a switchable camera that retains the same number of pixels in both formats.
The horizontal angle of view is related to the focal length of the lens and the width of the CCD image. In a dual format camera, if the CCD is optimized for 16:9 format, the angle of view will be smaller working in the 4:3 format compared to working in 16:9 using the same focal length of the lens. At the shortest focal length the loss is about 20 per cent. When switched to 4:3 working, there will be a 9 mm diameter image (see Figure c) compared to the 11 mm diagonal image when working in 16:9 or the 11 mm diameter image of the conventional 4:3 format camera (see Figure a).
This change in horizontal lens angle when switching formats can be remedied by employing an optical unit in the zoom (similar to an extender but producing negative magnification) which shortens the focal length when working in the 4:3 mode (see Figure d). This 0.8 reduction of focal length produces the same range of angles of view as a conventional 4:3 camera using the equivalent zoom lens.
It is not essential when working in 16:9/4:3 dual aspect ratio camera to fit a lens with a 0.8 convertor, but be aware that the lens angle will be narrower than its equivalent use with a standard 4:3 camera.
Zoom lens aspect ratio conversion
Switchable aspect ratio camera with aspect ratio conversion fitted to zoom reducing focal length of lens by 0.8
Power
The cable connecting the lens to camera varies between makes and models of equipment. It is important to match the correct lens cable to the camera it is working with so that the correct drive voltages are connected to avoid lens motors being damaged. Conversion cables are available if lens and camera cable pins do not match.
The servo-driven zoom motor is powered from the battery connected to the camera/VTR. If it is continually operated (i.e. zooming in and out to find the required framing before recording), it can add an unnecessary drain on the battery. To conserve battery power, find the required framing by manual control of the zoom (if fitted) before switching over to servo if you require to zoom on shot.