2 Introduction to Focus
Focus adjustment is one of the primary functions that must be mastered in order to produce video images of a professional standard. Obtaining correct focus is not only about ensuring the best quality images it is also a valuable device for directing the viewer’s attention to areas within the frame.
Whilst many video cameras have an automatic focus setting, this should really only be used in situations where gathering any form of image is more important than gathering an image with perfect focus.
Although automatic focus can be used to great effect, and is usually unnoticeable to the viewer, it does leave the decision of what is in focus to the camera and not the videographer.
Automatic focus will only provide an estimate of what should be in focus, the measurement of which is normally based on what is in the middle of the image. If you want to focus on part of the image which is off-centre, the automatic control will not allow for this. So, if your shot includes a tracking or panning movement with items in the foreground and background, the automatic setting will continue to adjust the focus as the camera moves and this might not be your intended result.
It is, therefore, important to master manual control of focus and to appreciate the role of focussing and how it can help to direct the viewers’ attention to aspects within the frame.
2.1 Depth of Field
Depth of Field (DOF) is the range of distance between the camera lens and infinity which is in focus.
The DOF can be anything from a few millimetres to virtual infinity – and is primarily governed by the Aperture or f-number ( The DOF can be anything from a few millimetres to virtual infinity – and is primarily governed by the Aperture or f-number (ƒ). The greater the f-number the smaller the aperture (or iris opening size) and, consequently, the more focused or concentrated the light will be passing through the lens to the camera’s imaging device. This is why in low lighting conditions your DOF will become much shallower, as the camera’s f-number will need to be reduced in order to produce acceptable images (see also section 10. Filming in Low Light).
The way to correctly set the focus on a zoom lens is:
i) Zoom in fully
ii) Adjust the focus so that the camera is producing a sharp image
iii) Readjust the zoom as needed to frame and compose the shot
Once the focus is set at full zoom (also known as the ‘long end of the lens’), the lens should remain in focus throughout the zoom range as long as the subject remains at the same distance from the lens. Focus can be set at any zoom setting (or Focal Length) but it is much easier to set a correct focus at the long end of a zoom lens as described. On cameras with detachable lenses, it is also important to ensure that the Flange Back (or Back Focus) has been set up correctly – which essentially adjusts the position of the lens in comparison to the image sensors. If this is not set correctly, focus will not remain constant throughout the zoom range.
It is important to understand that precise focus is only possible at a specific distance from the lens face. Anything which is not that exact distance from the face of the lens will be out of focus or ‘soft’.
2.2 Neutral Density Filters and Negative Gain
In order to reduce the amount of light entering the camera, most professional video cameras will have built-in Neutral Density Filters (ND) in varying strengths. In addition these can come in the form of screw-on attachments and filter plates that are placed in front of the lens to achieve the same effect. By reducing the light entering the camera you will need to increase the aperture size and therefore reduce the DOF.
Some cameras also have a further means of lessening the light sensitivity of the imaging device called Negative Gain. However, because of the electronic processing this can reduce picture definition, but can still be useful to “bridge the gap” between two different strengths of ND.
2.3 Focal Length
Another contributing factor to DOF is the Focal Length of the lens. All video cameras have a zoom lens which gives a varying focal length, ranging from its zoomed out setting (shortest focal length) to its zoomed in setting (longest focal length). The longer the focal length, the greater the amount of light is needed to produce a sufficiently exposed picture. Therefore, at the lens’ fully zoomed in setting it will produce its shallowest depth of field. This information is especially important when filming in low-light environments (see also section 10. Filming in Low Light).
2.4 Image Sensor Size
Whilst the videographer will not be able to alter the size of the image sensor/s within their camera, they should understand what impact this has on the possible range of the DOF. The basic rule is, the larger the image sensor the shallower DOF. Video cameras with very small image sensors can only achieve a relatively large DOF.
Traditionally, video cameras were not able to achieve the extremely shallow DOF that could be achieved on film cameras without the use of 35mm film adaptors. As well as the difference in frame rates and aspect ratio between film and video, the shallower DOF contributed significantly to the look of footage shot on film – which is often referred to as a ‘cinematic’ look.
Video imaging device technology has now moved on considerably and there are video cameras with much larger image sensors capable of producing a similar shallow DOF of that produced by film cameras. There are also 35mm Digital Single Lens Reflex stills cameras that have the ability to shoot video too.
2.5 Focus for Effect
Pull/Push Focus is one such technique. By using a long focal length and shallow DOF, the focus can be adjusted from an item in the background to an item in the foreground (pull) or vice versa (push).
In a pure pull/push focus it is not normally necessary to make any adjustment to the framing of the shot. It is only the action of adjusting the point of focus that redirects the viewer’s attention to another feature in the frame.
Using a soft focus, whether by lens adjustment, filters or attachments, is another way of creating mood or effect with focus alone. However, this can also be achieved in post production with the use of digital filters. The main benefit of applying digital soft focus effects in post production is that the integrity of the original footage will be maintained.
2.6 Hyperfocal Distance
It is useful to know the Hyperfocal Distance of your lens at any given f-number. Hyperfocal Distance is the distance beyond which all objects are acceptably sharp when a lens focus is set at infinity (∞).
