11 Introduction to Picture Ratio
The shape of the image produced by a video camera is referred to as its ‘Aspect Ratio’. This is represented as the image’s width in relation to its height. The most common image ratio is 16:9, but there are still many older cameras in use that produce a 4:3 image ratio. Most Standard Definition (SD) video cameras can produce either 4:3 or 16:9 ratio images (user-selectable), while all High Definition (HD) cameras produce 16:9 images only. Cameras that can be switched between SD and HD will provide both 4:3 and 16:9 in SD mode but only 16:9 in HD mode. Whilst there is the same number of pixels in a Standard Definition 4:3 image, these pixels are almost square in shape. In a Standard Definition 16:9 image the pixels are oblong in shape. Off-screen information (called ‘16:9 switching’) in the video signal tells the display or video monitor the correct way to display the image to ensure that it is not distorted.
It is important to match the aspect ratio of your images to the correct display ratio. Unless you alter the size of the image, a 4:3 image displayed on a 16:9 monitor will produce images which are horizontally stretched, whilst a 16:9 image displayed on a 4:3 monitor will be horizontally compressed.
The processes for altering the size of the image to fit the aspect ratio of the display device are called ‘Letterboxing’ and ‘Pillarboxing’. Letterboxing is the process of adjusting a 16:9 image to fit a 4:3 display. Pillarboxing is the process of adjusting a 4:3 image to fit a 16:9 display.
11.1 Pseudo 16:9
In addition to these main aspect ratios, there are some SD cameras which electronically produce a ‘pseudo’ 16:9 image by cropping the top and bottom of a standard 4:3 image.
Another method is for the camera to sample the central region of its sensor and scale that up to record ‘full frame anamorphic’ images, but the scaling is often of such low quality that the former method is more desirable.
You can also convert the 4:3 images in post production by zooming and cropping to fill the 16:9 frame – although this is not desirable as it will be at the expense of resolution and sharpness.
11.2 Optical Anamorphic 16:9
The use of an optical anamorphic adaptor makes a 4:3 camera shoot a true 16:9 image. However, it will reduce the effective focal range of your camera, create more problems with lens flare, effect the physical balance of the camera – and these adaptor are generally expensive.
11.3 Shooting in 16:9
Also referred to as ‘widescreen’ this is the most common aspect ratio of TV screens available today. All HD screens and cameras produce 16:9 ratio images. However, there are still many 4:3 TV sets in use today so there are some 4:3 considerations for the videographer when shooting in 16:9.
Broadcast companies will often shoot material in 16:9 ratio and then convert this to an in-between ratio of 14:9 for analogue transmission. This is so that viewers with a 4:3 screen will not experience pictures which are greatly reduced in vertical height (as a result of Letterboxing – see above). In this instance you should keep the action within the 4:3 centre portion of the picture. This is sometimes referred to as ‘Shoot and Protect’ or ‘Shoot and Protect for 14:9’. However, care should be taken in ensuring that the composition still works for 16:9 as the material might be simultaneously transmitted in both aspect ratios.
Many professional-level cameras provide framing guides in the viewfinder to assist the videographer in cross-ratio framing.