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THE CELLULOID IMAGE

IMPROVE YOUR CAMERAWORK

F Numbers and Focal Lengths

If you look at the lens of a cine camera (or almost any photographic lens), you will see small letters on the mount something like Som Berthiot A 123456 1:1.9 f=25mm. This indicates that the lens has been made by the firm of Som Berthiot; it has a maximum aperture of f1.9 and a focal length of 25mm. The number 123456 is a number unique to that lens, so that the lens can be readily identified - you may need to quote this serial number for guarantee repairs or for insurance. Any extra reference like 'A' for example will probably refer to the type of construction of the lens (number of components or quality).

Focal Length

So, the focal length of a lens - 25mm in this example of a standard lens for 16mm (and maybe 9.5mm) cine cameras - is the distance between the lens and the film when the image of a very distant scene is sharply focussed. The standard lens on a std or Super 8mm camera will be around 10 to 13mm. Because the 16mm camera lens has to cover a film image which is around twice as wide as the 8mm image, the angle of view (see diagram below) for each lens will be about the same. In other words the actual scene reproduced on the 16mm camera with 25mm lens will be about the same as the 8mm camera with 13mm lens. Incidentally the 16mm, 9.5mm or 8mm refers to the overall width of the cine film which includes the perforations. The actual image will be about 10mm on 16mm film compared to about 5mm on std 8mm film.

In fact the focal length of the standard lens on a cine camera is about twice or two and a half times the width of the image on the film. If we want to include more of the scene on the film when at the same distance from the scene, we can use a wide-angle lens. Alternatively if we want to include just a part of the scene on the film when at the same distance from the scene, we can use a telephoto lens. In fact one lens can do all this - a zoom lens which has a variable focal length. Other older cine cameras often had a triple turret where the standard, wide-angle and telephoto lenses were all mounted on a rotating turret on the front of the cine camera. If the standard lens on a 16mm cine camera has a focal length of 25mm (1 inch), the wide-angle lens will have a focal length of about 15mm and the telephoto lens will have a focal length of 50mm (2 inch) or 75mm (3 inch).

The focal length of the lens determines the angle of view, and therefore the amount of the scene that will be recorded. (see diagram below). The wide-angle lens takes in a wide scene; the normal lens takes in only a part of that scene; the long focus or telephoto lens takes in still less - all at the same distance from the scene. But however much or little is taken in, it still fills the frame of the film. So when the film is projected, objects filmed with a wide-angle lens look smaller on the screen than the same objects taken from the same distance with a normal lens. Similarly objects filmed with a telephoto lens will look much larger on the screen than the same objects filmed from the same distance with a normal lens. Using a zoom lens, it is easy to see this effect through the viewfinder when the focal length is changed from wide through normal to telephoto.

The wide-angle lens A takes in houses, church and trees. The standard lens B takes in church and one tree only. The telephoto or long focus lens C takes in only the church. The angle made by the converging lines is the angle of view.

It will be obvious, however, that though the scale of the picture is changed, the perspective is not. The picture seen on the screen will be the same whichever lens is used - the only difference is that you see more of it on a smaller scale, and in another you see less, but the part you see is enlarged. To change perpective it is necessary to physically move the camera closer or further away from the scene.

Aperture

The word aperture is used to denote the size of the hole through which the light passes, measured in relation to the focal length of the lens. So if the focal length of the lens is 20mm and the hole (which is normally in between the glass components of the lens) is 10mm in diameter, the aperture is said to be f2. (20 divided by 10). So if the hole is made smaller (closed down) to a diameter of 5mm then the aperture will be f4 (20 divided by 5). As the hole gets smaller and lets less light in, so the aperture number gets larger (f4 lets in less light than f2).

Apertures are commonly referred to as stops, and changing from f2 to f4 is called stopping down. Conversely, changing from f4 to f2 is known as opening up. f2 is quite a big stop and would only be used indoors or outdoors when the light was bad. For average scenes in good sunlight with Kodachrome for example, the stop might be f8. The size of the stop is usually continuously varied by means of an iris diaphragm, which is made up of thin metal leaves between the glass components of the lens and opens and closes rather like the human eye. Some simple cameras (like the 9.5mm Pathescope 'Pat' or 8mm Kodak Brownie) just have a series of small punched holes (Waterhouse stops). With the punched holes only the exact apertures can be set, with an iris the aperture may be set at intermediate values for more accurate exposure.

F numbers run in series, the most commonly used in the UK being: f2 f2.8 f4 f5.6 f8 f11 f16 f22. Each f number represents an aperture passing half the light of the previous f number. So opening up from f8 to f5.6 would give the film twice as much light. In technical terms the amount of light passing through the iris hole is proportional to the square of the diameter. In fact if you square all this series of f numbers you will see that the results are doubled with each change of f number. Anyhow just remember that each change of f number gives half (stopping down) or twice (opening up) the light of the previous f number!

An exposure meter, once the film speed and shutter speed is set, will indicate the aperture or f number required for good exposure. If this sounds really complicated just use a cine camera with auto exposure - this will give good results in most normal conditions. Enjoy the filming and don't forget to set the focus as well!!

First published March 2004 ........ Last updated: 05 January 2008 ........ cel05.htm .......... ©MMV-G.L. Newnham