The Importance of the Focal Length

By Stanley Stern

It is especially important, to know the true focal length of a lens that you are using for cirkut photography. The proper amount of film to be pulled past the slit is determined by this. Ideally the scale on the camera should be used to focus the camera. This can only be done if the true focal length is known.

Most lenses have the focal length printed on the inside of the barrel in the front of the lens. This may not be the actual focal length though. The focal length must be measured to be accurately determined. One way to do this is with a device called a nodal slide. The lens is mounted with its nodal point over a pivot point. The lens is pivoted about its nodal point. The nodal point is the point at which all the rays of light converge to a point. The distance from the pivot point to the focusing screen is the true focal length of the lens.

When making a cirkut photograph, or any rotation panoramic photograph for that matter, you are actually taking a picture on the surface of a cylinder. The cylinder has a circumference ( C ) which is determined by both the focal length of the lens and the focus distance. The focal length, which is also the distance of the nodal point of the lens from the film plane when focused at infinity, is the radius ( R ) of the cylinder. The formula for determining the circumference of the cylinder is 2(pi)R. What this means is that the further the nodal point of the lens is from the film plane the larger the circumference of the cylinder or the more film you need to pull past the slit.

Each of the different gears supplied with the cirkut camera is used for a particular lens and focus distance. These gears determine the amount of film drawn past the slit. Gears have a specific number of teeth. The amount of film pulled is fixed for each gear. This is why the scale is used. The scale shows the distance the nodal point of the lens should be from the film plane for that gear. The circumference of the cylinder is then exactly correct for the nodal point to film plane distance. The lens distance should be synchronized with the gear.

When making cirkut photographs it is best to have the equipment working properly. The synchronization of the pull of film is important to the sharpness of the picture. The synchronization is a function of the gear and lens to film distance. An accurate measure of the lens focal length is the first step to having the synchronization correct.

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