THE CONVERTIBLE LENS
As view camera equipment and lenses are comparatively large it can be a problem transporting them to a shooting location. To overcome this, convertible lenses have been designed for use with the view camera that allow the lens to be converted from one focal length to another by carefully unscrewing the front lens element. When this element is removed the lens is about 75% longer.
In this example the lens can either be used as a f5.6/ 240mm (denoted by the white numbers) or f12/420mm, denoted by the green numbers.
When the front lens element is unscrewed the focal length of the lens is changed from 240mm to 420mm but the trade off with this gain in focal length is a loss of lens speed, and the effective aperture is also altered from f5.6 to f12. These lenses have two aperture scales, one in white and one in green that relate to the specific lens configuration and the focal length of the lens being used.
Use care when removing the front lens element to convert the lens. Make sure there is always some means of catching the lens in case it falls. Placing one hand beneath to catch it while unscrewing it with the other hand works well.