Critical Focussing Screen
Bolex Product Code :- None
All Non-Reflex Bolex H16 Round base cameras with a 3 lens turret made after 1937 have this device fitted as standard.
On top of the camera immediately above the upper turret lens position there is a device consisting of a ground glass prism and a magnifier that permits very accurate focusing of any lens that is swung in front of it.
Before using the critical focusing screen remove the dust cap that is screwed in front of the ground glass screen. Place the conical-shaped plastic eyepiece supplied with the camera on the top of the magnifier, with a lens in front of the upper turret position open the lens diaphragm to its maximum aperture and place your eye over the eye-piece. The Image given by the lens will then be visible.
It is now possible to focus the lens so that the subject is absolutely sharp. The magnifier enlarges the image ten times. When the lens has been focused the turret can then be turned to bring the lens into the shooting position. Care should be taken when revolving the lens not to accidentally touch the focus setting on it. The focus setting obtained in this way will apply to any other lens of similar focal length although each telephoto lens should be individually focused before use through the critical focusing screen.
To focus a lens, swing the turret to bring the required lens in front of the screen. Open the lens diaphragm to its maximum aperture, and place your eye over the eye piece. The image given by the lens will then be visible. Revolve the focusing ring until the subject is sharp, and then return the lens to the shooting position to shoot the scene.
The problem with the critical focusing screen is that to focus a lens correctly you have to crane you neck over the top of the camera to see into the eye piece. Another drawback is that you actually don't see the full image you are focusing but a circular image that cuts off the corners of the frame. Picture showing dust cap fitted over Critical focusing screen eye cup.
These problems lead to the introduction of the Eye Level Focusser which works with the already installed critical focusing screen to make the camera easier to use by moving the place eye piece towards the back of the camera and showing exactly what is seen by the lens with no cut off.