e. diopter
This adjusts the viewfinder to the eyesight of the operator.

To set the diopter:

First rotate the turret so that there is no lens in front of the gate. It is easier to set the diopter without an image to distract you. An alternative way is to use a telephoto lens racked way out of focus with the aperture wide open.

The locking ring is now loosened by turning it counter-clockwise. This is the slotted ring, closer to the rubber eyecup.

Now the adjustment ring can be turned while looking through the camera. This ring is the one with the little screw mounted on the side and a white scribed line that can be moved up and down a scale with a “+” and “-.” While looking through the camera you will see different planes within the viewfinder come into focus as the diopter is adjusted. The groundglass is located the same distance from the lens as the film plane. The groundglass is etched with a field of grain, so when the grainy pattern is at its sharpest the diopter is set.

Feel free to take your time doing this. It can be easier to see the grainy pattern if the camera is pointed at a light source or bright subject. There is often dust visible in the viewfinder. This is not something to worry about, because it isn’t seen by the film. In fact, bits of dust on the groundglass can often help in focusing the diopter.

Once the grain of the groundglass is visible the diopter can be locked by turning the locking ring clockwise.