A rangefinder camera uses light to let the eye see through the lens. The amount of light determines the strength of the image.

Lens is composed of other lenses inside, where a mechanism is found. 

Diaphragm.

1.4 largest

2

2.8

4

5.6

8

11

16

22

32 smallest

Shutter Speed.

8

4

2

1

.5

.25

.125

etc.

ISO -- sensitivity of the sensor, when there is a lot of light we should use low sensitivity and vice versa