Qualitative drawing of a 24-70 mm zoom lensMove the pointer sideways
Zone in focus, focused at 2 mLens groups moved by the slotted cylinderCone of the diagonal angle of view, at 1:40 scale against the lensThe lens and the body are cut in half by a vertical plane through the axis and seen from the side. In the model the zoom ring turns a cylinder with two slots. Each slot guides the pin of one lens group, so the two groups travel along the axis by different amounts and the focal length goes from 24 to 70 mm. From 24 to 70 mm the rear group moves forward, while the front group first moves back and then forward again. The diagonal angle of view narrows from 84° to 34°, and the depth of field changes too. The cone in front of the lens has this diagonal angle: its edge reaches the corners of the frame, and on the sensor plane it ends on the circle that touches those corners. The aperture ring closes the blades and widens the zone in focus. With Focal length selected, moving the pointer sideways changes only the focal length. With Aperture selected it changes only the f-number. The left and right arrow keys do the same. While Focal length is selected the zone in focus stays visible but dimmed, because it also depends on the focal length. The drawing is qualitative and comes from a simplified model with two lens groups.
