In most cases, changing a lens will not require adjusting the back focus of a lens. But when a lens does not hold focus at both ends of the zoom range, you may need to adjust the back focus. By adjusting the back focus, you are changing the distance of between the pick-up device and the rear element of the lens. Technically, it’s the distance from the vertex of the rearmost element of the lens to the focal plane, but we’ll keep it simple. Note that not all lenses have a back focus adjustment.
Before starting, put the camera on a tripod and adjust your camera’s viewfinder so it is in sharp focus. Ideally, you’d want a test pattern chart (looks like a dart board) to be at least 75 feet from the camera. Otherwise, as far as possible. If you don’t have a test chart, Use a page from a magazine.
- Set the iris to manual.
- Set the zoom to manual.
- Open the iris to 1.4 or its widest aperture. If the illumination on the test chart is too bright for the open iris, reduce the light or move the chart to a darker area.
- Turn the zoom barrel to extreme telephoto.
- Focus on the chart.
- Set the zoom to wide angle.
- Loosen the back focus ring retaining knob.
- Adjust the back focus ring for the sharpest focus.
- Repeat steps 4 through 8 until focus is consistently sharp.
- When it is, tighten the back focus ring retaining knob to secure the ring.
Note: Most lenses are at their sharpest focus at about a middle iris position like F5.6.