Canon Lens Zoom Brush

A zoom brush is a metal brush with typically around four to six pins located within the zoom assembly of many zoom lenses. It's a part of a system which makes it possible for the lens to know exactly what focal length it is set to at any given time. This is important information that is necessary for the autofocus to function properly.

Zoom brushes are not unique to one camera lens brand, and can be found in the lenses of a variety of brands including Canon, Nikon, and Sigma. Since the purpose of a zoom brush is to determine the position of the zoom ring, you will obviously not find them in any prime lenses. Some lenses will also utilize a focusing brush which works similarly to the zoom brush and gives the lens the position of the focus ring.

The zoom brush is typically connected to the zoom ring, so that as you turn the zoom ring, the zoom brush turns with it. It is typically held in place with one to two screws. The zoom brush is positioned so that the pins slide over top of a flex cable (also referred to as the zoom FPC) and covers the entire surface that the zoom brush is able to move across.

The flex cable has several lanes which correspond to the number of pins the zoom brush has. The lanes contain different patterns of exposed traces which allows a signal from the zoom brush to be passed to a specific lane on the flex cable. This system allows the lens to know which of the pins on the zoom brush is making contact with an exposed trace.

Often times, such as the one in the image to the left, the first lane has an exposed trace going across its entirety. This lane isn't detecting when the zoom brush pin comes into contact with it, but instead is what's sending the signal through the zoom brush. The other lanes are then the ones that receive the signal when a zoom brush pin comes into contact with it. The other lanes also have a specific pattern so that no matter where the zoom brush is, it will come into contact with a different combination of traces. In other words, it is able to produce a binary number and the number of bits it has corresponds to the number of lanes there are (minus the signal lane). The autofocus system is then able to use this number to know the exact position the zoom is in.

Some zoom brushes are designed to allow for a bit of adjustment in it's position, before being fully screwed down into place. As explained above, this is important so the autofocus system has the accurate zoom ring position for the focal length. Unless the screw holding the zoom brush in place comes loose over time, adjusting the zoom brush is typically not something that needs to be done unless it had to be removed during a disassembly.

The below procedure is based on the procedures described in the Canon zoom lens service manuals I have access to which range from the years 1982-1998.

Prerequisite: Have the lens disassembled as needed to have access to the zoom brush.

1. Set the zoom to its maximum focal length. Keep the zoom in this position for the remainder of this procedure.
2. On one of the lanes, you should notice a trace that has a very small cutout in the middle of it and that is different from the rest of the traces in that lane. It will typically be found at one of the ends of the flex cable. Also while it looks to be a part of the traces in that lane, if you look closely you should notice that the trace is isolated from the rest of the traces in the lane and do not connect with them.
   Loosen the screws to the zoom brush and adjust the zoom brush as needed so that the part of the zoom brush pin that contacts the flex cable falls within that small part that is cutout from the trace. In the example on the left below, this area is marked with "A".
3. Confirm the zoom brush is positioned correctly using a multimeter. One probe should be connected to the zoom brush. The other probe should be connected to the check pad. The check pad should be located a bit further up the ribbon cable. The image on the right below shows an example of what that may look like.
   If you are unable to find the check pad, you could also try connecting the second probe to the trace next to the cutout as long as there is space. Be sure not to let the second probe come into contact with the zoom brush.
   If the zoom brush is positioned correctly, there should be no continuity between the zoom brush
4. Once the adjustment is complete, apply screw lock glue to the head of the screw(s).

A potential alternate method to adjusting the zoom brush position can be achieved by using an external camera flash. The flash would need to be able to display the focal length on its display. The advantage of this method is wider range of lenses and brands this might work with. However, the accuracy of this method might be a bit more questionable.

Digital Canon Repair Blog