Is it normal for EF 24-105/4L IS to autofocus when switched to manual focus?

Question

I'm using a Canon 6D mark II. I recently got a Canon EF 24-105mm f/4L. I wanted to try using manual focus, so I switched the lens to manual focus, but it keeps auto focusing when I half press shutter button. I have reset my settings, but it still keeps doing the same thing. Is this normal? Could there be a problem with my lens or camera?

Answer 1

Are you certain that the camera actually changed focus while set to MF? With an IS lens on the camera, you’ll often hear a noise when you half-press the shutter release, but that noise is the image stabilizer kicking in, not the AF system.

Try setting the lens to MF with IS switched off. This time you shouldn’t hear or see anything when you half-press the shutter release.

Answer 2

If the switch is set to MF and the AF motor is engaging there is something wrong with the lens.

Depending on:

• The specific lens model and how much it is worth versus how much it will cost to have it repaired
• Whether it is still covered under warranty
• Whether it might have been damaged in a way that would preclude warranty coverage even during the warranty period
• Whether you are willing to use 'back button' AF to work around the issue

You can either decide to have the lens repaired, you can decide to replace the lens, or you can decide to use the lens as is and work around the problem using custom camera settings to separate AF from the shutter button half press using 'back button focus'. For Canon cameras that do not have an 'AF-ON' button, you can use the menu to set the 'AE-Lock' button, the one with the little six sided star [*] next to it, to act as an 'AF-ON' button.

I recently encountered an EF 85mm f/1.8 that needed some internal cleaning (well beyond dust that does not affect the image). In the process of disassembly/reassembly the plastic prong of the actual electrical switch soldered to the lens' main circuit board beneath the external AF/MF switch was damaged. It can be moved, but the lens must be partially taken apart to do so. The prong is no longer long enough to engage the guides on the back of the external switch that sits on top of it. I chose to set it to AF and reassemble the lens. Since I use back-button AF most of the time anyway, if I want to manually focus the lens, I only need to not press the 'AF-ON' button before taking the photo. The EF 50mm f/1.4, EF 85mm f/1.8, and EF 100mm f/2 all use the same basic construction including the same less than elegant interface between the external mechanical switch on the lens' exterior and the actual electrical switch underneath it that is directly soldered to the main PCB. It's very easy to damage the switch when assembling the lens.

The switch on the EF 24-105mm f/4L IS is different, but my guess is that the AF/MF switch has been damaged and the lens is stuck in AF, regardless of the position of the external switch.