Why doesn't the focus stay intact after I change the lens setting to manual on my Canon 77D?

Question

My Canon 77D exhibits a strange behaviour with any lens:

1. I half press the shutter button to trigger auto-focus
2. I then change the lens switch from Auto to Manual, while holding the trigger button
3. Instead of staying intact, the focus suddenly changes and the picture is no longer focused

What am I doing wrong? Isn't switching to M from A supposed to lock the focus in place? I know I can still manually focus instead, but using the auto-focus is slightly faster.

I've experienced this with the Canon 24mm 2.8.

Answer 1

What am I doing wrong?

You are holding down the shutter button half-press while moving the 'AF/MF' switch on the lens. It seems somewhere between the "AF" and "MF" position some contacts are being made in the switch that is providing current to the AF motor in your lens, causing it to move.

Assuming you are using 'One Shot' AF mode (which is what you should be using to do what it seems you are attempting to do), all you need to do is:

• Half press the shutter button (or better yet, remap AF to the 'AF ON' button on the back of the camera so that the shutter button half-press only affects metering)
• Wait until the camera confirms focus by displaying the green dot in the lower right corner of the viewfinder
• Release the shutter button half-press (or the AF ON button)
• Move the 'AF/MF' switch on the lens from 'Autofocus' to 'Manual Focus'.

If you remove AF from the shutter button half-press and assign it only to the 'AF ON' button, you can do the same thing when in 'AI Servo' AF mode. As soon as you release the 'AF ON' button the camera will stop focus tracking.

There is a difference if your lens is the EF-S 24mm STM, rather than any of the EF 24mm f/2.8 lenses with USM AF motors. STM lenses have a 'focus-by-wire' system with no mechanical connection between the focus ring and the focus elements of the lens. When the manual focus ring is moved it sends electrical signals back to the camera which then uses the STM motor to actually move the lens' focusing elements, even when the 'AF/MF' switch is set to "MF." The focusing elements of an STM lens can not be moved when metering is not active. (Metering is active any time the exposure information is lit up in the viewfinder. Metering is inactive when the exposure information is not lit up in the viewfinder.)

Although there is no current 24mm 'nano USM' lens, for the purpose of completeness for others that might find this question and are using a 'nano USM' lens: Nano USM lenses are also all 'focus-by-wire'. They use a linear, rather than ring, USM motor to move the lens' focusing elements.

Answer 2

Isn't switching to M from A supposed to lock the focus in place

No! At least not on any camera i know of. Switching to manual tells the camera to shut off auto focus and disengages the AF motor so you can manually focus.

There may be a way to lock the focus after AF has focused with a menu command or button but switching to manual would not be it.

Seems the simplest way would be to manually focus and then not touch the focus ring again. OR - Take the photo when you have achieved the focus you want. Is there a reason you want to lock the focus but not take the photo? If you do not want the camera to refocus while in AF mode than do not continue to hold the shutter release button half way down. I.E focus by holding it down and after you have achieved the focus you desire release the button and the focus should not change. If the object you want to be in focus moves then you will need to repeat the process. In my camera if i do this then take the photo by quickly pressing the shutter it will take the photo with out re-focusing because i pressed the shutter button quickly and did not give it time to re focus. Meaning i believe there is a set time the shutter button has to be depressed half way down in order to activate the AF, pressing the shutter button all the way quickly does not give the AF time to activate.

One of the reason for AF is to let camera change focus in a changing scene or to track an object in the scene and keep it in focus.

Answer 3

Just let go of the shutter button once the camera has focused - no need to carry on holding it.

If you're moving the camera (even slightly) to switch from AF -> MF then having hold of the shutter button is going to potentially cause it to try and re-focus when you move the camera. If you're then disengaging the AF motor midway through that process you're going to get an out of focus shot every time.

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If you think there's a genuine fault then try using a monitor (or custom firmware if you understand the potential consequences and are happy to do so) for focus peaking and check for consistent changes.