Showing results for 
Show  only  | Search instead for 
Did you mean: 

Telefocus and movie servo

Rising Star

I have a Canon T8i and a 55-250mm lens.

My camera is set at Movie Servo and continuous autofocus.

I have a problem with autofocus at long telephoto distances. If I aim the camera at something long distance, it focuses easily, but if I shift the camera to something closeup, my lens will slowly focus on the subject, but if I then lift the camera and aim at something far away again, the lens refuses to focus. I have to start zooming back out before it will begin to focus on that far away subject.

Just for the heck of it, I slapped a 50mm prime on my camera, and the movie servo worked flawlessly.

I did a little reading in the manual this morning, and read that movie servo on my camera doesn't work when zooming or if the image is magnified.

Even though I wasn't technically "zooming", I think my camera has a problem when "zoomed". The servo mode kicks in when I start zooming back out.

I also read that some cameras have a movie servo sensitivity scale. Mine isn't that sophisticated. It's either enabled, or disabled, period.

Also, just for the heck of it, I slapped on the 18-55mm kit lens that came with the camera, and didn't have any problems.

It's just with those long telefocus lengths, at least with the 55-250mm lens that I have.

Is this normal?

Steve Thomas



The autofocus system needs to have sufficient light and a subject with good contrast. Frequently when zoomed in the subject fills more of the sensor but may not have sufficient contrast, like edges. This may be your issue, particularly when you say that wider angle compositions focus OK.

John Hoffman
Conway, NH

1D X Mark III, Many lenses, Pixma PRO-100, Pixma TR8620a, LR Classic