Why is my Canon 500D with kit lens having trouble focusing at higher focal lengths?

Question

I have a Canon 500D with the kit lens. Recently I have observed that its not able to focus at higher zooms but it works at min zoom. To get it to focus, I have to zoom out or recompose and it works. Why would this happen?

Answer

If this is the f/3.5-5.6 lens, then as you zoom out the max aperture is reduced ( f/3.5 at 18mm, but f/5.6 at 55mm). Autofocus will struggle in lower light at the f/5.6 end.

You can try to find something high contrast to focus on, or use manual focus if you have to.

If you focus at 18mm then lock focus and zoom in, that won't necessarily still be in perfect focus at 55mm.

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