EF 75-300mm never achieves focus in AF mode

[jonwilton]

I having been using this lens on my Canon base camera since new, never knowingly damaged it and look after my camera.

On AF mode the lens never settles on a focus but is constantly whirring back and forth, never acheiving a focus point to shoot. in MF mode it continues to work fine and the smaller lens I have works fine in both MF/AF modes.

Is this fault repairable/worth it / doable at home or needs a specialist?

Thank you for any guidance,

Jon

[Tronhard]

Hi and welcome to the forum, Jon

The symptoms you describe are usually associated with the camera's focusing system not finding a focus lock. That could be because of low light, not enough contrast, or no object with a distinct shape or boundary to focus.

What I suggest is try pointing at something that is very diatinct and in high contrast to its surrounding and see if it focuses. That should tell us if this is the case.

[jonwilton]

Hi Trevor,

thanks for your help.

unfortunately on trying that theres no improvement. constant whirring back and forth and sometimes a knocking noise which sounds like the af is being obstructed somehow??

either way, it doesnt stop at a settled focus point, and shooting results in blur

[jonwilton]

.. the problem started in costa rica.. took lots of decent zoom af pictures before this and previous with no issues but this suddenly started for no aparrent reason                    

[Tronhard]

Thanks for your quick response.

To narrow down the possibilities, do you have any other lens we can test to check the camera's focusing?

[jonwilton]

i also have a smaller lens which works normally in af/mf.. no problem with that

[Tronhard · Accepted solution]

I seems to me that the lens may have suffered some issue during your time in Costa Rica, possibly from the ingress of dirt or from humidity as the lens has no weather sealing.  Unless the camera is under warranty, I would suspect that a repair would cost more than the lens is worth - it is arguably Canon's cheapest telephoto lenses.
If you were considering a replacement, I would suggest looking at the EF 70-300 MkII lens - a far superior optic, both in terms of construction and optical performance.  It also has very efficient image stabilization and super fast focus.  See: 70-300 Canon Lenses In-Depth Analysis - Canon Community.  you can get one refurbished from Canon at this link: https://www.usa.canon.com/shop/p/ef-70-300mm-f-4-5-6-is-ii-usm 

[jonwilton]

thank you for your help, this was my conclusion too unfortunately