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[KmH]

What camera is the lens mounted on?
What AF mode is set and what AF Area mode is set?

The AF module is in the camera, though some of the electronics, like the motor that drives the focus mechanism, is in the lens,

Mounted on an older consumer grade camera body might mean a less capable AF module too.

It's a consumer grade lens that when zoomed all the way out has a maximum aperture of f/5.6.

AF is done with the lens at it's max aperture, but in low light situation f/5.6 may not let in enough light that the AF system can 'see' sufficient contrast to work well.

Your camera user manual probably has a page that describes several scene types that any contrast-detect AF system will have trouble with.

 

[WhaleDaughter]

I recently purchased a Canon EF-S 55-250mm lens but have been finding on many occasions that it is very slow to focus and doesn't always focus when it tries too, when using it with Auto-Focus switched on.

On average, it takes around 20 seconds of focusing and I'm lucky if it will focus on 3/10 attempts.

Has anyone else experienced this with the same lens or a similar lens? Or know of a way if this can be rectified?

I have one of these and it does take a while to focus, and has a tendency to hunt. Kind of makes it a crummy choice for spontaneous moments. I also really hate that there's no stop on the focus ring when manually focusing. It just keeps spinning and spinning and spinning...


"Rule 408: Time is not the boss of you"

 

[Derrel]

I used one on a two-hour photo lesson I gave a couple years ago on a Canon Rebel-something (T3 maybe?) my student had...it was a bit of a hunter on focus...not a sure, positive, emphatic focuser on the camera it was on. I think it was a combination of the slow, variable maximum f/stop the lens has, and also a rather anemic AF module and processor in the low-end Canon.

It's not that uncommon to have a slow lens, like an f/4~5.6 for example, or an f/4.5~5.6 lens like Nikon's 70-300 VR-II model, that will have some hesitation or trouble when the AF target is only marginally clear and contrasty. Nikon's older 50-200 and 50-250 were also rather slow, unsure focusing lenses, especially on low-end cameras.

Rectifying this issue: In Nikon-land, dropping a $149 kit tele-zoom on the $5,000 to $8,000 class bodies was always a good remedy for me, compared with older cameras like the D40 and D70, which had weak AF modules and limited CPU bandwidth for the data processing, so there **might** be a remedy, that of moving up to a higher-class camera that has a stronger AF module and faster CPU to run the camera and to evaluate the AF data.

MANY times however, its a user issue. One of the biggest issues can, at times, be that of using the camera with Center-point AF ONLY; using ONE single AF point can put a huge performance hit on some weaker AF modules! Seriously, this might seem counter-intuitive, but the issue is that if you have a camera that has a 3-, or 6-,or 9-point AF system, at times switching to a group AF mode will increase speed, sureness, and minimize hunting, by feeding more, comparative data, to the AF module.

KmH in Post #2 really lays out the issues for you. What camera? What AF mode? Consumer camera? Focusing at f/5.6? Type of subject matter? All these things, and a few more, can come into play. My first suggestion would be to read the manual's AF sections very diligently, and then try experimenting, and if you have not done so, switch to a Group or Multi-point AF mode and see if that improves things.

 

[beagle100]

I still have the original 55-250, it's good (once used it for air shows) but the newer 55-250 STM is better.
Or get a used 70-200 f/4, 100-400, 70-300, etc.

Canon digital rebel XSi and old original 55-250

 

[table1349]

Canon EF-S 55-250mm f/4-5.6 IS STM mounted on Canon EOS 7D Mark II : Tests and Reviews | DxOMark

 

[beagle100]

Canon EF-S 55-250mm f/4-5.6 IS STM mounted on Canon EOS 7D Mark II : Tests and Reviews DxOMark

I'm thinking those using a 7D2 would probably not be using a $50 telephoto lens .............

old digital Rebel XSi with old original 55-250 ......... look Ma - no feet !