Difficulty in focusing!

[Bob Wright]

I have a Canon D50 with 100-400 f5.6 USM IS long lens bought in October 2009. Everything was fine until about a week ago when I found it impossible to focus on my subject (always tends to be birds). The image would lock-on when focusing then blur as soon as I pressed the trigger home. I rest the camera to factory default thinking that I had, perhaps, unwittingly touched something. Then again, earlier this week, I had an exposed view of a "Common Roller" at the top of a dead tree and try as I might I just could not focus on the bird. Above the sky was a clear blue (in Spain) and very warm. The battery was within an hour or much less of expiring.
The camera was set to AV at 100 ISO with A1 servo and spot metering. The lens was set to AF and 6.5 to infin.

Any suggestions about what might be going wrong?

[Papathanassi]

Re: Difficulty in focusing!

Try knocking it off A1 servo and try one shot, It could also be heat haze, worth a try any way?
Papa.

[colin C]

Re: Difficulty in focusing!

I would agree with Papa, but another thought would be: Reset the focussing point to centre point only. You may have all points activated, or center point plus assist points. Changing to centre point only may resolve it.

Another option would be to obtain focus, then press "focus lock" before taking the shot.

Another thought would be to clean the contacts on the lens and on the body, vigorously rub with a microfibre cloth. There may be a miscommunication between the body and the lens.

Final thought - does this happen with any other lens?

Colin

[Bob Wright]

Re: Difficulty in focusing!

Many thanks papa and Colin for your suggestions. Will now be checking the manual most carefully to make sure that I follow your recommendations, etc and then have some practice shots from my studio overlooking the mountains and lakes. With a bit of luck a wandering eagle or something might just drift into view! Will also have to check about a "microfibre cloth"; is this the same as comes with my specs?
Another thought, could the blurring be the result of high temperature or even heat haze picked up by the camera if not me/
Bob Wright

[jhdee]

Re: Difficulty in focusing!

Bob - whilst a micrfibre cloth is the same as you would use for cleaning your specs, the normal recommendation is not to use the actual cloth you use for cleaning your specs - it can and will transfer grease cleaned from the lenses of your specs defeating what you are attempting to achieve .... grease free contacts.

[colin C]

Re: Difficulty in focusing!

Bob

It could well be down to heat haze. You may hardly notice it, but it is a problem for large telephotos. At 400mm you lens is an 8x magnification and it will effectively compress the heat haze by the amount. In those conditions, manual focus.

Colin

[Bob Wright]

Re: Difficulty in focusing!

Thanks to all her offered support. Have not used "cloth" yet and will not unless absolutely necessary. I have checked the focus and re-set to single, centre spot. Now know how to change to multi-point in readiness for when the big birds pass over! have also take some distant shots of flowers, etc in the midst of bushes, growth and other distractions to check that camera is only using said centre focus. All seem OK at present. Will have to pop down to river bed tomorrow or Monday to try out for real. Many thanks to all. End results can be seen on my blog at http://birdingaxarquia.blogspot.com

Bob Wright