af point not working

[VirtualAdept]

I'm sorry if this has been covered before, I did have a look but couldnt see anything.
The far right af point on my 30D mostly refuses to focus at all, constantly hunting for a lock but failing, going from one extreme to the other amd back.
Its worse on my sigma 18-50, though to be fair, that lens is back focussing, but it does the same to a lesser degree when using the nifty fifty.
Is this a body problem, dirty af sensor, faulty hardware etc?

Im due to get the sensor cleaned before long, and im wondering if perhaps the af sensor needs cleaning also, so id appreciate any advice you could give me.

Mads

 

[33L]

the outside AF points are no the best (compared to the center AF point) I dont know about the sigma lens but the nifty is not exactly the most accurate at focusing to start with. To make sure that enviroment is excluded from the equation what are you trying to focus on. need something something grided across the frame and well lit, Also make sure you are not focus near the minimum focus distance for said lenses too.

 

[VirtualAdept]

Thanks for the reply.
Its not so much that the focus is out (though thats true of the sigma whatever af point I use) its more that it'll go from one end of its focal range and back hunting for a lock and failing, where any other point will lock very quickly.
Its also only the one point, not as you might expect, both outside points.
Ive tested it on various subjects in various light with invariably the same results

 

[33L]

well it seems that you have covered the basics, may there is some dirt on the focus screen thats not quite obvious in the viewfinder. maybe have a word with you local camera shop if they can give it a good clean out.

 

[Canon Bob]

....may there is some dirt on the focus screen thats not quite obvious in the viewfinder.

The light path to the AF sensors doesn't go near the focus screen Lee, it goes through the mirror and down to the sensor via the sub-mirror.
Maybe there's some contamination on the mirror (visible) or the sub-mirror (not easy to see)

Bob

 

[33L]

The light path to the AF sensors doesn't go near the focus screen Lee, it goes through the mirror and down to the sensor via the sub-mirror.
Maybe there's some contamination on the mirror (visible) or the sub-mirror (not easy to see)

Bob

yup put screen instead of mirror, i.e the sub mirror! thanks for the correction

 

[VirtualAdept]

The light path to the AF sensors doesn't go near the focus screen Lee, it goes through the mirror and down to the sensor via the sub-mirror.
Maybe there's some contamination on the mirror (visible) or the sub-mirror (not easy to see)

Bob

Would that ne easy to clean/ get cleaned?