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Focus Point

Alan_Coles 12 178 11 Wales
30 Jun 2010 1:52PM
For some time I have been pondering about what i think are focus problems on my 50D. Attached is a picture I took on Skomer recently...looking at it in DPP I switched on the focus point indicator and noticed although the focus red spot was on a foreground flower the actual focus point was behind and to the right? Am I missing summat or is there something wrong?

Can't upload an image but here is a link to an image showing the problem http://www.flickr.com/photos/thevalleywalker/4748966066/

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Coleslaw 12 13.4k 28 Wales
30 Jun 2010 2:14PM
Did you focus and recompose?
The red square only shows which focus point you used, which is not necessary where you focused on.
Alan_Coles 12 178 11 Wales
30 Jun 2010 2:21PM
I don't think so.... I will shoot some stuff later and report back - thanks for replying.
Phil_Morgan 11 182 1 Wales
4 Jul 2010 6:22PM
You have E mail Alan Smile
Nick_w Plus
10 4.3k 99 England
4 Jul 2010 6:46PM
I don't know the 50d but on Nikons there is a focal point adjustment.

I used a ruler inclined to 45 degrees I then used a 6 inch nail in a cork perpendicular to the ruler. I focused on the nail and read off the scale, from memory I had back focus issues. I made adjustments in camera till it was spot on. After then it was incredibly sharp.

Oh and if using a zoom calibrate at the long end as this willbe the most demanding (reduced dof etc).


discreetphoton 13 3.5k 20 United Kingdom
4 Jul 2010 6:51PM
is that a 400mm lens? Have you checked that you were outside the minimum focus distance for the lens? Focal problems aren't uncommon, but that's a big shift, and I think that there may be some other things to rule out first.
cambirder 14 7.2k England
4 Jul 2010 7:05PM

Quote:Have you checked that you were outside the minimum focus distance for the lens?

I suspect that could be the problem. The 400mm f5.6 does not focus that closely and I suspect it has focused down as far as it will go which is why the nearest flower is OOF despite it sitting directly in front of the single focus point in use. Other pictures taken with that lens in your portfolio do not seem to show this problem.
Graywolf 10 1.0k United Kingdom
4 Jul 2010 8:53PM
I've just checked my 400 f5.6 on my 7D body just to be sure. If the lens is struggling to focus it may hunt, or refuse to fire the shutter or both. I imagine the 50D will be the same. Did you experience either or both of these phenomena. If so then I would certainly agree with the above comments. MInd you not having experienced them doesn't mean that that isn't the problem.
cambirder 14 7.2k England
5 Jul 2010 12:47AM

Quote:If the lens is struggling to focus it may hunt, or refuse to fire the shutter or both.

In One shot or AI Focus modes you can't shoot until focus has been achieved, but in AI Servo mode that is not the case and if you also have Focus search switched off you won't get a lot of hunting either, so looking through the VF you may be fooled into thinking you have achieved focus and happily snap away.
colin beeley Plus
15 1.2k 10 England
5 Jul 2010 8:30PM
i only ever use the centre one then re compose the image .
miptog 12 3.6k 61 United Kingdom
7 Jul 2010 12:28PM
With the focus/recompose technique its possible that by recomposing you have changed the distance between you and the subject, hence the shot can be out of focus. This is more likely with smaller apertures like f/2.8.

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