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OK with SLR AF if the lens is not too far from focused the camera can work out how out of focus the lens is and in which direction to turn the focusing ring to achieve focus. The longer the focal length of the lens the shorter the depth of field and the greater the precision required ot focus it.
With a very long lens it is possible that if the lens is very out of focus, say focused @ 3M and you want to focus on something 100M away it cannot work out if the lens is focused too close or too far away. the way to overcome this is for the camera to decide to sweep the lens through its focusing range in the hope it can find something to focus on. This is often referred to as sweeping the focus or hunting.
On some cameras this hunting is an automatic feature on others it is something you have to enable. If you use a long lens to follow birds in flight you may not want the hunting as if you track the bird and it does a turn then you may suddenly find the AF point no longer on the subject. In that condition it may be better for the lens to not change focus rather than go on the hunt for AF as it could just be second till you get the AF point on the subject. So if you have sweeping/hunting off then if the lens is a long way from focus you need to manually focus till the Af gets a contrast subject.
And if you were following birds from far away to close this could explain why your AF was off in the bright light as it was not expecting to do a sweep of all focus settings if it saw nothing to focus on. Have a look in the AF custom settings in your manual. Not at home so I cannot look at my manual to see what the correct Canon term is.
It is possible that on the 70-200 the focus range you were using was in the acceptable range while on the 400 it may be too much of a difference near to far.
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I can't see it myself in the manual, John.
I have posted a link to 5d2 manual in first page. Feel free to have a look, John.
Is it page 178, cfn III - 1 set to 'focus search off' ?
Page 178 The canon title is Lens drive when AF is impossible.
Sorry my terms were not the canon ones. I think you have CFN III-1 set to focus search off.
If not it was worth a try.
*** Edit yes that is it***
Sorry, John. Do you mean I should set it to 'off'?
Ps: just checked. Mine is 'ON'.
Err sorry I would set it to on, but sounds like you are already there, so sorry no idea. It was what fooled me when I tried a 7D so I thought it was the same issue.
Back to square one then.....
Cheers for trying. Appreciated.
Psst! Try manual focus, with pre-focusing, like they did in the olden days.
Yeap, you go and try that on puffin inflight, CB.....
My Nikon D3 would focus on the Puffins that were passing us side-on and on the ground or even when they were slowing down and almost landing but it just wouldn't keep up with the ones coming towards us.
It was the first time I'd tried shooting birds in flight with my D3 except for seagulls down my local beach and they don't fly as fast as Puffins do lol.
I even tried pre-focussing the lens where I thought the birds would be to save the auto focus working too hard but to no avail.
I was disappointed with my results when I got home and phoned Mark Davies as he's an expert on wildlife photography with the D3 and he went over my camera settings with me.
I discovered I had the "Focus tracking with lock-on" set to OFF in my menu system so basically I was trying to track Puffins with focus tracking turned off doh!, not something I'd recommend lol.
He said it should be set on short for birds coming towards me and long or off for side-on shots.
I googled "birds in flight camera settings" and found some interesting forum comments on this subject and it seems I also had a few other camera settings wrong.
The light was particularly harsh though and up at the Wick the grassy background was very similar to the bird in contrast, it was difficult to follow the birds by eye yet alone with the camera that day, the worse light I've ever had on Skomer so there's hope.
I'll have to go back again for another try now won't I?
I have also experienced what your talking about.
Not entirely sure why it happens but def find that bright harsh conditions can fool the auto focus system.
If it was hot I have also found that the heat waves given off from the floor can influence auto focus and the further from the subject the worse the results.
It could be that due to the bright light, the finer details of the background are illuminated and the camera is constantly searching to lock onto something..... where in a duller light the camera only has the bird to focus on as the background is less defined...
I have even found this happen on non moving subjects in a really harsh light at times, and you can here the af hunting to aquire a lock.....
I pretty much aim for overcast weather now and where possible try to ensure the light is behind me....
Sorry I can't be of more help, but don't think there is anything or setting that can be adjusted to assist with this issue... if you find out there is then let me know
Quote: "Focus tracking with lock-on" set to OFF
Ei? What is the point of that setting then?
Quote: I'll have to go back again for another try now won't I?
I haven't looked on mine yet. Will do that tonight.
Cheers Lee. Glad it wasn't only us.
Apparently Cole it sets the speed of the continuous focussing and once I've initially focussed on the bird it will then use whatever surrounding focus points I've selected to follow or "track" the bird to keep it in focus for me.
I had it set to 21 points which was probably right, (I think there's also 9 and 51 points too) but I had the tracking turned off under a different menu setting.
I'm not sure if it would've solved the problem but it may well have.
Too complicated, Geoff, I will go back to point and shoot, I think....
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