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Old December 6th, 2006, 09:41 AM   #1
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strange focus problem

I've been struggling with the XL2 this autumn and have today found the reason. I first saw anomalies with a sigma 50-500. Tried the 100-400 and finally the standard 20x. All show focus wobbling in and out. The camera is on manual focus, stabilisation is off, no presets, and is on a heavy tripod.
I've put a quicktime movie on www.gigrin.co.uk/focus.html and would much apreciate your views.

Many thanks

Chris
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Old December 6th, 2006, 10:09 AM   #2
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Chris, this is quite common when you are using large ef-lenses at the telephoto end. It's called air waves and I don't think you can get rid of this without loosing the long focal lenght.
You shouldn't get this when you use the 20x lense, not so heavily even at the telephoto end. Are you sure that you got this air waves when using the 20x?
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Old December 6th, 2006, 11:16 AM   #3
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Per, the movie was taken with the 20x, the wall was only about 10ft away and the temperature is only 8 degrees C.
Would air waves present themselves in this is instance?

I've previously used an XL1 with these lenses and have only ever seen anything like this on hot days and at long focal lengths.

When the feed is viewed directly on a monitor it looks as though focus is being lost for a moment then recaptured - all very odd.

Cheers

Chris
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Old December 6th, 2006, 11:21 AM   #4
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Per is right, this is not showing a shift in focus but almost certainly warm air waves filmed at extreme distance between the lens and subject (can happen over snow as well as over terrain in warm claimates).

It does show up sometimes with the 846mm range (35mmsLR equiv) of the 20X lens, but is far worse when super telephotos in the 2,340mm-6,240mm ranges (X7.8 factor), such as when 300mm, 500mm, and 600mm and 800mm lenses are used.

The best advice is to try to avoid atmospheric conditions or parts of the day when air-waves are atb their most frequent or active, and above all, try to get closer to your subjects.

In your short clip, it looks like the side of a house or building. This waving affect may be the warm air escaping from inside the wall, or the sun's rays hitting the damp or cooler wall and causing evaporation, etc.
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Old December 6th, 2006, 11:54 AM   #5
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I'll have another look tomorrow but it does throw me as the building is a stone barn and it was cloudy with showers today - definitely no sunshine to warm up the air.
Heat haze is something I have often seen on warmer days and at long focal lengths but never before in winter.

I'll see what the morrow brings and come back to you.

Many thanks for your help.

Chris
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Old December 6th, 2006, 12:02 PM   #6
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Evaporation off a dry stone wall is happening continually, even during overcast conditions, and especially during and after damp conditions or rain.
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Old December 6th, 2006, 04:43 PM   #7
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Quote:
Originally Posted by Chris Powell
I've been struggling with the XL2 this autumn and have today found the reason. I first saw anomalies with a sigma 50-500. Tried the 100-400 and finally the standard 20x. All show focus wobbling in and out. The camera is on manual focus, stabilisation is off, no presets, and is on a heavy tripod.
I've put a quicktime movie on www.gigrin.co.uk/focus.html and would much apreciate your views.

Many thanks

Chris
Chris - got exactly the same problem here - see

http://wildlifeinpixels.com/clips/blur1Mbps_Stream.mov

So you are not alone!

Cheers

Andy

PS - saw you on TV tonight - was thinking of comming to see you sometime!
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Old December 6th, 2006, 04:55 PM   #8
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Quote:
Originally Posted by Per Johan
Chris, this is quite common when you are using large ef-lenses at the telephoto end. It's called air waves and I don't think you can get rid of this without loosing the long focal lenght.
You shouldn't get this when you use the 20x lense, not so heavily even at the telephoto end. Are you sure that you got this air waves when using the 20x?
Per - I raised this effect on the DVdoctor forum this lunchtime - see http://forums.dvdoctor.net/showthread.php?t=40378 - I was wondering if it was air turbulence or a mirage type effect.

I shot some curlew footage the other day using the Sigma 500mm on my xl2 - clip at

http://wildlifeinpixels.com/clips/blur1Mbps_Stream.mov

and the whole footage is rendered useless because of a repetetive blur that effects the frame every 8-12 seconds.
The field the curlew were feeding in was partially flooded.

I had the sneeking feeling it was air turbulence but just wanted to check I hadn't got a cam fault.

Thanks for the reassurance!

Andy

PS - love the muskox footage - I'm jealous as hell!
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Old December 6th, 2006, 05:08 PM   #9
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Andrew, I have no idea what can cause this, I think this is very strange! I have never experienced any thing like this with the XL2.
As you are telling, all was set to manually? To me it looks like an auto focus issue, but if the lens was in manual mode this should not happend!

Maybe some others will help you out.

Thanks for your replay about the muskoxen. I will soon start to shoot with a XLH1 HD camcorder, hopefully even better pictures. I'll put some footage online in a while.
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Old December 7th, 2006, 01:07 AM   #10
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Quote:
Originally Posted by Per Johan
Andrew, I have no idea what can cause this, I think this is very strange! I have never experienced any thing like this with the XL2.
As you are telling, all was set to manually? To me it looks like an auto focus issue, but if the lens was in manual mode this should not happend!

Maybe some others will help you out.

Thanks for your replay about the muskoxen. I will soon start to shoot with a XLH1 HD camcorder, hopefully even better pictures. I'll put some footage online in a while.
Oh Per, I was happy until I read your post!!

Manual everything - the Sig 500mm is your 300's bigger brother but in Nikon fit from my D200 stills camera.
It mounts to the xl2 with an Optex mount converter.
Xl2 set to manual.
So the cam isn't trying to talk to the lens or vice versa.

I'd had the same sort of effect but less exagerated the day before when filming Pied Wagtails feeding on a rotting muck heap. The effect was constant and identical to Chris's clip but only effected the image above the top of the heap - heat generated by the rotting process is what I put that down to.

When I shot the curlews the wind was rather gusty, the sun was at a really low angle at 3pm and there were quite a few big puddles of water between the cam and the subject - all in all the conditions for picture taking were not ideal.

The only other thing I can think of, and I'm really clutching at straws here is that there are two telecom towers and a main electric rail track within a couple of hundred meters of where the cam was - surely its not possible that they could effect the focus motors in the lens - wouldn't that level of interference show up as banding on the tape?

I suppose the only answer is to try again and see what happens!

Thanks

Andy

PS - Chris - sorry to have hijacked your thread
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Old December 7th, 2006, 01:27 AM   #11
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I will not let you out in the dark Andrew! Even if it seems to me like some extraterrestrial activities caused this!

My question will be how many times have you experienced this?
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Old December 7th, 2006, 04:48 AM   #12
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Andrew - there is usually a vast difference in shooting with a 300mm and a 500mm on the XL2, and I've noticed that there is often a lot more air-waves ect shown on the tape footage if I shoot with a 600mm Nikkor rather than a 300mm Nikkor.

To test that your XL2 body is working OK, just film some tape footage of a static object such as a large tree or park wall (not house wall) with the 500mm lens fixed to a sturdy tripod locked-down. (Focus on the central column base of the tree, not a moving branch). After manual focus on the bark, click the Exp. Lock button on the side of the XL2, then start the tape by using the Canon WL-D4000 Wireless Controller. Now change the lens to your 50-500mm and set it to the widest 50mm setting (or preferably a prime fixed 28mm or 50mm lens). Repeat the procedure for more footage. Change again to the 20X lens and take more footage at the widest, middle, and longest zoom settings.
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Old December 7th, 2006, 06:18 AM   #13
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Quote:
Originally Posted by Per Johan
I will not let you out in the dark Andrew! Even if it seems to me like some extraterrestrial activities caused this!

My question will be how many times have you experienced this?
No one lets me go out in the dark Per!

Just the once - it was the last time I used the cam and I've not had chance to try since as the weather in Cheshire has been foul and I've been waiting to take delivery of my Miller t'pod - which should be this afternoon.

With a bit of luck I'll give it a whirl tomorrow and let you know what happens!

Thanks Per

Regards

Andy
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Old December 7th, 2006, 06:25 AM   #14
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Originally Posted by Tony Davies-Patrick
Andrew - there is usually a vast difference in shooting with a 300mm and a 500mm on the XL2, and I've noticed that there is often a lot more air-waves ect shown on the tape footage if I shoot with a 600mm Nikkor rather than a 300mm Nikkor.

To test that your XL2 body is working OK, just film some tape footage of a static object such as a large tree or park wall (not house wall) with the 500mm lens fixed to a sturdy tripod locked-down. (Focus on the central column base of the tree, not a moving branch). After manual focus on the bark, click the Exp. Lock button on the side of the XL2, then start the tape by using the Canon WL-D4000 Wireless Controller. Now change the lens to your 50-500mm and set it to the widest 50mm setting (or preferably a prime fixed 28mm or 50mm lens). Repeat the procedure for more footage. Change again to the 20X lens and take more footage at the widest, middle, and longest zoom settings.
Got that - will do though it'll just have to be a comp between the 500prime and the 20x canon - my other Nikkor lenses are DX no aperature ring and I cant control the exposure at all.

Thanks

Andy
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Old December 7th, 2006, 06:41 AM   #15
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Sure looks more like the focus is going in and out than it does "air waves"
Air waves can happen anywhere there is a more than 18-20 degree temp difference. It could be 30 below and a 18 or more temp difference could show up particularly in telephoto shots 2X normal or greater. Air waves like heat off of black top in the summer at 105 degrees in Arizona. Or heat off of Ice Flows in Antartica on a 25 below day. Both have potential for heat waves.

I could swear I saw the image actually go out of focus on the Curlews.
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