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 Ghostly zoom and focus issues 
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Joined: Sun Oct 09, 2011 7:34 pm
Posts: 116
Location: Melbourne, Australia
Post Ghostly zoom and focus issues
One of our clients has a long term time-lapse unit that is exhibiting very strange behaviour. Across three photos taken at 30 minute intervals, the frame has zoomed out and the depth of field/focus has changed! Here (http://bit.ly/1bjlVdl) are five photos taken across the weird change, along with Exif data. First two photos (and previous) the focal length is recorded at 19mm, third photo is 18 and then last two photos (and following) are 12mm...

If you have any ideas what's happening with the lens, let us know!

Other bits and piece of information:
  • camera: Nikon D3100
  • lens: Sigma 10-24mm f3.5-4.5
  • focus: manual
  • camera is not pointing up, so gravity can't have pulled the zoom out
  • No one has touched the camera (that we know of)
  • Re: Exif data, the "distance to subject" keeps changing, but I'm assuming, given that (I'm pretty sure) the lens is in manual focus, that this just means the distance it is metering from. Or, might this indicate that it is actually in auto focus?
  • the enclosure is as weatherproof as you can without immersing it one metre under water, so there's no way a mouse or something has gotten in :)

The only thing I can think of is that there was some massive vibration next to the pole that vibrated the zoom and focus rings and turned them. Seems incredibly unlikely, but straws are all we have to clutch at! Of course, "the most obvious explanation is the most likely" so maybe someone did sit up there for half an hour slowly changing the zoom and focus...

Anyone able to shed any light on this spooky phenomenon?

Matt

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Mon Nov 25, 2013 6:10 pm
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Joined: Tue Aug 26, 2008 3:30 am
Posts: 824
Location: Sydney, Australia
Post Re: Ghostly zoom and focus issues
Had something similar happen with remote camera set up on roof of a tall building.
We figured it was RF interference (or something electronic/data burst telecoms) switching on and off.
Also double check the client has actually set manual focus - the problem described could be focus 'hunt' when camera is set to Auto :oops:
Hope you sort the problem.

EDIT: just re-read original post - is the zoom lens in manual mode?


Mon Nov 25, 2013 6:22 pm
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Joined: Tue Apr 29, 2008 2:48 pm
Posts: 1144
Post Re: Ghostly zoom and focus issues
I am going to bet money that the auto focus is on - that totally sounds like it and the changing distant to subject just seems to confirm that.

tt

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Tim T


Mon Nov 25, 2013 7:33 pm
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Joined: Sun Oct 09, 2011 7:34 pm
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Location: Melbourne, Australia
Post Re: Ghostly zoom and focus issues
The client says he's "97% sure" that it is in manual focus. It seems too big a change to be an auto-focus adjustment - you get a tiny bit of frame adjustment with auto-focus hunt but not 7mm. That and the fact that this is the only time it has done it. If it had been on auto-focus for the whole installation (months) then we would expect to see it more often. The mystery continues...

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Also Aussie distributors of eMotimo TB3 motion control head: eMotimo in Australia


Mon Nov 25, 2013 7:47 pm
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Joined: Tue Apr 29, 2008 2:48 pm
Posts: 1144
Post Re: Ghostly zoom and focus issues
yeah sorry just looked at the files not auto focus. So my second guess is a big jolt to the pole or surrounding area - so I'm assuming that the lens stayed in that final position of 12mm or did it change again? - I once had a zoom that was the older push pull style and it was slowly (and I mean slowly) zooming in as I was shooting some plants - in a closed room that I only went in at the beginning or the end of the shoot - it was eerie!

I guess I would look for evidence of a possible source of the jolt - maybe there was a crane that hit something (again grasping at straws) I guess when this shoot is over you could see how touchy that zoom lens is.

tt

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Tim T


Mon Nov 25, 2013 8:14 pm
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Joined: Tue Nov 24, 2009 7:55 am
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Location: UK
Post Re: Ghostly zoom and focus issues
Hi, looks like the camera has moved and is pointing downwards. If only the zoom had changed then one would expect the image size to change but the centre of the images to remain same.

The centre of image 1131am and 1201pm are the same - expected. Zoom19
The centre of image 1201pm and 1231pm are the same - expected. Zoom 18
The centre of image 1231pm and 1301pm has changed - not expected. Zoom 12
The centre of image 1301pm and 1331pm are the same - expected. Zoom 12

Between 1231pm and 1301pm the camera has moved downwards.


Load the images into Photoshop as layers, make sure all images are aligned. Draw diagonals from corner to corner, then turn off the layers one by one.
Hopefully you will reach the same conclusion :?:

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Sun Dec 01, 2013 10:39 am
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Joined: Wed Dec 14, 2011 1:56 am
Posts: 75
Location: Sydney - Australia
Post Re: Ghostly zoom and focus issues
I've experienced something similar with a Canon 1100D and the stock 18-55mm lens. It was mounted at a slight downward angle with a protective glass filter attached. Over a period of a month the zoom mechansim slowly moved out. I've found that the cheaper lenses tend to have quite loose zoom mechanisms and so now tape them down to prevent them moving. The scenario of a large vibration or hit to the pole seems the most plausible explanation as this could have moved a loose zoom mechanism.


Sun Dec 01, 2013 5:40 pm
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Joined: Tue Jan 11, 2011 8:12 pm
Posts: 121
Post Re: Ghostly zoom and focus issues
I always suggest to our customers to wrap the lens rings with gaffer tape.
Funny how many of them say "Of course, what a great idea!"
I wish all problems were that simple!


Sun Dec 01, 2013 6:39 pm
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