Ghostly zoom and focus issues - help!

[photoSentinelPro]

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 (Ghostly zoom and focus issues - photoSentinel Time-Lapse System | photoSentinel Time-Lapse System) 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

 

[SCraig]

My suggestion would be to use some electrical tape to tape the zoom ring so it won't move after you get it set. If nothing else that will tell you for certain whether the zoom ring is moving or not.

 

[Derrel]

As SCraig suggested, I suspect the very common "zoom creep" is what is occurring. On MANY zoom lenses, the lens will change focal length, as gravity affects the zooming mechanism. This can happen on both push-pull zoom designs and also rotating ring designs. I have a Sigma 18-125 that has HORRIBLE zoom creep. Same with my Nikon Series E 36-72mm f/3.5 and 70-150 f/3.5 lenses....pretty noticeable zoom creep on all three of those lenses. Zoom creep is actually the proper word for this, not just something I made up. On some newer zooms, the manufacturers have built in a locking mechanism that locks the focal length to keep the lens as compact as possible when the lens is in transport or being carried.