What is causing this breathing-like lens motion effect?

Question

I was shooting some video this week with my Canon Rebel T3 (1100D). This camera only has one video mode, which has auto focus and auto exposure, but I have the AF switch on my lens switched off and I set the focus manually. I was shooting with a tripod and zoomed in. When I reviewed my footage later, it was almost unusable, because ever couple of seconds there is some sort of shift in the lens, the focus goes a little blurry, and the zoom changes slightly, and then it goes back to where  I had it set. The effect looks as if the lens is "breathing", like the lens is literally expanding and contracting, although I think that term may have a slightly different meaning that what I am experiencing. I am not moving the lens or touching the camera at all. Auto focus is turned off, and I am not zooming, so there should be no motion in the camera. Is this caused by some sort of image stabilization gone wrong? Or is there some other techinical glitch that it might be?
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Answer 1

First you said you "zoom in" then you said you didn't zoom in/out at all. I'm not sure if you zoom the lens during recording or not. But if you did, it could be like Cicopo said. Most photography lens are not parfocal.
Here is the excerpt from Lensrental.com
"Does The Lens Breathe?
A lens that breathes changes the size of the image slightly when your focus from near to far or vice-versa. Most photography lenses do. If you look at the Specs we post for various lenses, any lens that has “rear element focusing groups” will definitely breathe, sometimes dramatically. Lenses with a “floating element” (used to improve close up sharpness) will also breathe.
Is The Lens Parfocal?
Some lenses are parfocal, meaning that zooming does not change the focus point. All cine zooms are parfocal, but very few photography zooms are — for the same reason that they breathe, and for lower costs. If you want to zoom while filming, its important to make sure the lens you choose is parfocal, and remember, most are not.
The following lenses are constant focus while zooming. If its not on this list, it is not parfocal.
Tokina: 11-16mm f/2.8
Canon: 17-40 f/4 , 16-35 f/2.8 , 70-200 f/2.8 Non-IS
Nikon: 17-35 f/2.8, 24-70 f/2.8 AF-S , 70-200/2.8 VR Mark I (not the Mark II)
Micro 4/3: Panasonic 7-14 f/4
Standard 4/3: Olympus 11-22 f/2.8-3.5"
Full article here: http://www.lensrentals.com/blog/2011/02/photo-lenses-for-video
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