PiGraham wrote:The most noticable difference between those images is change in contrast. In the 'blurry' one the dark areas are not very dark. Could it be light on the window or lens as the sun moves?
If you right-click on those images and see them in full resolution, you can see that the focus has changed substantially. (And it gets worse over the day.) I'm less concerned with the contrast change (as the sun has moved a bit), though the changes may be related.
The camera is behind a window and is shielded with a hood to prevent reflections from the inside making their way into the image.
As far as the lens distorting, that's a possibility. If you compare the two images you can also see a shift of the whole image, as well as a non-uniform smear of the image (directional, not just unfocused blur).
I did look at the full size images.
Note that a soft image is not the same as poorly focused image. Poor focus will give an soft image, but other things can produce similarly soft images that refocussing will not improve.
As an example, 'soft focus effect' can be acheived by putting a lightly diffusing filter over the lens (cling film is sometime used) . This does not alter the focus, but produces a softened image.
I'd look at scattering on the window and in the air, which will soften an image and reduce contrast.
If the plastic lens housing gets hot it could distort, but it seems unlikely that it would be sufficient to notice.
The Lens-sensor distance is small, but it's the proportionate change that matters. Any thermal expansion of the tiny housing will be tiny.
If this is a focus problem you should find that you can eliminate the effect by refocussing the lens later in the day. If that is not possible the effect is not lens focus.
You could try a shroud on the outside of the window.
Monitor temperature to look for correlation with temp while eliminating lighting changes.
If you have a widow facing away from the sun late in the day I suggest you set the camera there for a test.
Another thing you could try is to use a camera phone to take pictures of the scene at the same time of day from the same spot and see if they show something similar or not.