There is always the possibility that I did something wrong when taking the picture, which would account for the differences in the crops. If that's the case, I would appreciate your feedback. I have to admit that, looking at the cropped pictures, it might look like the one with the filter on would have been totally out of focus, and not just the cropped part.
In this context, I downloaded in the links below the full images from where the crops were taken. You can judge for yourselves and, once again, do not hesitate to comment so as to improve my abilities, if that's what it turns out to be.
This first picture is the full image. Lens WITHOUT filter (the crop was taken from the top-right corner of the picture):
http://farm3.static.flickr.com/2336/150 ... abe5_o.jpg
This picture is the same, but this time with the UV filter ON (crop was taken from same area as pic above):
http://farm3.static.flickr.com/2023/150 ... 6468_o.jpg
Note that I did not change a thing between the two shots, except for taking the UV filter off very carefully so as not to move the camera from the tripod. But even if that would have been the case - that I would have moved the camera - then the picture without the filter should have been the one out of focus, because I took first a picture with the filter ON (in focus), then with the filter OFF (which thus should have been out of focus because of movement). I thought it might have been the wind that caused the blurriness, but then look at the railings next to the foliage...Just as blurred.
Also, note(a) the difference in sharpness of the bricks in the centre of the pictures and (b) the difference in exposure between the pics with the filter on (more washed out in my opinion) and without (more saturated).
Oh dear...my first day on this forum, and I feel like I opened a can of worms already. Gordon, is this out-of-topic?