The same image, before and after applying lens effect.
You can find this filter through
Filters->Glass effects->Apply Lens
After applying this filter, a part of the image is rendered as through
a spherical lens.
“Apply Lens” filter options
Parameter setting results are interactively displayed in preview.
Scroll bars allow you to move around the image.
Keep original surrounding: The lens
seems to be put on the image.
Set surroundings to Background color:
The part of the image outside the lens will have the
Background color selected in Toolbox.
Make Surroundings Transparent: The
part of the image outside the lens will be transparent.
Lens Refraction Index: Lens will be
more or less convergent (1-100).