Any question answered...

A question to do with Optics:

As the iris of the eye gets larger to adjust to low light levels. Why does this not create a very shallow depth of field for our vision (or does it?).

It does. Wikipedia seems to be a bit confused about whether the maximum aperture of the human eye is f/3.2 or f/2.1 http://en.wikipedia.org/wiki/F-number#Human_eye but you might not have noticed because it has to be pretty dark for your pupil to expand that much.

And for animals who can really adjust their iris's from pin point small to very big (like cats) do they get a change in focal lengths?

Focal length is a separate thing from pupil size. The only direct link is if the expanded pupil allows light into the eye through the edges of the lens which might have a different focal length to the centre, making it impossible for the eye to focus perfectly. The edges could contribute more light to the image than the centre, so in that case including them could change the effective focal length of the eye.

I think the animals with non-circular pupils are interesting. By contracting their pupils to a vertical line instead of a point, a cat probably retains the distance clues from limited depth of field even in bright light. I wonder if that's an advantage to them, or just a side effect of the mechanics needed to allow such extreme ranges of pupil area.