Great idea to get in close on the eye - it works for me. I would suggest however handheld with extension tubes is going to make things a bit tricky and probably numerous shots would have to be taken before the focus was really nailed unless you got lucky of course. I do think however you have the eye nicely focused, my only slight reservation is that possibly the blacks are a little bit too dense.
Hi Bob. Good sharpness within your DOF - especially for HH in natural light. I like the cool patterns in the eye. Marina's suggestion for a vertical crop is intriguing - you might give it a try as an alternate presentation.
This is a case where the shallow DOF works quite well. The eye looks nice and sharp and since it's the main focus of this image, I would lighten it up quite a bit and maybe boost the contrast so it really grabs the viewers eye.