I use 2.8 or 3.2 with my 50mm.
It is a lot more likely that the eyes will not be in focus at lower then this. I would rather get sharp eyes and fix anything else in photoshop. If I have a subject that can hold really still, like adults or my friend's chameleon then I will shoot at a lower 1.8. It's really personal preference and how still you can stand when you shoot.
For focusing, one way I do it is I set my camera on single spot focus where the camera only uses the center focus point. Then I focus on the eyes and then adjust my composition in the frame then snap the picture. You have to be careful with the lowest f stops because you are moving the camera after you focus but for most others it works great. Really you are only moving the camera and inch or less horizontally or vertically. The focus should only change if you move forward or backward.
I would be interested in hearing better ways. I am still learning how all the focusing options on my D70 work.
Best Wishes,
Melissa
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