Question: I am trying to get DOF portraits of my daughter with my Nikon d7000, as per my [previous post](https://photo.stackexchange.com/questions/5971/can-i-get-dof-using-nikon-d7000-kit-lens-18-105-f3-5-5-6).

I have zoomed in to 100mm or so and tried to get a large aperture but as I zoom, the camera decreases f# to about 6.

I have also tried in aperture priority mode and still have the same problem lowering the f#.

What am I doing wrong?

Answer: 
The aperture range on your lens indicates the maximum aperture at the shortest end to the maximum aperture at the longest end. So, at 18mm you can have f/3.5 but at 105mm you can only have f/5.6. That's pretty typical for a kit lens, the much more expensive zooms will offer a constant aperture across the entire range.
Frankly, I don't care if this gets downvoted (and it seems I'm headed in that general direction with this topic), but how difficult would it be to zoom out to 50-60mm and get twice as close? Just to run an empirical experiment, you understand. Yes, getting faster glass will help (a *lot*), and there's no way you'll get nearly as shallow a depth of field with you current lens as you'd get with an f/2 or faster lens, but give it a shot. The only thing you can possibly lose is a bit of time. (It cost me a lot of money in film to learn what I've learned.)