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: 
You can't zoom to f/3.5 at 100mm on your lens, your lens will do at best f/5.6 at 105mm.

You can however get a shallow DOF effect quite easily, for example by shooting at f/5.6 at 105mm, with your subject quite close (say 6ft) and you background say 10ft behind the subject. 

See this [set of photos](http://www.flickr.com/photos/noirs/sets/72157625541149359/with/5289107008/) I took with a Nikon D70 (much older predecessor to the D7000!)

You can see the shot at f/2.8 has a shallower depth of field than at f/5.6, but at f/5.6 you can clearly see the subject in focus and the background blurred, you can even see that at f/8. (They aren't all sharp, I was on a tripod, long exposures, and caused the camera to move when I released the shutter, i used a 5s timer after that..!)

At f/5.6 at 200mm the effect is even more pronounced.

I'm sure at f/5.6 at 105mm you could do even better than I did, getting closer to the subject, and having the background further away.

[![DSC_9335](https://farm6.static.flickr.com/5123/5289106404_9bea3bd4d6.jpg)](http://www.flickr.com/photos/noirs/5289106404/ "DSC_9335 by AlexBlack, on Flickr")

f/2.8, 105mm

[![DSC_9336](https://farm6.static.flickr.com/5282/5289107008_442592ed7a.jpg)](http://www.flickr.com/photos/noirs/5289107008/ "DSC_9336 by AlexBlack, on Flickr")

f/5.6, 105mm

[![DSC_9337](https://farm6.static.flickr.com/5287/5289107568_ac1d72314e.jpg)](http://www.flickr.com/photos/noirs/5289107568/ "DSC_9337 by AlexBlack, on Flickr")

f/8, 105mm

[![DSC_9341](https://farm6.static.flickr.com/5285/5289108290_0a6ed29e4f.jpg)](http://www.flickr.com/photos/noirs/5289108290/ "DSC_9341 by AlexBlack, on Flickr")

f/5.6, 200mm
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.