How can I get bokeh with an aperture of 3.5 as my max?

Question

I am trying to get DOF portraits of my daughter with my Nikon d7000, as per my previous post.

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 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.

f/2.8, 105mm

f/5.6, 105mm

f/8, 105mm

f/5.6, 200mm

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