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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?
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
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.
It all depends on the lens, not so much the camera. My suggestion, buy a better lens, there's several that'll do the trick for only a small amount of cash. Take a look at Nikon's Lens, and here's a few specific ones that might catch your attention. Sorry, I'm a bit more familiar with Canon's line-up, but they seem to be adequate.
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.)
Depth of field for a portrait (which will be at a medium distance) depends basically on two things: the aperture and the size of the subject on the sensor. Therefore, to minimize the depth of field you want to (a) get as close to the subject as possible without being so close that her features are distorted so that (b) you can get the widest possible aperture obtainable with this variable-aperture lens.
There's still going to be a fairly wide depth of field with this lens. For example, if you have to be zoomed out to 105 mm then your maximum aperture is f5.6. Suppose the subject's near eye is 4.00 meters from the camera. If you focus on that eye, the calculated depth of field (using, for reference, a circle of confusion of 0.03 mm diameter) will lie at distances between 3.77 m to 4.26 m. This will put her entire face and head in focus, with the near eye and the plane of the cheek the sharpest. To make this depth of field shallower, just focus a little closer. (You will need to focus manually to do this.) If you were to focus at 3.85 m, say, then the depth of field would range from 3.64 to 4.09 m. If you have a clear line of sight to your daughter, everything from 3.64 to just under 4.00 m will be air, so this part of your DoF range is irrelevant. The parts of your daughter in focus (back to 4.09 m) will include her face and any hair framing it, but everything behind that will be (relatively) out of focus.
The price you pay for this workaround, besides having to make the simple manual adjustment to the focus, is that no point on the subject's face will be perfectly sharp. However, you will be able to visually isolate her face from the background better than before. If the resulting image still does not meet your needs, you need to find a lens that offers a better combination of longish focal length and large maximum aperture (small f-stop). In the Nikon (and Canon) lines the least expensive options are the 50 mm f/1.8 and 85 mm f/1.8 primes. The former is quite cheap while the latter will attain half the minimum depth of field and is an excellent focal length for portraits.
