Which aperture is best for portrait photography?

I've been taking pictures of couples/families, etc and I'm having trouble with which f/stop to use. Sometimes, both people are at the same distance from the camera (they are next to each other), but the camera seems to focus on only one person. DOF is supposed to be in distance from front to back, right? And not side-to-side? I ussualy shoot on Aperture Priority Mode and my camera is a Nikon D40, with a Nikkor 18-200mm lense. It seems as if I need to use at least f6 or more (f8, 9...) for both people to be in focus. However, this does not always blur the background as much as I would want. Could this be because the D40 only has 3 focus areas? I asked a professional photographer and she told me f4 would be fine... but once on the computer, I see that someone is out of focus... Also, when shooting two or more people, where on the subject should I look the focus area? Any help would be good! Thanks!

Focus on the eyes..

http://farm3.static.flickr.com/2164/2168...

Canon 5D with Canon 100 mm Macro f 2.8 at f 4 at 1/180, ISO 400.

Had there been two persons, I would have used a slightly smaller aperture--but not that small--and I think I would have made sure they were standing at the same plane--wonder if it would work..

You can shoot with your lens at two stops down. It will also give you the sharpest image.

Be sure you place your focus point on the subjects eye and that will ensure the best focus. ... even if you have to manually focus your camera .. although with practice you will be able to use the auto-focus area in the right spot and then recompose

The aperture and DOF also depend on how far from your victims you are. Focus on the eyes. F1.4 - F16 are all great what focal lenghts are you using and how far are you from your subjects?

A few photography classes might be helpful.

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When you shoot multiple faces in a portrait, they really should be approximately in the same plane.
Try f11.

For taking portrait photography,the best apertures at f/11 or f/16 for the sharper image,once you using Zoom Lens or Tele Lens, you will use a closer image for the portrait.if you used bounce flash ,an aperture at f/4 or f/5.6 to blur the background.Good Luck to you.

Victims: I love that one.

Another thing to think about, if you are using your super-zoom at or near the WA end, you might be getting some lens distortion, especially near the edges.