So, here's a tool that I have found quite useful lately. It's called Simple DOF and it help you determine how much of your subject/s will be in focus. This is critical to understand while shooting portraits of more than one subject.
To illustrate... let grab an image...
So my girl is in focus. My boy... well, there goes the picture... he's not in focus. The boy is about 2 feet further from the camera than the girl.
Clearly I intended for both to be in focus. But it appears that we are giving some precedence to the girl here. I need to explain what I could have done to avoid this error.
First of all, because it was in the evening and I wasn't willing to push my ISO up (it is at 125 for this shot), then I had to use a larger aperture (f3.5 here). So I should have pulled out my DOF calculator (Simple DOF). I punch in that I am roughly at 34mm on my D300s camera (51mm on a full sized sensor), I was roughly 5 feet from my subject in focus (girl). The calculator tells me that my DOF is 0.85 feet. That's 10 inches. In other words, everything from 5 foot 2 inches to 6 feet away from camera will be the sweet spot of focus. Because the boy was more like 24 inches away, he falls in the out of focus zone.
So what could I have done differently? Well, I could have placed them equal distance from the lens. Pull the boy closer or move the girl further, or simply repositioned myself to be equal distance from both. Well, 10 inches isn't much room for error in estimating distance.
But here is an example of the same area with the same fstop. Notice both subjects are in focus here.
Well, that was a lot of talk... but hopefully you've learned something about calculating the dof. Google DOF or depth of field and read up a bit. It is very critical. I will end with this one where I shot 23 people and needed them all to be in focus. Fortunately, I had my ipod with me with Simple DOF on it, I used it, and it worked out. I was about 35-40 feet from the subjects, f2.8, and 70mm. That gives me around 7-10 feet of focus. I also tried to place everyone semi-equal distance from the camera by pulling in the sides and pulling forward the people on the top row. And it was nice to still have some background blur to contrast with the focused people.