I don't know about you, but I'm bad at math and other technical things like that. When I first started shooting I was always getting confused with the technical details. Well dealing with the focal plane sometimes confuses people but don't fret - it's really not. that. bad. What the heck am I talking about? Just a little something that looks like this...
So yeah, there are these focal planes in a photograph. Basically if the subjects in a photo line up on the same focal plane at the point of focus - then they will all be in focus... depending on the depth of field. Big wide open apertures like f/2 have a very shallow depth of field. What this means is in focal plane land, there is a very small area where things will be in focus. After such the point of focus drops off really quick.
That's why it's tricky to nail focus when shooting wide open. Now if you're shooting with an aperture that's smaller, it will have a more increased depth of field and you have much more of that subject in focus. Confused yet? That's ok here are some pictures...
I love shooting portraits (singles or couples) in wide open apertures, but it can be really tricky to get two people in focus with apertures like f/2. In the photo above I asked Andy to lean forward until his head lined up with Christine's. Yep, I totally put their faces/heads on the same focal plane. Now if he dropped back some, he would drop out of focus really quick. Likewise, if I happened to hit focus on their shirts, then that area would be in focus and not their faces. That's because of the wide open aperture and the very shallow depth of field.
So when I shot this image we were running out of natural light. Since I didn't want to use flash, I knew that I needed to stick with a bigger aperture. I could have shot this photo with f/2.8 however for the size of the group and how I have them posed someone would not be in focus. I ended up using a high ISO to get the look I wanted with f/4.5 and had just enough depth of field to get everyone lined up and in focus.
f/2.8 = shallow depth of field = focus on the cool Marilyn painting in a thrift shop. The clothes in the foreground were on a different focal plane so they would be blurred (just like I knew they would.) This composition technique is called framing the subject and works great when you want to highlight your subject.
In my last example I wanted to blur quite a bit of the background. For lighting reasons my couple had to be close to the background, so I shot in a wide open aperture of f/2.8. As usual with big apertures it was really important to line them up on the same focal plane if I wanted them both in focus. The result? My couple are both in focus and the green industrial wall behind them is more blurred, making them stand out.
So remember if you're shooting in wide open or want a certain look in your photos, keep in mind the focal plane to help get what you want in focus.