Depth of Field, or Why One Fence Post Came Out Sharp
Okay. Last lesson we worked through the f-number backwards logic, small number means big opening, big number means small opening, and I know that still takes a second to click. That's fine. It clicked for me too, eventually.
Today we're talking about what aperture actually does to your picture, which is depth of field. That's the term for how much of your photo is in focus, front to back. Some pictures have basically everything sharp, foreground to background. Some have one thing sharp and everything else soft. Aperture is the main dial that controls which one you get.
Here's the thing. A wide open aperture, something like f/2.8 or f/4, gives you a shallow depth of field. Thin slice of sharpness. Good for portraits, good for making one flower or one bug or one fence post pop out from a blurry background.
A closed down aperture, f/11, f/16, f/22, gives you a deep depth of field. Sharp from way up close all the way back to the mountains behind it. Good for landscapes, good for anything where you want the whole scene readable.
How I actually learned this
I've told you I learn slow but it stays learned once it does. Depth of field is a good example. I read about it, understood the words, nodded along, and didn't really get it in my hands until an accident.
It was February, cold, frost still on everything that morning. I've got a wood fence along the garden and the frost had done this thing on the posts I wanted to shoot. I set up, dialed in f/11 because that's what the book said to use for landscape-type shots, and I worked my way down the fence line trying to get the whole thing sharp, post after post after post.
Got home, pulled up the photos, and every single fence post was soft except one. One post, close to the camera, sharp as anything, with the rest of the fence dissolving into a nice soft blur behind it. I'd been trying for the whole fence and instead I got one post.
That bothered me for about ten minutes and then it clicked. I hadn't understood that depth of field isn't just about the f-number, it's also about how close you are to your subject. Close up, even at f/11, your zone of sharpness is thinner than you'd think. That one accidental post taught me more about depth of field than the manual did. I still go back to that photo sometimes when I'm explaining this to somebody, because it's a perfect example of a mistake that turned into the lesson.
What to actually do at home
Same as always, one dial at a time. Put your camera in aperture priority for this lesson, that's the A or Av mode on your dial. The camera will pick the shutter speed for you so you can focus completely on what the f-number is doing.
Find something with depth to it. A row of fence posts is genuinely perfect if you've got one, or a line of books on a shelf, or your kid's toys lined up on the carpet, anything with a near thing and a far thing and some stuff in between.
Shoot the same setup at f/2.8 or whatever your widest is, then f/8, then f/16 or f/22 if your lens goes that far. Don't move the camera between shots if you can help it. Look at all three side by side after. You'll see the sharp zone get thinner or wider right in front of you.
Pay attention to how close you are to the subject too. Get in close on something small, a coffee mug, a screwdriver on the workbench, and shoot it at f/4. Then back up ten feet and shoot something at f/4 again. The depth of field will look completely different even though the f-number didn't change. Distance matters as much as the aperture number does, and that's the part most people skip over.
One opinion, since we're here
I'll be honest with you, I think people chase the perfect aperture setting way too hard when the light or the moment is actually the thing that matters. I missed a moose crossing a meadow up Provo Canyon a while back because I was standing there adjusting my f-stop instead of just taking the picture. Moose walked off into the trees while I was still fiddling with dials. Got nothing.
A slightly wrong photo you actually took beats the perfect photo you missed. Learn this stuff so it becomes automatic, so you're not standing there thinking about it when the moose, or the kid, or the light on the ridge, isn't going to wait for you.
Before next time
Shoot that fence-post-style test, near to far, at three different f-numbers, and bring the best one in to show the group. If you don't have a fence, a row of anything will do.