It’s all just chunks of space in focus. These excercises show you how much.
With my f/1.2 and f/1.4 lenses I basically worship at the altar of shallow depth of field; bang my gong for the gods of bokeh. But, I do also occasionally stop. Well, stop-down actually [stop-down being the technical jargon for making one's aperture smaller]. It’s good to know what sort of chunk of space will be in focus when you do select f/8. So here are some simple ways of figuring it all out.
Shoot an aperture test-strip
The galleries below have in them a bunch of shots of the same subject shot at all the full-stop apertures on the lens. This is a great way to actually see how much more depth of field you get with each aperture down the scale. Better to do it on a tripod. I didn’t have one on me at the time so the shots aren’t all in the same exact register, although I did focus on exactly the same spot each time, which is the main point of the excercise.
Choose a subject that suits you being to easily see the depth of focus. Lines of posts or pillars, lines of tiles on a wall… etc etc
If you don’t have an older lens, where all the apertures are moveable in single, whole stops, then check how your camera is set up. Most of the DSLRs these days, out of the box, have the increments in exposure [exposure steps] set up in thirds of a stop. You also usually have the choice to set them up in halves or whole stops. I’d suggest leaving the camera at one-third increments as it allows for fine-tuning of exposure and all you need to remember is that three clicks of either aperture or speed is equal to one, full stop of exposure.
Get into the habit of using the depth of field preview
On most dslrs and pretty much all film cameras I have ever used, there is something called a depth of field preview. This allows you to see the amount of depth of field you are going to get in your scene. It takes some practice to get used to but once you have found it and used it a little, you’ll swear by it in certain situations.
On the D300, it’s the top left of the buttons you see around the throat of the camera
Things will get dark
When you look through the viewfinder of a camera you are looking through at the maximum aperture of the lens. This allows the most light in, to give you the brightest possible view. When you press the shutter-release button, a switch in the camera is mechanically connecting with one on the back of the lens to stop the lens down to its working aperture; i.e. the one you set. Using the depth of field preview button allows you to stop the lens down to that working aperture and actually see the dof you will get. The viewfinder gets darker, of course, as you are now looking through the working aperture. But you’ll see that the areas behind and in front of the plane of focus will become sharper. Try it at f/16 to really see the dof.
Macro: hold it down whilst focusing
This is particularly useful if you are working, say, in macro mode when even at small apertures [e.g. f/11] you have very shallow dof due to the degree of magnification of your subject. I often find it useful -although it takes a bit of dexterity – to hold down the dof preview button as I am focusing. When the dof is so shallow, and with the dof preview held down, you can actually see the sliver of focus moving across your subject and can therefore place it in the optimum spot.
That’s it for now. Any questions, drop me a line.
I’ll be making video lessons on various topics relating to depth of field as soon as my new assistant starts in a week or three. The next one will be ‘focusing by thirds’.