Judging Depth of Field in a Modern Digital World
One of the variables with the most impact on depth of field is the aperture setting. But it can be hard to tell exactly what that impact will be since the lens is generally wide open when you look through it regardless of what the aperture is set to. Here are three options you should be aware of.
You press the shutter release to take a picture and the lens aperture stops down to what you've selected. After the sensor does its job to capture the image, the aperture returns to its fully open position. But at the same time the aperture stops down, the reflex mirror moves up, blocking the viewfinder image. And at the mirror doesn't move back down until the aperture returns to fully open. With the mirror down, light can't reach the sensor and is instead reflected up through the pentaprism and towards the viewfinder. With the mirror up so the light can reach the sensor, it blocks the viewfinder image. This is why the viewfinder goes dark the moment the shutter fires. But since the aperture doesn't stop down until the viewfinder goes dark, you can't easily judge depth of field using the viewfinder image. Either the mirror is reflecting light to the viewfinder and the aperture is wide open, or the aperture is stopped down and the mirror is flipped up, blocking the viewfinder image.
The traditional way to tackle this problem has been to press the Depth of Field Preview button on the camera body. This forced the lens to stop down to the aperture you chose without actually taking a picture. With this button held down, it was possible to look through the viewfinder to see the depth of field as it would actually be rendered. But while this does let you more accurately gauge depth of field, the image will also get significantly darker since the smaller opening lets through less light. Fully stopped down, the image will be mighty dark indeed. Many photographers dislike the DOF Preview control and end up not using it for this reason. It doesn't matter if the depth of field is thereby correct if you can't see it.
Out in the field, I've been known to pull my jacket hood up over my head, not for warmth of protection from the elements, but in order to block out extraneous light so I can have a fighting chance to see anything through the viewfinder. When you first go from a brightly lit area into a darkened room it takes a short while for your eyes to adjust. In the same way, if you can block out all the light other than what is coming through your camera's viewfinder, you can start to see things if you have a bit of patience.
This used to be the only way to see depth of field back in the film days. The moment we all moved to digital photography though, a lot of those old rules began to go by the wayside.
Digital provides for instant gratification. Take a picture, and you can immediately see the results on the camera back LCD. If you don't like the way it came out, you can hopefully still take another attempt before your subject or the magic lighting on it changes. This gives you a second chance if the horizon was crooked in your first try, if the image came out blurred or if you think you can do better for some other reason. This includes depth of field problems. There's no reason why you can't just guess at the proper aperture and see what you end up with. At that point, if you decide you need to you can adjust the aperture one direction or the other and shoot again. If at first you don't succeed, try, try again. You get the idea. Then just delete the images that didn't come out as planned. This can be a viable option if your camera doesn't even have a Depth of Field Preview button or if the lighting is just too harsh to see anything useful with button pressed and the lens stopped down.
Even newer cameras provide for a third option to check depth of field by means of Live View. The feature differs somewhat between Nikon and Canon, but both call it Live View. With Live View enabled, the reflex mirror raises up blocking the normal viewfinder. But since light can now reach the sensor, the camera electronics displays the rendered image on the LCD camera back in real time. That is, it replaces the traditional optical viewfinder with an electronic viewfinder.
If your camera has a movie mode to shoot video, Live View is the only viewfinder option available. In camera/photo mode, Live View can be used to see the image as the camera sensor sees it, complete with the impact of the selected aperture and other settings. So, rather than pressing the Depth of Field Preview button and looking through the viewfinder, you can press the Live View button and look at the camera back LCD panel.
Not all cameras with Live View allow you to see the effects of changing the aperture with Live View enabled. I believe all Canon cameras do, but only a few newer, high end Nikon models do. If you change the aperture with Live View enabled, it will still affect the final image when you press the shutter release, but the Live View display won't change as the aperture setting does. It seems rather shortsighted and stupid for Nikon to leave this capability out of most models, but so be it. It's really not the end of the world. All you have to do is press the Live View button again to exit Live View mode, change the aperture, and then press the Live View button again to re-enable the LCD display and check the result. Just keep exiting Live View, making the desired aperture adjustment, and then re-enter Live View to see what you changed. This isn't really all that dissimilar from repeatedly pressing the Depth of Field Preview button to re-check how things look. Still though, I'm jealous of my Canon fiends.
I'm not specifically advocating any one of these options for previewing depth of field. Live View is the state of the art method of course, but holding the aperture stopped down and powering the LCD display back for a prolonged period definitely consumes more battery power than the other options. When cameras first started popping up with Live View capability there was also an overheating problem that could the camera makers have since overcome with newer models. But it's great to have this option available. Having more choices is always better.