Depth of Field in Photography
Depth of field refers to the amount of the object that is in focus (the depth that is in focus). When you look through the camera’s viewfinder on a close – up shot you may notice that part of the object is in focus and part is not.
You can change the focus using the lens so that the front, middle or back zone of the object is in focus. You will, however, not normally be able to see the entire object in focus at the same time throughout the viewfinder. However, what you see in focus and what the camera (and film) see in focus is not the same thing. Luckily we can arrange matters so that, even if we can’t see everything in focus at the same time through the viewfinder, the camera and film can. In a final image the amount of the object that is in focus is very important. In general we want all of the object in focus.
On your camera lens, you have f-stops. What the f-stops do is open and close an iris diaphragm inside the lens that allows more or less light inside the camera (this is called varying the aperture). If we have the camera lens set on f-2.8 or f-4 or so, we’re letting a lot of light into the camera: the hole that lets the light in is open wide. If we have it set on f-16, f-22, f-32, then we’re letting a lot less light into the camera: the iris diaphragm is closed up very small. In order to take the picture if we have the lens set on f-22 or f-16 (so there’s not much light coming in), we have to take a longer exposure to get the same image (keep the shutter open a longer time). If we had it set on f-8, letting a lot of light in, then we would be able to use a much shorter exposure time for the same image. The less light you let in, the more of the object is in focus.
The less light you let in (the smaller the aperture), and the longer exposure you need, the more depth of field you will have and the more of your object will be in focus for the camera and the film. On the other hand the more light you let into the camera, the less depth of field you have, the less of your object will be in focus. Again, most of the time with three – dimensional objects we are concerned with having all of it in focus, so that most of the time with this system we will be shooting if possible with the f-stop at f-22 or at f-16. There is little point to going beyond that; even if your camera goes to f-32, as I understand it, f-22 would be the best choice for us.
The smaller an object, and the closer you are to it, the less depth of field you have available, so that, for instance, in microscopy, where they’re taking pictures of bacteria and really tiny objects on a glass slide, their depth of field is almost non – existent; it’s a single plane. In our case here, where we’re pretty close to an object, depth of field becomes an issue. Again, most of the time we will solve this by setting the f-stop at f-22 and adjusting the shutter speed until we get the correct light meter reading to allow us to take the picture. Most often, that shutter speed will be one second or so, but of course you’ll have to check for the right combination of f-stop and exposure time yourself when you are taking a specific photograph.
Commercial photographers will go to extremes to obtain greater depth of field; they may have an exposure of a minute, or sometimes a number of minutes long in order to have a large object or a tabletop composition all in focus. This kind of long exposure requires very good equipment, absolutely no vibration, high light levels and lots more experience and theoretical knowledge than can be dealt with in this book. With a 35 mm SLR camera you are generally be dealing with a one second or so exposure for the system we are dealing with. Dan Gordon points out that tungsten films will handle exposures of 30 seconds or more without problems.
I described earlier how we could deliberately use depth of field to arrange things so that our rear support stand is out of focus and our object is in focus, and here’s an example: let’s say that I had a rather flat object, and it was on a rod projecting from a stand at the back of the drop shadow shooting surface and I could see some of the stand behind the object. I might set the camera on f-8 or f-4, and reduce my depth of field so that only the flat plane of the object was in focus, making the visible parts of the stand go strongly out of focus and thus blend in with the background. We would now use the depth of field preview button to see what the camera is actually seeing.
If we increase the local light levels, then this allows us to use a faster shutter speed with a small f-stop (small f-stop equals better depth of field), and that offers a little more flexibility. This is one of the reasons why professionals often bring the lights in close. The closer you bring the lights, the more light, and hence the more control you have (more or less) of the depth of field; the more light you have, the better depth of field you can have at a given shutter speed. This does however have drawbacks. A photographer friend of mine who does a lot of food photography told me one time about shooting a bowl of strawberries and ice cream. Basically they had a four – sided translucent white box around it and were shooting straight down at the bowl. The lights were all about a foot away from the bowl of ice cream and fresh strawberries. They went through 50 bowls of ice cream, because every time they put one in to take a shot, the heat from the lights would melt the ice cream almost immediately, so it was like, "Quick, quick, move now, shoot and do it again – let’s have another bowl."
Again, for our drop shadow box I suggest sticking to the formula given for awhile and having the lights at some distance from the object; that’s what allows us to use our mirrors and it is the mirrors that make the system excellent. After awhile, experiment with bringing the lights in closer and see what you think of the approach.
How to use the Depth of Field Preview button
The depth of field preview button is usually on the front right of the camera. You push it in order to see what the camera and film are really going to see, and to find out what’s in focus and not. Again, what you see through the viewfinder is not what the camera and film see.
So, if I am looking at an object, and I’ve focused on it, and I’ve set the camera lens at f-22 perhaps, and perhaps it’s at a one second shutter speed, depending upon what my lighting is like, I will still see most of my object out of focus through the viewfinder. To see what the camera is seeing, I press the depth of field preview button. Everything immediately gets darker because I’m now seeing what the camera’s seeing through this little tiny hole – f-22, not much light coming in; however, with the depth of field preview button pressed, I will see whatever is in focus, even though it’s dark.
Begin by focusing about one third of the way back into the object. Because what you can see darkens quite considerably when you press the button (and it is hard to see if the object is in focus or not), the way you deal with it is to look through the viewfinder and establish some bright spots on your object, some light streaks, or glints of light at the front of the object that are out of focus, and at the rear of the object that are out of focus. Look at the edges where you placed your side lighting to define and frame the object against the background to find such bright glints of light. Now press the depth of field preview button, and if those sharp light spots come into focus, you know your whole object is in focus. So the depth of field preview button allows you to see what the camera sees, and that’s really, really useful
Standard Focus Position (1/3 in)
A general rule of thumb that works most of the time is to use f-16 or f-22 and to focus through the viewfinder to one third of the way back into the object. That means that when you look through the viewfinder, a point one third of the way back into the object is in focus. You can check to see how much is really in focus using the depth of field preview button but most of the time this ‘one third back’ rule works out well.
What you see is not what you get
When you look through the lens, as I said, you don’t necessarily see what the camera sees; another thing you don’t necessarily see is how much image area you’re actually going to get on the film. Some cameras give you more surface area than you see through the viewfinder, and some cameras give you a little bit less surface area than you can see. If you’re using a camera that you’re not familiar with, I recommend taking a sheet of gridded paper, writing numbers on it, and taking a picture of that grid. When you get it back from the photo processor, and after it’s slide mounted, have a look and see how much you’ve gained or how much you’ve lost, because you need to know exactly what you’re going to get on the slides. The Nikon F2 and F3 are apparently the only cameras around which actually show you same amount of image through the viewfinder that will appear on the film. You should also take into account that if you’re shooting slides, after the transparency goes inside the slide mount, you lose some of the area around the edges of the shot; the slide mount will crop your image a bit. This is why you don’t have an object too large in the shot, too close to the edges of the frame.