The Depth of Field Challenge
Depth of field is the distance of sharpness in depth from close to far. It is not an on/off, either/or part of sharpness. Depth of field changes continuously from absolutely sharp to absolutely out of focus with a continuous range of nuances in between. With close-up work, it will be important how sharp the background appears, not necessarily whether it is actually sharp or not. Just because a background is out of focus does not mean it is unimportant. Also, deep versus shallow depth of field should never be an arbitrary decision. Choose what is appropriate to the subject and what you want from your photo, as shown in Figures 4.6 and 4.7.
ISO 400 • 1/100 sec. • f/2.8 • 12–35mm lens (MFT)
Figure 4.7 In this image, greater depth of field gives more emphasis in the composition to the setting and environment around the flower.
ISO 400 • 1/6 sec. • f/11 • 12–35mm lens (MFT)
No matter what you do, depth of field is always relatively shallow when you’re up close. Because of this, many photographers think that they have to stop the lens way down to f/16 or f/22 in order to get a good picture. That’s not true. In fact, that may cause problems for you that can reduce the possibility of you getting a great shot. Depth of field is a lot more than whether or not you shoot at f/16.
To fully understand depth of field, it is good to understand the three major factors that affect depth of field as you are shooting: aperture or f-stop, focal length, and distance to the focus point.
- Aperture: The aperture that you choose for a given shot has a big impact on depth of field. Small f-stops give more depth of field, while large f-stops give less depth of field. This gets a little confusing because of the way numbers are shown for apertures. A large f-stop will show as a small number such as f/2.8 or f/4. A small f-stop will show as a large number such as f/16 or f/22. The reason the numbers work like this is that they are actually fractions. Just keep this in mind: Small numbers mean small depth of field and large numbers mean large depth of field.
- Focal length: The next two chapters deal with focal length and close-ups with much more detail than I can give right here. However, it is important to keep in mind that wide-angle focal lengths give more apparent depth of field whereas telephoto focal lengths give less. This is one reason you see different focal lengths for macro lenses, i.e., choosing a different focal length changes the depth of field you get.
- Distance to focus point: Depth of field changes with your distance from your focus point, increasing as you get farther from your subject and decreasing as you get closer. In fact, as you move in closer to your subject, depth of field declines rapidly. When you are at true macro distances, there is nothing that you can do to get more than a fraction of an inch of distance that is sharp. This is a very good reason why you need to pay careful attention to where your focus point is.
Looking at Depth of Field Changes
In the photos you see in Figure 4.8, the basic composition does not change, but the choice of aperture does change, which then affects how you see the composition. This is an important exercise for photographers to do if close-up and macro photography is something they really want to get better at. Doing this at least once to get a better feel of the craft of choosing an f-stop will teach a photographer more about depth of field then anything else I know.
Figure 4.8 This series of images of a little wishbone bush flower changes only in the aperture used. Notice how the depth of field changes; compare not only the extremes, but also the more subtle, yet still important, changes from f-stop to f-stop.
ISO 200 • f/as shown • 1/varies depending on f-stop • 60mm lens (MFT)
The difference in depth of field, and the look of the background, is quite dramatic when you compare the widest f-stop at f/2.8 to the smallest at f/16. However, if you look closely, you’ll see there is a change in the relationship of the subject to the background with every change in aperture.
One time when I was writing for a photography magazine many years ago, I wrote about changing your f-stop to affect the appearance of the background and how even the change of a single f-stop could make a difference. The editor got an angry letter from a reader that said I didn’t know what I was talking about because it was only if you change from a very wide f-stop to a very small f-stop that you would actually see any difference. Obviously he had never done this exercise! Those differences from f-stop to f-stop can be very important to an image because they affect emphasis within a composition as well as whether a background is distracting or not.
So how do you know what aperture to choose? First, you need to consider if deep or shallow depth of field is more important for your subject. Second, take the picture and look at the image on your LCD. Take two or more and change the aperture if needed. With practice, you will start to see differences that matter as you change your aperture.
Using Deep Depth of Field
Since depth of field is always shallow close-up, using the adjective “deep” for depth of field might be pushing it a bit. Still, sometimes it is appropriate to use deep or at least deeper depth of field than others.
Deep depth of field allows you to emphasize relationships among details of your subject because you’re able to see more of it in focus. It also allows you to create relationships between a subject and the background because now you’ll actually be able to identify elements in the background (even if they are not sharp). Deep depth of field can tell your viewer whether a mushroom clump is growing in a pine forest or a maple forest because of what shows up in the background, as shown in Figure 4.9.
Figure 4.9 By using a wide-angle lens and a small aperture, I was able to connect this clump of mushrooms to its environment.
ISO 200 • 1/13 sec. • f/11 • 12–35mm lens (MFT)
This is also about composition. Many photographers think that composition is only about what is framed within the viewfinder or on the LCD. Composition is much more than that. It is also about emphasis and depth of field, which strongly affect emphasis.
When you get really close, deeper depth of field can simply help you capture more of the subject in focus. When your subject is significantly bigger than the shallow depth of field of a close shot, even just a little deeper depth of field can help the viewer better identify what the subject is and what the picture’s about.
Something you really need to keep in mind about deep depth of field is that as you allow more things to become defined by sharper details, you are also encouraging your viewer to look at them. That’s great if you want them to look all around your picture and not just at your subject but it can also be a problem if you don’t. Think carefully about why you might want deep depth of field and then check your LCD to make sure that that’s really what you expected.
Using Shallow Depth of Field
Shallow depth of field is very easy to get when you’re up close because you don’t get a lot of depth of field anyway. However, to really use shallow depth of field effectively, you need to go beyond thinking that an out-of-focus background is enough. As you can see from Figure 4.8, sometimes even a slight bit of depth of field can be too much if you’re trying to isolate and emphasize your subject.
Shallow depth of field is really useful in doing exactly that—isolating your subject within the photograph. There is no question that when your subject is sharp against a very blurred background, you’re helping your viewers see exactly what you want them to see, as shown in Figure 4.10. There are no distractions of sort-of-sharp things in the background to attract the viewer’s eye.
Figure 4.10 By shooting with my lens wide-open at its maximum aperture of f/2.8, I could create a simple, beautiful background to isolate my subject, a daddy longlegs.
ISO 400 • 1/125 sec. • f/2.8 • 60mm lens (MFT)
Beautiful soft backgrounds also come from using a shallow depth of field. This is most pronounced when your background is most out of focus. Since distance affects depth of field, one way that you can ensure that your backgrounds are strongly out of focus is to move your camera position so that anything behind your subject is farther away.
For example, photographers often photograph flowers pointing their cameras down at the flower at a 45-degree angle. When you do that, the background is fairly close to your subject and will show up even when depth of field is limited. If you drop your camera position to a lower angle, often the background will now be much farther behind your subject. That immediately creates a softer background that can contrast nicely with your subject.
The Diffraction Problem
Small apertures have a problem that many photographers are not aware of. If you are arbitrarily stopping your lens down to f/22 or smaller so that you get more depth of field, you may actually be reducing the sharpness of your photograph.
What happens is this: As apertures get very small, the rays of light coming through the lens bend or diffract along the edges of the diaphragm blades that make up your lens opening. This diffraction softens sharp edges so that the picture starts to lose its sharpness, image brilliance, and contrast. This gets worse as you focus closer. You can see the effect of diffraction on sharpness in Figure 4.11, which shows details of the scene shown in Figure 4.12.
Figure 4.11 This piece of petrified wood made a nice test subject to look at the diffraction effect on a 12–25mm lens with extension tubes. This is the entire shot.
ISO 200 • 1/100 sec. • f/8 • 12–35mm lens (MFT)
All lenses are affected by diffraction through small f-stops because this is an optical issue due to the physics of light, not to any lens quality issues—but there is no way to make a blanket statement about diffraction problems for a particular f-stop. This depends on the lens and especially the focal length. The only way that you can know what your lens will do at small f-stops is to do some tests.
For this reason, if I don’t absolutely need the smallest f-stops, I will often shoot at f/8 or f/11. On some lenses, I don’t have any problem stopping down to f/22, but I have also owned lenses where the change from f/16 to f/22 was so significant that I never used f/22.
Focus stacking is a unique digital way of dealing with depth of sharpness independent of traditional ways of dealing with depth of field. With this technique, you shoot a series of pictures as you change your focus point from near to far (near to far will depend on what your subject is), then you bring those images into the computer and use software to combine the shots into one image with deeper focus.
I used this technique for a shot of a green lynx spider just after it molted its old skin (Figure 4.13). Both spider and old skin are sharp. This would be impossible with just one shot. I shot several images, changing the focus slightly each time, to be sure the spider was sharp in at least one image and its old skin was sharp in another. Then I combined them in Photoshop to give a great range of focus without making the background too sharp. Luckily, there was no wind and the spider was resting after its molt.
Figure 4.13 Focus stacking made this shot of a green lynx spider just after it molted its old skin possible.
ISO 200 • 1/80 sec. • f/10 • 60mm lens (MFT)
I don’t do this a lot because it is challenging and time-consuming. Your camera has to be locked down and the subject can’t be moving. You need to change your focus point quickly through a series of shots that can be used in the computer later. David FitzSimmons is a master of this technique and uses it extensively in his Curious Critters series.
If you have Photoshop, combining the shots is fairly easy to do. There is also some dedicated software to help simplify this process. The actual techniques are not within the scope of this book, but you can learn about them by Googling focus stacking online.