Depth of Field in Macro Photography

One of the greatest challenges for macro photographers is achieving sharp focus for all of the scene's important elements. In this image the wings of only one of this pair of caper whites are in focus.

The defining characteristic of macro photography is of course that subjects are shot at close distances. While this close camera-to-subject proximity can lead to visually arresting images captured from an intimate perspective, this sort of photography presents unique technical challenges as well.
In this article I'll address one of the most significant of these challenges - controlling depth of field (DOF). The term depth of field refers to the area in front of and behind the point on which focus is set that can be rendered in sharp focus. As we'll explore throughout this article, DOF control plays a very prominent role in macro photography.
The cute creature in the image below is a cicada nymph, by definition the larval or sub-adult stage of an insect with partial metamorphosis. For me, this image is a failure. Why? Almost all the interesting parts and features of the nymph are are out of focus - its abdomen, wing buds, legs, even the front of its head.

In this image of cicada nymph , satisfyingly little of the subject is rendered in sharp focus.

Why are so many of the image elements blurred? It's not due to poor focusing technique. If you look carefully, you’ll see that I placed focus on the cicada's eye, always a good choice whether photographing people or insects. The lack of sharp detail results from insufficient DOF. That is, the range of objects in front of and behind my point of focus that can be simultaneously rendered in sharp focus is extremely shallow. The result? We see sharp detail in just a tiny portion of the whole image.

Here's another example of shallow DOF. When focus is placed on the mantis’ eye, the rest of its body is out of focus.	As you can see, the problem isn't solved simply by focusing on another area. If I place focus on the rest of the head area, the is eye then out of focus.

Understanding depth-of-field

Before we can begin to figure out how to better control DOF, we must first understand the factors that make it so problematic in macro photography. Depth of field is dependent upon three factors: aperture value, focal length and subject distance. When each of the other two variables are fixed, setting a larger F-stop number (which actually means a smaller aperture opening) will result in a larger DOF. Using a longer focal length will result in a smaller DOF. And shooting at a closer subject distance means a smaller DOF.
In macro photography, however, DOF depends primarily on just two factors: aperture value and magnification. At any given aperture value, the higher the magnification ratio, the smaller the DOF. And this explains why DOF is so shallow in macro;  the magnifications are simply much larger than in any other type of photography.
With this in mind, let's go back to the cicada image that began this discussion. When photographers see such a shallow DOF, they instinctively think the aperture was set very wide (a small F-stop number). But this shot was made at f/9.0 which, outside of macro photography, is considered to be a narrow aperture. That leaves magnification as the main contributor to shallow DOF. This nymph is only 2 or 3mm in length, and since I wanted to photograph it filling a large portion of the frame, I had to use an extreme magnification ratio – in this case, of 5:1, meaning that the cicada's projection on the sensor was 5 times its actual size! Extreme indeed, and so DOF is extremely shallow, at only a fraction of a millimeter.

One might think that a too-shallow DOF appears only in extreme macro. This is not true: even in this image of a devil’s horse nymph, shot using a magnification much lower than 1:1, a larger DOF would definitely be welcome.

Since DOF is affected by aperture and magnification, let's see what happens when we alter them. First - aperture value. The robber fly below was shot using a very small aperture: f/16. In fact, this aperture is so small that is causes a significant loss of sharpness due to diffraction. And it still doesn’t help this image much; the DOF is too shallow with most of the subject out of focus.

Here, even a very small aperture couldn’t help making the DOF large enough to have the whole subject in focus.

The second thing that can be done is to lessen the magnification by stepping back from the subject and making it take up less space in the frame. This most certainly works to increase DOF. Yet I have two major problems with this 'solution'. Having the subject fill a smaller part of the frame than intended forces you to crop the image in post processing. And while a large  crop may make the subject appear as if you shot it at closer range, you end up with less detail, eliminating one of the most appealing aspects of macro photography.
Furthermore, as a wildlife photographer I always wish to capture my scene in as close a state as possible to the final image. Using a small magnification and then making a significant crop collides with this ideal and personally I avoid this unless there is simply no other choice.