Why Auto-Focus Has a Hard Time with Macro
I'm a big fan of autofocus. It's simple, fast, and generally quite accurate. But when shooting macro photography, it's time to switch to manual focus. Here's why.
Let's start by looking at how autofocus actually works. At first glance, it can seem almost like magic, but the technology behind it is actually quite understandable. It's all about contrast. A typical camera these days has many autofocus sensors distributed around the field of view. In some cases, cameras can use more than one sensor at a time to measure focus at multiple points. Regardless of how many get used though, the concept behind each is the same. When an image is in sharp focus, edges can be seen in high contrast. When focus is less sharp, edges become fuzzy and contrast drops. All the camera has to do measure contrast across the width of the sensor and move the focus until it finds the point where the contrast is highest.
And this is where the problem lies attempting to use autofocus when shooting macro. At high magnification, the total depth of field can be quite narrow indeed. Even when stopped down to a small aperture, you will likely have a slice just a few millimeters thick in focus. Everything in front of that or behind it will be thrown out of focus. And out of focus means low contrast. If you are photographing people, landscapes or any "normal" scale subject, your depth of field in most cases will be large enough that the camera should easily be able to find focus. Not so with macro.
Focusing with macro is difficult. As the camera moves the focusing helicoids forward and back looking for improvements in the contrast it measures, it often misses entirely the limited range of distance where focus is at all close. You've probably experienced it doing so: the lens focus hunts back and forth, in and out, seeking what it is unlikely to find. It moves the focus further away, overshooting the mark, then focuses back the other direction, overshooting it once again. After doing this a few times, it gives up, never noticing that focus was possible if only it slowed down and took smaller steps on its quest for the elusive goal of high contrast edges.
The only practical approach to focusing for macro is to focus manually. Looking through the viewfinder, you can see the entire frame. As things come into focus and start to go beyond, you can see what the focus sensors in your camera miss. Once you start to get close, you can shift focus by the smallest of increments while continuing to watch until you find the perfect focus point. The camera really has no idea what you are pointing it at and when to take extra care when focusing. You do.
Manually focusing with large scale subjects can be time consuming and will rarely achieve better results than when using autofocus. The reverse is true when shooting macro. Even when autofocus does work, it will often take longer than manual focus will due to the overcompensation and hunting for focus.