Close-up: Depth of Field and the Film Plane
In macro shooting, your biggest enemy is depth of field, or rather the forces that conspire to rob you of it. I've written in general about depth of field before, but it's worth a second look as we focus (gee, I just love that pun) on macro.
So what is depth of field? In reality, when you focus on your subject, objects in front of or behind that point are technically not in focus. To some extent though, at least some range of distances will appear to be in focus anyway. The distance between the closest and farthest thing that looks acceptably sharp is depth of field. At normal subject sizes, depth of field extends roughly one third in front of your plane of focus and two thirds behind it, but at macro sizes, it becomes closer to fifty-fifty, making your actual plane of focus in the middle of the perceived depth of field. That's just the way the physics of it works out. The exact reasons why aren't so much important as it is that you understand what you have to work with. Don't merely focus on the closest point of your subject or you will be wasting that portion of your depth of field in front of this plane. The DOF Preview button on your camera can help you maximize your use of this zone of acceptable sharpness.
The wider your chosen aperture, the less depth of field you will have. More importantly for the topic at hand though, the greater your magnification the less your depth of field. This magnification can come from using a longer focal length lens or from getting closer, but it can also come from any overt steps you take to intentionally increase magnification. Macro lenses, diopters, bellows, stacked lenses and many other options exist to increase magnification, and they will all work to rob you of depth of field.
As an example, at 1/10 life size (an area about 10 by 15 inches on film or about 7 by 10 inches on Nikon's DX format digital), your depth of field will range from around an inch and a half at f/5.6 to over six inches when stopped down to f/22. At life size (a subject around an inch by an inch and a half on film or two-thirds of an inch by an inch on digital) you will be down to less than a millimeter at f/5.6 or still only around three millimeters at f/22. And if that weren't scary enough, at 6x life size your depth of field will be a microscopic quarter of a millimeter, even at f/22.
Gosh. What's a macro photographer to do?
In order to maximize the benefit you get from the narrow depth of field you have to work with, your camera position becomes of paramount importance. As magnification increases, it becomes increasingly critical that you place your camera's film plane (essentially the back of your camera) parallel to the predominant plane of your subject. This will allow you to keep the greatest percentage of the important details of your subject within the range that appears acceptably sharp. The more your camera and lens are positioned off axis from your subject, the more of the zone of sharpness will be wasted on areas that either don't contain your subject or contain less important parts of it. Not all subjects are flat of course and it becomes part of your job to determine what is important.
Suppose you want to photograph some ugly PowerPoint clip-art as I am here. By positioning your camera correctly, you can align the subject with the plane of focus and maximize the portion that will appear sharp. Not doing so will result in only the middle of the image being in focus, leaving areas on either side of that blurry. You wouldn't want blurry clip-art, now would you?
When photographing something measured in millimeters, small changes in camera position can have a big impact. Learn to take your time and do it right. For high magnification work, a sturdy tripod is a must. As a rule of thumb, the time needed to position your camera optimally increases as you work at higher and higher magnification. The results though, can be well worth it.
As a footnote, I wrote last week of the benefits of longer focal length lenses for macro work. You may now be wondering if shorter lenses aren't better after all if depth of field decreases as focal length increases. For any given magnification though, depth of field will not be different. When shooting from the same position, a 200mm macro lens will see a narrower angle of view than a 105mm macro will and will thus be magnifying what it sees more and give you less depth of field. However, if you move closer with that 105mm macro so that both give you the same effective magnification, say life size for instance, your depth of field will be the same. It's just that with one, the magnification comes from using a longer focal length while it comes from getting closer to the subject with the other. And background control and working distance are still significantly improved with the 200mm.
We'll look more at camera positioning and focusing techniques next week.