Focusing in Macro
Over the past three decades I have witnessed the birth and maturation of autofocus photography. Although the early incarnations were not terribly useful to me, the autofocus technologies available in recent years have been tremendously helpful for many of my photographic subjects. The one exception to this is macro photography.
My macro lens is autofocus, but I usually focus this lens manually. One problem with macro photography is that there is often a hard choice to be made: upon which feature (among the multiple reasonable candidate features in the scene) do I bestow prime focus? Although I regularly face this difficult choice, I do choose to face it — I don’t trust the technology in autofocus cameras and lenses to make the same choice that I would make. In some respects, I would like to get the whole flower in focus (and nothing else); we don’t usually imagine our flowers as out-of-focus entities. However, only rarely can I get enough depth-of-field (hereafter “DOF”) to put all of the flower (from front to back) in focus — especially if there is any breeze-induced movement and I have to sacrifice some DOF by opening my aperture to get a faster shutter speed.
There are a couple of technologies which can help, as I learned from the renowned photographer, George Lepp. The first technology is a tilt-shift lens, a variation on the theme of “movements” available to view camera photographers. By tilting the front elements of the lens, you can tilt the focusing plane (the theoretical plane at which all points are in focus); here you could tilt the focusing plane forward to help get more of the front and rear edges of the flower in focus simultaneously. George recommended a 90mm tilt-shift lens in conjunction with an extension tube, and I can see that this would be a powerful combination for the flower photographer. The second helpful technology can be found in software, and allows you to merge multiple images into a single image, each image being focused at a different distance, and the resulting image including all the sharp parts of the constituent images. To get the best results, you would want to consider how much DOF you have, and shoot your sequence of photos in such a way as to ensure that the DsOF are overlapping from image to image; I somehow expect that tomorrow’s cameras will support the capture of just such a sequence of differently-focused images. The results of this technology, as demonstrated on the web page for Helicon Focus, are truly stunning.
For those who lack both the tilt-shift lens and the image-merging software (along with the ability/appetite to take several differently focused photos), there are still things you can do to get better results: hyperfocusing and careful selection of feature to receive prime focus.
Hyperfocusing
When you focus on a point, you have some DOF which means that some things closer to and some things farther away from your lens are still reasonably sharp; the smaller your aperture (the higher the f-stop), the more DOF you will have. In fact, approximately one-third of your DOF is in front of your focal distance, and two-thirds are behind it. You can use this distribution of DOF to your advantage by shifting your focus (to the “hyperfocal distance”) in such a way that the resulting DOF will cover all desired features in your image. The hyperfocal distance might not correspond to any of the features in your image, which adds to the challenge since you can’t see what you’re focusing on; in such cases, I will approximate this by rotating the focusing ring to focus on the nearest feature and then (without letting go!) rotating the focusing ring to focus on the farthest feature, and then rotating the focusing ring back (toward the nearest feature) about two-thirds of the way; it’s a crude approximation that works surprisingly well. To do this more precisely requires knowing how much DOF you have, and that is a function of the focal length of your lens, the distance at which you are focused, and the aperture; to help, some lenses still show DOF ranges on the barrel. (My first autofocus camera, the Canon EOS 5 (similar to the A2E), had great functionality to automatically perform such hyperfocusing.)
Careful Feature Selection for Prime Focus
Earlier I mentioned that often you have multiple candidate features vying for prime focus. You want to make sure that the most important (eye-catching) feature in your image is in focus — either at prime focus or within the DOF. Therein often lies a decidedly non-technical challenge: what is the most important feature? This is the very decision that I don’t trust technology to make. Curiously, there is a commensurately non-technical solution which I recently learned from Laura Arlov, an expert in usability: squint your eyes until you can just barely make out forms; what stands out in that abstracted view will be the most eye-catching feature of the scene. Laura’s context was the information rendered on a computer screen, but her technique applies equally well to photography.
In the photo above, there was no use of a tilt-shift lens nor employment of multiple-image merging. I determined that the inner structures of the flower would be the most interesting feature, and was particularly drawn to the pollen which had fallen to the petals of the flower. Fortunately, at the angle which I had on the flower, the central structure of the flower was approximately one-third of the distance between the front edge and the back edge, so this central structure was approximately at the hyperfocal distance. Not all of the flower is in focus — that will often be the case — but this isn’t overly distracting because the most important features of the scene are in good focus.
Macro focusing is one of those areas of photography that feels much more like an art than a science, and yet, science (coupled with experience and a willingness to experiment) can be a great help in our quest to create beautiful art.
Tip of the Week
2007.03.19
