Precision Automated Focus Stacking With Stack Shot
Ever tried to photograph a subject, only to find out that you don’t have enough depth-of-field to get the whole thing in focus? Typically our first reaction is to stop down (choose a greater aperture) to increase our DOF. Unfortunately that doesn’t always give us the results we expect. The first issue is that even with our lens stopped down to its maximum aperture, we still may not have our subject completely in focus. The second problem is that when we stop down our aperture, we are often trading that increased DOF for decreased sharpness. This is something known as diffraction. Every lens has a sweet spot that gives us the absolute best image quality possible from that lens. But imagine if you could use that sweet spot of your lens (somewhere around f/5.6, f/8, or f/11 on most lenses) and still get the entire image in focus. This is where product, food, jewelry, and even nature photographers (macro shooters) employ a technique called Focus Stacking. The technique is pretty simple; take a series of photos, each focused on a different portion of the subject, and merge them all together in post-production to get one REALLY sharp image. This kind of sharpness is normally not possible with just a single frame. But when we set our aperture to that sweet spot, and shoot several frames, we are able to achieve the impossible (or at least our clients will think so).
Sure there are other factors in getting sharp images, such as camera shake, and shutter slap … but these issues are easily resolved by using the right tools and techniques. More on that later.
In one of my previous videos on Jewelry Photography for Catalogs, I introduced the technique using a Really Right Stuff Macro Focusing Rail. There are other manufacturers, but I like RRS products b/c they’re bulletproof. Basically a focus rail is a rail on which a camera mount slides forward and backward via a threaded shaft that’s turned with a knob. The movements created by turning the knob are very fine adjustments, allowing the photographer to have finite control over how much movement is being used. Basically, you back the rail all the way up, focus on the near portion of your subject, take a shot, turn the knob so the camera moves a little closer to the subject, take another shot … repeat as many times as necessary to move through the entire subject. Sometimes it can be as many as 20–30 images, depending on focal length, aperture, distance, and size of subject. Then we take all those images and dump them into software that detects the portions of each image that are sharpest, and combines them all into one REALLY sharp image. Adobe Photoshop is capable of this, although I’ve found third-party software like Helicon Focus to be more accurate.
Alternatively, if you don’t have a macro focus rail, you could just refocus on the subject, taking photos after each turn of the focus ring. This is not as accurate and can produce some less than optimum results due to the optics changing in the lens as you turn the focus ring, distortion of your subject, large difference in perspective, and a myriad of other problems. But, if you’re just testing the waters, it’s worth a shot!
The problem with a macro focus rail is that it doesn’t do anything to combat the biggest complication in the technique … human error. With a macro focusing rail the photographer has to physically touch the rail (which is connected to the camera) introducing the possibility of camera movement or misalignment. It also relies on the photographer’s estimate of how far to turn the knob between each shot. Using a shutter release cable and mirror lock-up will eliminate the possibility of camera shake, but you have to remember to actually use them. But the biggest problem I have with the technique is repeatability. Often times, I create several versions of a subject, lighting different portions of the subject or making small changes in each image, with the intention of compositing them into one final masterpiece. This technique relies on having absolutely identical images in order for my retoucher to be able to paint/mask the areas we went to incorporate into the final. When we aren’t focus stacking, I simply don’t touch the camera and just introduce a bounce card, additional light, prop, whatever. When focus stacking, I need to do the same thing, but it’s not just a single frame and the camera is not stationary. So, we need each stack to be identical in movement. As a human, it’s just not possible, this is where this video comes in. A colleague of mine introduced me to a motorized focus rail called Stack Shot.
Stack Shot is basically a macro focus rail that has a motor and is controlled remotely. This eliminates much of the human error I talked about earlier. It allows me to get repeated incredibly accurate, repeatable movements, over and over and over again. The video describes it pretty well, but in short, you program your starting point, stopping point and tell the unit how many images you want to capture between those two points. The unit automatically splits up the distance into equal movements, and does all the work for you. This is completely customizable and the device is actually WAY more capable than what I’ve described in my video, but that’s how it works in it’s simplest form. One of the coolest features of the Stack Shot is the ability to connect the brain to your camera and have the unit trigger your shutter after each movement. You can even program a pause between each movement before triggering the shutter so that any vibration from the movement has stopped. Badass.
If you want to know more about my technique for focus stacking, check out my tutorial over at RGG EDU: The Complete Guide to Product Photography, Lighting, & Retouching where I cover the subject in depth and demonstrate how to capture, use Helicon Focus, and composite multiple stacks in post.