Thursday, August 22, 2013

Focus stacking cont'd.

A picture I made today by method 2 explained further on
Focus stacking (FS) is like 3D scanning. In such, human or other parts are scanned and images of virtual slices of the object are created at discrete positions along an imaginary axis. Software solutions could be further used to blend the obtained slice images into high resolution 3D virtual objects in which experts navigate to establish medical diagnoses or other useful 'stuff'. In FS, we similarly shoot a number of photographs of the same object, and like in traditional scanners, the frames are only sharp (and useful) on discrete planes perpendicular to the lens axis. Each such frame has a very shallow depth of field (DOF) that spreads incrementally in front and on the back of the focus plane. Shallow DOF is typical in macro photography where FS is mainly used for. Next, FS software maps out the blurred (out-of-focus) areas in each of the frames, and only keeps the sharp parts. By finally blending the latter together, a razor sharp photograph is obtained with a 'humongous' DOF (almost spectacular compared to conventional photography).

There are two ways to focus a camera on the aforementioned focus planes (incidentally, those same planes are parallel to the camera's CCD capturing element plane, if you haven't guessed it yet):

1. Keep the camera immovable on a steady tripod, pointing at the object, and use the lens ring to focus and shoot frames on a number of equally spread imaginary planes across the lens axis of the camera.

2. You focus at one extreme of the object (say, its front end), and then micro-displace the camera at new focus plane positions, for a discrete number of incremental steps, at which points you obviously shoot subsequent new frames, until you reach the object's other extreme (say, its back end).

I added those arrows on the 454's knob for better control
After all is said and done, (a) I still don't know which of the two methods yields better results, (b) whether indeed better results can be obtained by one of the two methods, and (c) what would be the reasons for possible differences in quality. If there are any differences, they must be the consequence of the blur/sharp region mapping algorithms used in the FS apps, right? I can only say that I tried both methods and found that the two of them equally yield great results. I should probably do a formal test with them, and systematically compare the results in a more formal manner. Until that is done, one thing I found to be true however:

To systematically focus on those incremental planes by precision turning of the focus ring, one does need indeed to tether the camera to a computer and control it with a dedicated app (like Helicon Remote). It's virtually impossible to obtain similar increments between focus planes by manually turning the focus ring. If you can manage to do it, then you're a champ. Also, the measurement scales typically depicted on focus rings are not terribly accurate to be used for such micro adjustments. For all practical reasons, only computers could manage this precision focusing properly. So, for method 1 you really need to tether your camera to a computer and control the shoot with an app. Furthermore, most people own cameras that are not readily supportable by the available tethering software (eg. Helicon Remote). So, you could really be left out in the cold...

Manfrotto 454 Micrometric Positioning Sliding Plate
To do 2 is much simpler. You mount a micro-positioning sliding plate like the one shown here on the head of your tripod, and then mount your camera on the sliding plate. After you focus your camera on one extreme of the object, and shoot a first frame, you slide the camera forwards (or backwards, depending which extreme you shot first) at regular increments by carefully turning the finger-tip control for precise micro movements; I used three entire revolutions of the knob in a few tests I did. It turned out to be fine. The larger the increments the less frames you'll have to shoot, obviously.

In other words, method 2 only requires you to invest in a sliding plate (plenty of solutions for a few bucks, less than 100 anyways) and any camera of your liking, without bothering about availability of tethering control apps. The FS you can eventually do with any dedicated app (available from free, via shareware, and a few good commercial solutions like Photoshop and Helicon).

Camera setup for shooting the coins
Of course, in case you have the proper gear supported by solutions like Helicon's, it's a much simpler and far more fine-tuned experience as you control everything from the computer, including all detail camera settings and shooting parameters. At the same time you can maintain the camera really amovible vis-à-vis the object and avoid the need for frame alignment later. Indeed, even using the sliding plate described above, a remote control to trigger the camera release button, and a super steady tripod like the 055XPROB I use, the resulting frames may still need to be further precision aligned for perfect results. The FS apps I use, Photoshop and Helicon Focus, are able to perform such alignments, but the lesser the need the better the result. I think...

No comments: