I am also a user of Camranger, but find its focus stacking facility somewhat limited. One can set the near focus precisely, but attaining focus at the far range is problematic. One can specify the step size (small, medium, or large) and the number of shots, but there is no good way to tell if one is taking too few or too any shots in the stack. One way to estimate this is to see how many steps in the focus adjust tab are necessary to reach the far focus point, but the step size there may not be equal to the step size in the focus stacking tab.
Hi Bill,
I don't know how the Camranger software calculates the number of steps, but it would need to know the right parameters for the lens because each lens is different. A small/medium/large step can be different depending on the (focal length of) the lens. You can use the procedure below to calculate the required number of shots. I don't know if/how that interacts with the step size in the Camranger interface though.
Helicon Remote automatically determines the step size and number of shots according to the f/stop and other shooting parameters. Unfortunately, there is no iPad version of Helicon Remote, but they say they are working on it. I bought a small Nexus tablet to use with Helicon Remote, but find the Android interface of the program not to my liking.
There's always room for improvement, but in my experience the folks at Heliconsoft are open to suggestions. Not that they would implement anything thrown at them, but if it makes sense to them (given all considerations they have to balance) it might happen.
I also use a Nexus 7 with Helicon Remote. The only thing that one should figure out is the Correction factor to be used for the calculation of the number of steps that HR will use. The easiest way, rather than finding out by trial and error, is to calculate the DOF at the nearest focus position, and divide the total DOF depth to be covered by the single slice depth. The correct number of steps can then be created by tweaking the 'Correction factor' until they roughly result in the same number of shots.
To calculate the DOF for a single slice, I use the following formula that's most suited for Macro photography because it avoids focus distance:
Total DOF = (2 x C x N x (1 + M / P)) / (M^2 - (C^2 x N^2 / f^2))
where:
C = the Circle of Confusion, which I set to the sensel pitch in millimeters,
f = the focal length in millimeters,
N = the aperture value set on the lens, so
not the effective aperture due to magnification, but the nominal one,
M = the magnification factor, as can be accurately measured from the size of the subject on the sensor divided by the actual size of the subject,
P = the pupil factor, the Exit pupil to Entrance pupil diameter ratio, or use 1.0 if unknown.
If all (C and f) units are expressed in e.g. metres instead of millimetres, then the result will also be in metres.
If the closest focus distance is used for the calculation, the farther distances will not have gaps in DOF range between the slices. Since the CoC is set equal to the sensel pitch, there will be no DOF gaps possible between slices, and the stacking software should be able to achieve optimum focus at each position through the stack. This achieves the highest quality at the pixel level, but for down-sampled image sizes one can relax the parameters with an amount equal to the down-sampling factor.
With extremely narrow apertures, diffraction will also reduce the resolution which will then also allow to relax the pixel quality requirements a bit.
Shooting with a focus rail that moves the camera+lens assembly, or the subject stage, leaves the magnification factor constant. That means that the same effective slice DOF is used for each shot, whereas a refocusing by lens will gradually increase the DOF as one focuses farther away. Therefore, in the latter case it's best to calculate the narrower DOF for the near focus distance.
Cheers,
Bart
P.S. EDIT: I've modified/rearranged the formula a bit to make it shorter and perhaps faster to calculate.
