Tuesday, November 12, 2019

More on tilted focus stacking with Tilter

In the previous post I mentioned tilted focus stacking, and I hear some may say, but why? 

Afterall, is not one of the advantages of a TS-E lens that you achieve an ‘optimum’ focus state/plane that you can’t achieve in a single non-tilted image?

Whilst this is may be true sometimes, especially if you are seeking out a single zone/wedge of focus within an image that will be ‘out of focus’ elsewhere, it is not generally true. For example, say you are in a Cathedral and you are shooting close to the ground and you wish to have the beautiful flagstones on the floor captured, from near to far.

With a TS-E lens you might seek to position your hinge point on or very near the ground plane and focus towards/at infinity. Doing this will achieve a focus wedge, with the blur at the edges of the wedge equal to the, so-called, circle of (least) confusion, setting: typically anything from 10-30 microns on a full frame, eg anything less than two sensor pixel widths being ‘pointless’. This scenario looks like this:

Here we see the (yellow) field of view of the TS-E lens, positioned at the appropriate J height, and the tilted depth of field of the TS-E, both relative to the cathedral floor.

But what if there was a upright tombstone that we also wished to get into focus and was within the FoV of the lens, for instance, but positioned at, say, H/3, as in this scenario:

Here we see a focus failure. An alternative approach could be to focus normally, ie non-tilted, on the tombstone, but, of course, the far depth of field would drop off too much (assuming we don’t wish to push the aperture down too much because of diffraction; as the tombstone is positioned in front of the hyperfocal/2. This would lead us to consider (planar) focus stacking, which, of course with Magic Lantern and my in-camera focus stacking script(s) is easy to achieve :-)

But if we reverted to a focus-stacked, non-tilted approach, the artistic dimension will be changed, eg no differentiation in focus, as focus stacking with a non-tilted lens will ensure everything from the near focus to, say, infinity, will be ‘acceptably’ sharp.

This is where tilted focus stacking could be a useful (artistic) tool, ie allowing you to keep a lowish (HQ) aperture setting, eg two stops down from the maximum aperture. Illustratively things look like this:

Here we see a second tilted image (red), relative to the first one, shown in blue. We have now achieved our goal: the cathedral floor is tack sharp, the near tombstone is in perfect (acceptable) focus and the rest of the image, above the second image’s top/near depth for field boundary is ‘out of focus’.

Before progressing, let’s remind ourselves of the (general) hinge model we are using, thanks to the work of others such as
Emmanuel Bigler and Harold Merklinger:

Here we see an important feature of the model, namely, at the hyperfocal (H), the depth of field parallel to the sensor is simply J, ie the hinge height, which sits vertically under the front principal plane of the lens. Also, if we are focused at x (x is also projected on the ground plane above for clarity, as it is in Tilter) in front of H, as above, we will be tilting away from the ground plane, and if we are focused beyond H, we will be tilting towards the ground plane.

All this tells us that the angle of plane of maximum sharpness, relative to the ground plane, may be simply estimated (ie we should not use the above for doing macro work) as simply atan(J/x). Thus at x = H, the far/lower, tilted DoF just touches the ground plane, with the plane of sharp focus at an angle of atan(J/H); and focused at infinity, atan(J/infinity) = zero, so the plane of sharp focus is along the ground plane, with the near/higher or far/lower DoFs equally positioned either side of it.

From the above, it is also relatively simple to estimate the number of brackets from the current focused position, ie upwards to cover the top FoV, behind H, and downwards to cover to the ground plane.

I hope in a later version to account for camera tilt, but, for now, Tilter assumes a zero camera tilt. However, the Tilter screen does now shows the estimated number of brackets to cover from the current position to the ground plane. For example, in the screen image below we see that the plane of sharp focus is positioned at an angle of 52 degrees and the lower DoF is at 27 degrees. We thus need to bracket down from here, if we wish to also have the plane of sharp focus along the ground. Tilter tells us that we need one more image, ie a total of two tilted focus brackets.

In this example, Tilter also tells us that the (U = Upper) half FoV is 26 degrees. This FoV feedback provides FoV estimates for landscape, portrait, fully shifted landscape and fully shifted portrait.

As usual I welcome feedback, including corrections [:-)], and any ideas to make Tilter ‘better’.

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