Friday, January 6, 2023

A Rule of Thumb approach for perfect landscape focus bracketing with any (WA) lens/camera combination

Those that read my blog know I love 'playing around' with focus stuff. I'm always looking for hacks to help me, pragmatically, use all my gear: both AF and manual lenses.

In the last few posts I've shown how we can use a split, thin lens model, together with an estimate of the position of the front principal, to create any required depth of field scale.

In this post I will show how we can focus bracket by exploiting the lens itself. So, there is no more need to follow incorrect, urban myths, eg focus a third into the scene.

First, let’s remind ourselves of the Rule of Ten, that gives us an estimate of the position of the hyperfocal from the front principal.

The hyperfocal RoT says that at f/10 and at a circle of confusion of f, as microns, the hyperfocal distance, in meters, is simply the focal length of the lens divided by 10.

As an example, a lens with a focal length of 16mm, set to f/10, will have a hyperfocal at 16/10 = 1.6m, using a CoC of 16 microns.

The hyperfocal RoT is therefore ideal for wide angle lenses, ie where the focal length is less than the largest acceptable CoC in microns; however, it is simple to use any lens length or CoC by adopting the following form of the RoT, where the terms in the square brackets are optional:

H = (F/10)[*(10/N)*(F/C)]

As an example, take a 50mm lens at N=10. Without adjusting things, the H would be 5m, but at a CoC of 50 microns! To adjust to a more sensible CoC, say, 25 microns, all we need to do is adjust the base H by (F/C), ie 50/25 = 2. Thus, the hyperfocal, at 25 microns, is 10m.

Once we have an approximation for the hyperfocal, we can now make use of hyperfocal bracketing:

From the above we can see an estimate of the number of brackets from the current focus (x, as measured from the front principal) to H/2 is simply H/(2*x): which we should round up.

For example, if we have a 16mm lens set at f/10, the RoT hyperfocal is 1.6m at a CoC of 16 microns.

If we now have a near point of focus at, say, 200mm from the front principal, we can estimate the number of focus brackets from x to H/2 as 1.6/(2*0.2) = 4.

From this all we need to do is to exploit the lens helicoid to give us our perfect focus bracketing. That is, divide the lens throw, ie rotation angle, between the near point of focus and infinity by 4; rather than guess where to focus.

Thus we end up with 5 focus brackets: four covering from the minimum point of focus to the H/2 point of focus, with a far depth of field at H; and one at infinity, ie with a near depth of field at H. 

In lens rotation space things look like this, where we see the two end focus points, ie at the near point of focus and infinity; and the three other points of focus, positioned at the quarter throw locations:

Clearly, once the number of brackets starts going beyond, say, 4 or 5, things get a little out of hand. However, the above hack, exploiting the lens throw/helicoid, is obviously better than guessing where to focus; and, on a wide angle lens, you will nearly always only need a few focus brackets to ensure deep focus capture.

As usual I welcome any comments on this post or any of my posts.

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