Monday, November 12, 2018

Focus bracketing using the Rule of 10

In the last post I introduced the ‘Rule of 10 (ROT)’ for those that are looking to manage focus from infinity to a near field subject, eg landscape photographers. The ROT is simply hyperfocal focusing without Apps or look-up tables, ie all in your head.

The ROT simply says that at an aperture of F/10, if you focus at one tenth of the focal length of the lens (in meters), your infinity blur (ie the circle of confusion, CoC) will be the focal length in microns. That is the ROT focus point is the hyperfocal focus point for a CoC of FL in microns.

Focusing at the ROT focus points gives a depth of field between ROT/2 to infinity.

Further, to adjust the quality, to smaller or greater infinity blurs, all one need do is adjust the focus distance by the ratio of the required blur to the (ROT) FL-based blur.

Finally, to move away from using F/10, all one need do is adjust the focus distance by the ratio of the required aperture to F/10. That is, if you wish to use F/5.6 (half of F/10) you would simply refocus at double the F/10 ROT distance.

As an example, with a 10mm lens at F/10, one would focus at 10/10 or 1m, giving an infinity blur of 10 microns and a depth of field from 0.5m (1.0/2) to infinity.

If one wished to relax the blur criterion to, say, an OK 30 microns (triple the high quality 10 microns ROT figure), all one would need do is simply refocus at one third of the basic ROT number, ie 1.0/3 or around 0.34 m. Thus, giving a depth of field of 0.17 m (0.34/2) to infinity, at an infinity blur (CoC) of 30 microns, ie still an OK number for a full frame sensor.

The previous post also, pragmatically, suggested that infinity blurs between 10 and 30 microns is where one should be seeking focus solutions using a full frame (say, 20-10 microns on a crop sensor). Noting that for on-screen viewing one could relax the blur beyond 30, but in doing so one would eliminate the ability to print to a satisfactory quality. Thus, I suggest one sticks with the 10-30 micron blur (or CoC) criterion unless you really will only project on screen.

In other posts I also introduced a simple procedure for focus stacking, once again based on what this post calls the ROT focus point, ie the FL/10 focus. See

Thus, if we call our ROT focus point, R, if one wished to achieve a greater near field focus cover, ie better that the R/2 near field point; all one needed to do, to ensure each focus bracket touches the last one, is to focus at the odd number fractional values or R, ie R/3, R/5, R/7 etc.

The depth of field would then, following focus stacking in post, become R/(x+1) to infinity. Where x is the final focus bracket denominator. For example, if one took four focus brackets, at R (ie R/1), R/3, R/5 and R/7, the depth of field coverage would extend from R/8 to infinity. We also see how the focus coverage narrows rather quickly once you focus short of the ROT (hyperfocal) point (eg at R/5 the depth of field is R/4 to R/6) and therefore, one needs to really need the extra focus coverage to go past one of two additional focus brackets.

As an example, let’s assume we are shooting with a 24mm lens. The ROT focus point, at F/10, is 2.4m, at an infinity blur (CoC) of 24 microns. Giving a depth of field of 1.2 m (2.4/2) to infinity. Let’s further assume that 24 micron blur quality is an acceptable one, but that we need to do better in the near field (as the focus falls off rather fast here, compared to the far field). That is, say, we have a feature of interest at 0.5 m we wish to bring into focus.

Without focus stacking the focus field looks like this, where a unity blur is 24 microns. This plot also shows the usual defocus characteristics: the far field asymptotically going to the infinity blur (24 microns here) and the near field quickly going towards infinity. We can also see that a single image won't meet our goal of covering a feature of interest at 0.5m.

Using our focus stacking rule, we know the first focus bracket should be set to ROT/3, or 2.4/3 = 0.8 m, giving a depth of field of ROT/4 to ROT/2 = 0.6 to 1.2 m (that is ROT/( x +/- 1). This gives us two focus brackets looking like this:

[BTW the above two plots are screen captures from the online version of cBlur: To set unity to specific blurs you need to do the following: put cBlur's d
esired image resolution into sensor resolution mode and use the following Mpixel number: 3333/(FL*FL). Thus at 24mm focal length the Mpixel you should use is: 3333/(24*24) = 5.786. Doing this the unity blur in cBlur becomes the 24 micron in this case.]

Looking back at our example, as a single extra focus bracket is not enough, we need to take one more at the next position, ie ROT/5 or 0.48 m, giving an additional focus cover between ROT/4 and ROT/6 or 0.6 to 0.4 m.

Thus we end up shooting the following focus brackets, using a blur (CoC) of 24 microns:

  • Bracket 1 @ 2.4 m, covering infinity to 1.2 m
  • Bracket 2 @ 0.8 m, covering 1.2 m to 0.6 m
  • Bracket 3 @ 0.48 m, covering 0.6 to 0.4 m
For those looking for a two-image focus strategy, I would recommend the following:
  • Take the first image as twice the ROT value
  • Take the second one at half the ROT value
In the 24mm example above that means, one image focused at 4.8m and one at 1.2m, giving the following result, ie a higher quality background, plus some extended near field:

Finally, we need to recognise that knowing the ROT doesn’t do the focusing for us! The ROT information simply informs you where you need to focus. You will still need to focus your lens at the indicated ROT distances, eg using auto focus and pointing to an area on the scene that you believe to be at the required distance or using Live View and focus peaking at the required distance.

What the ROT does is give you the best information to inform your focusing, eg better than assuming one third into the image is (always) OK.

In this post we have seen that, by using the Rule of 10, we can estimate all our focus information in our head using simple arithmetic, whether we wish to focus bracket or not. In future posts I will carry on exploring the ROT focusing strategy and its power. 

As usual I welcome any feedback on my posts.

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