Simply focus at the HFD and everything from half the HFD to infinity will be ‘in focus’ according to a specified criterion, or more correctly not so out of focus that you can see the defocus.
If we ignore diffraction, the HFD is simply a function of the focal length, the aperture number and a blur criterion, usually called the Circle of Confusion (CoC).
If you have read my previous posts, you will know that the HFD is only ‘just’ acceptable, ie at infinity the defocus blur is the CoC. You will also know that you can achieve a far superior result by simply focusing at, say, Y times the HFD, as calculated with your ‘just acceptable, CoC: which will reduce the blur at infinity by the CoC/Y. Thus at a focus of 2xHFD(CoC), the blur at infinity will be the CoC/2.
With a typical (just acceptable, and no diffraction accounted for), full frame CoC of 0.03mm, ie 30 microns, you can therefore get a far field focus, all the way to infinity, that is always better than, say, 15 microns throughout the focus field: dropping to 0 at the point of focus. But of course in doing this you have reduced the near field depth of field.
Of course if we focus at infinity we achieve a blur of zero at infinity, but at the cost of reducing the near depth of field, ie HFD to infinity. Also defocus blurs less than two sensor pixels are rather meaningless. Thus you are over focusing with a digital camera if you focus at infinity.
To achieve the ultimate, of (very) near to infinity focus, you need to use focus stacking. That is you focus overlap various images and post process them to achieve an image with as large a depth of field that you want.
As readers of my blog know, if you have a Canon DSLR or EOSM, you can use my Magic Lantern Lua scripts to automate the process: taking all the guess work out, and automatically accounting for diffraction.
But what if you don’t have Magic Lantern. How do you know where to focus and how many focus brackets to take?
This is where knowing the HFD for your focal length and aperture helps.
As most photographers seeking out large depths of field will be ‘landscapers’, they will know that you need to ‘balance’ the blurs caused by defocus (from the lens) and diffraction (from the aperture alone). Without proof, you want to be ‘in the middle’ with your aperture, ie not at F/2.8 or at F/16, if you wish to balance out the two blurs. Say, between F/8-F/11 on a full frame DSLR.
So we have now set one of the HFD variables: the aperture number, eg, say, F/8 (N). The focal length (FL), of course, will be fixed once you have composed: so that is known (and permanently fixed on a prime lens). This leaves the CoC, which we will leave at the accepted ‘just acceptable’ number of 0.03mm for 35mm full frame format.
Note for on screen/web presentation we could increase this and for high scrutiny print exhibition/judging we should consider reducing this ‘base’ CoC. For now, we will assume normal quality is an infinity blur or 30 microns and a high quality print blur at infinity will be between 10-15 microns.
Previous posts gave the HFD formula, which, in its (approximate) simplified form, is FL*FL/(N*CoC). Thus for a FL of 24mm, at F/10 and a CoC of 0.03, the HFD comes out at 1920mm (ie about 2m). Giving a depth of field of 1920/2 mm, ie 960mm through to infinity (at a defocus blur criterion of 0.03mm).
But what if you had a ‘point of interest’ at, say, 400mm, what focus bracketing strategy should you use? How many brackets should you take?
Once again, not wishing to frighten off readers off with lots of equations, the number of brackets can be estimated from dividing the closest point of interest (400mm above) into half the HFD (and round up to the next integer.
Thus, in the example above we simply do the following calculation: 1920/(2*400), or 2.4, which we round up to 3. That is we need to take three brackets to capture the full near field.
But this begs the question: where do I focus those brackets?
Once again, the HFD comes to the rescue.
Without proof, and assuming each focus bracket just touches its adjacent bracket, the nth bracket needs to be focused at HFD/(2n-1). For example in our above three bracket example we would focus at HFD, HFD/3, HFD/5. The near and far depths of field of each bracket being HFD/(2*n) and HFD/(2(n-1)).
For the ‘perfect’ bracketing set, I would also take a bracket at 2*HFD, which would result in the following (illustrative) focus strategy. Note in this example we have not addressed the bracket to bracket overlap. We will deal with that in a subsequent post.
Bottom line: for those that tend to do landscape photography and use a ‘sweet’ spot for focal length and aperture, it is relatively easy to remember the HFD (or HFDs). Once you know the HFD, all you need to do is multiple or divide this by integers, typically between 2 and 9.
In future posts I’ll deal with focus overlapping when focus bracketing.