In previous posts I've shown how you can 'calibrate' your in-camera focus bracketing, such that you can set the focus step in terms of the overlap optical blur that you wish to use: Part 1.
But what if you don't have a camera with an in-built focus bracketing feature, for example my EOS M3. Can you still focus bracket?
The answer is clearly yes, but we need to be aware of a lens limitation: namely, the lens can't be a pure focus by wire type.
To understand why this is the case, we first need to appreciate how a 'normal' lens works.
As we know, a modern lens is composed of many lens elements and some of these need to move relative to each other as we focus, say with a prime lens. Things get more complicated if we consider a zoom lens, ie more lens elements need to move relative to each other.
Since the early days of complex lenses, manufacturers have made use of a so-called helicoid mechanism:
In the above, taken from this lens rental post, we see the helicoid component, converts lens barrel rotation (focus or zoom) into axial lens movements. The key take away here is that there is a mechanical, which can be manually or electrically driven when using AF, relationship between lens rotation and axial lens element movement.
In addition, such a design will exhibit hard stops at the minimum focus distance and at infinity, although on some lenses they may 'focus beyond infinity', ie going out of focus beyond the optical infinity, until the helicoid infinity hard stop is reached.
In a previous post I showed how one can add a depth of field scale to any lens.
An alternative way of looking at this is to use the simple expression (k+3)/2. Where k is the ratio of the short hyperfocal distance, ie (f*f)/(NC), divided by, say, the minimum focus distance, as measured from the front/entrance pupil, ie the no parallax point of the lens; as shown here (from Photons to Photos) for an EFM 11-22mm lens, at 11mm, ie in blue:
The (k+3)/2 expression then gives the number of brackets to take to cover from the (minimum) focus distance to infinity. Once we have k, we can then simply put some gaffers or masking tape around the lens, mark the minimum focus distance and infinity focus points, then divide that distance up, to create k tick marks, eg using Thales Theorem. The tick marks giving you the perfect locations for focus bracketing, using the overlap circle of confusion that you decide to use, ie not the one the manufacturer engraved on the lens:
The downside of the above is that it is only good for one focus length, which is not a problem with a prime, or with a zoom, when you focus bracket at, say, the short end. It is also limited to a single aperture value and one circle of confusion value. Once again not a problem if you are shooting landscapes, eg, say, at f/10 and with a CoC of 15 microns on a full frame camera.
But what about if you have a focus by wire lens, such as the EFM 11-22mm (shown above).
Will our gaffers tape hack work?
The answer is no: as such lenses decouple the focus rotation from the lens movement. That is, the lens senses the focus ring being rotated and then electronically instructs the lens mechanism to move.
The issue is not that there are no hard stops, as you can work around this; the issue is that the lens will move focus in a variable way, according to the focus ring rotation speed. Although some focus by wire lenses allow you to switch the lens into, so-called, linear mode, where the lens behaves more like a helicoid lens, where rotation of the focus barrel is speed invariant in moving focus.
Although I don't like to say, never; in this case, from my experience, I can never guarantee to rotate the lens at the same speed each time, thus you may find that rotating the lens barrel does not move the lens elements in the controlled way it does with a helicoid mechanism lens.
So, the bottom line is: forget manual focus bracketing with a pure focus by wire lens, although a focus by wire lens that can be switched to so-called linear mode, to make focusing rotation speed invariant, could still use the masking tape hack.
As usual I welcome any comments on this post or any of my posts.
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