The Swaro. SLC HD focus knob goes from the min. focus of ~ 2 m to infinity in 2 turns, and at halfway point in the rotation is focused at ~ 4 m, acc. to a friend who tested this and estimated, not measured, that distance.
That’s the behaviour I would expect. What’s more, refocusing from 4 m to 8 m would require turning the knob halfway from the 4 m position to the infinity position (i.e. half a turn with your friend’s binocular); and refocusing from 8 m to 16 m would require turning halfway from the 8 m position to the infinity position (i.e. another one-quarter of a turn); and so on and so forth.
This behaviour is:
- a consequence of the physics of how lenses focus (roughly following the thin-lens equation)
- a requirement for the binocular to offer a consistent speed and precision of focus across its range.
The frequent calls for binoculars to focus faster in the close-focus range are just that: requests for the binocular to focus faster (relative to its depth of field) when focused closely, which is the same as saying the focus precision should be lower in the close-focus range.
So why do people ask for it? Because:
- subjects at close range often move quickly, demanding frequent, large, quick changes of focus. Moving from one close subject to another (e.g. 2 m to 4 m) also requires a large change of focus. It may be better in such circumstances to have a quick approximate focus than to have a super-precise focus that can’t keep up
- typical close subjects, being three-dimensional, have many possible ‘correct’ focus distances, so high precision of focus is less important, and missed focus is less obvious, than with distant two-dimensional subjects (e.g. a mountain landscape, all of which is at infinity: the slightest hint of focus imprecision throws the whole view out of focus).
To illustrate the latter point, have a look at
these flowers. If the photographer had chosen a slightly different focus point the viewer would likely not have questioned the alternative focus. Different flowers, or different parts of the same flowers, would have been in focus instead, but the photo would have been much the same. A similar situation arrises when viewing close subjects with binoculars, and this greatly reduces the precision of focus required compared to distant subjects.
If binoculars were typically used to focus on flat, two-dimensional subjects in the close-focus range (such as test charts!), the required focus precision would be the same as it is when viewing distant subjects, and there would be far fewer calls to trade precision for speed in the close-focus range.
Of course they are not and therefore the calls make some sense, though I would probably find a non-linear focusing arrangement confusing to use.
