Focuspocus: Why Adjustable Focus Is Not Adjustable Working Distance
- Stuart Anderton
- 2 days ago
- 3 min read
Updated: 2 days ago
There seems to be a growing misunderstanding in the loupe industry that “adjustable focus” means “adjustable working distance”. It doesn’t, that's just focuspocus.
That is not opinion or marketing language. It is basic geometry.
Prismatic deflection loupes are built around a fixed convergence angle between the left and right optical channels. Those optical axes are physically aligned to intersect at a specific point in space, which becomes the designed working distance.
Focus and convergence are completely different things.
Focus changes image sharpness. Convergence changes where the left and right optical axes meet.
One affects clarity. The other affects geometry.
That matters because binocular vision depends on both eyes converging onto the same target point. If the convergence geometry is wrong, the brain struggles to fuse the two images together properly, even if each image individually appears sharp.
The Geometry
Let’s use a fairly standard setup:
PD - Distance between your pupils: 62 mm
WD - Working distance: 400 mm
Fixed prism convergence geometry
The required convergence angle is determined by:
θ = 2arctan ((PD/2) / WD))
At a 400 mm working distance:
θ400 = 2arctan ((62/2) / 400)) ≈ 8.87∘
So the system is designed around:
8.87° total convergence
4.44° inward rotation per optical channel
Now reduce the working distance to 200 mm:
θ200 = 2arctan ((62/2) / 200)) ≈ 17.64∘
The required convergence angle is now:
17.64° total convergence
8.82° per side
That means halving the working distance requires nearly double the convergence angle.
The difference is:
Δθ ≈ 17.64∘ − 8.87∘ = 8.77∘
So the optical channels would need approximately 8.77° more total convergence to maintain proper binocular overlap at the shorter working distance. That increase cannot be created by focusing.
Focus does not rotate the optical axes inward. It only sharpens the image inside each optical channel.
For anyone unfamiliar with arctan, it simply means:
“Given this PD and this working distance, what angle must the optical tubes point inward to meet at that location?”
What Actually Happens When You Move Closer
If the loupes remain physically aligned for a 400 mm convergence point and you move the object to 200 mm, the optical geometry no longer matches the target position.
The optical axes are still intersecting at 400 mm because the prism alignment has not changed.
As a result:
the left and right images begin separating laterally
binocular overlap decreases
stereoscopic fusion becomes more difficult
depth perception degrades
eye strain increases
The user may still say:
“But the image looks sharp.”
Of course it does. Focus and convergence are different optical functions.
You can produce a sharp image that is geometrically incorrect for binocular fusion. The brain may partially compensate for this temporarily, but compensation is not the same thing as proper optical alignment.
That is where a lot of confusion comes from in the industry. People mistake the eye’s ability to tolerate bad geometry for proof that the geometry somehow changed.. It didn’t.
The Important Part
You cannot change the working distance of a prismatic deflection loupe by adjusting focus alone, because shortening the working distance requires a physical increase in convergence angle between the optical channels so the left and right optical axes intersect at a new closer point in space, and without changing that geometry the loupes remain optically aimed at the original working distance regardless of what the focus ring is doing.
The Good News
The good news is that the working distance of PENTAX loupes can genuinely be changed by our engineers in Sydney, because the optical convergence geometry itself is physically adjusted and recalibrated in the workshop, not simply refocused, with a typical turnaround time of 1–2 days.
PENTAX loupes are the first system on the market capable of independently adjusting deflection angle, PD, working distance, pupil height, and focus within the same optical platform. The optical geometry itself can be mechanically recalibrated rather than relying on simple focal adjustment alone, while most competing systems are fundamentally fixed-geometry designs attempting to imitate features without actually changing the underlying stereoscopic convergence system.
Check out PENTAX loupes, designed and supported by the best and brightest loupes and the best and brightest team at Osseo Group.
Click HERE to get in touch woth your local Account Rep for a demo.
