Filters Change Focus: The Working-Distance Shift Nobody Budgets For

Light filters aren't "just accessories" in machine vision—they're active optical elements. Any filter you insert behaves like a plane-parallel glass plate and lengthens the optical path. The practical consequence is annoyingly mechanical: your lens may now need a longer working distance to reach focus, even though "nothing changed" in your camera or lens settings.

Where the Filter Sits Changes Everything

The size of the shift depends strongly on filter placement. For typical optical glass with a refractive index around 1.5, here's the rule of thumb:

Filter in front of the lens: Working distance increases by roughly one-third of the glass thickness.

Filter in front of the sensor: The shift becomes glass thickness divided by three times the square of your magnification (β²).

That β² term is deceptively powerful. At high magnification, a "thin" piece of glass can create a surprisingly large working-distance change in real machine builds.

Why This Breaks "Perfect" Systems

This is exactly why filter swaps can wreck a machine that was flawless in CAD. Think about common scenarios: removing an IR-cut filter for IR illumination, or adding a daylight-cut filter to suppress ambient light. In either case, the working-distance shift can be large enough to force changes in camera mounting distances and mechanical construction.

And here's the part that really stings: even when you successfully refocus, inserted filters can introduce artificial astigmatism. So "it focuses" doesn't always mean "it measures cleanly."

The Bottom Line

Budget for focus adjustment in your system design whenever filters might be added, removed, or swapped. The time to discover this isn't during commissioning—it's during the design phase when you can still accommodate the mechanical changes.

This is part of KUPO's technical resource series on optical filters for machine vision. For custom filter solutions, contact KUPO CO. LTD.