Light slows in glass. In a converging beam, that displaces the focal plane away from the objective by a fixed amount that depends only on the thickness of the glass and its refractive index — not on your focal ratio, not on your aperture, and not on the filter's bandpass.
The arithmetic
The shift is thickness × (n − 1) ÷ n, where n is the refractive index of the substrate. For the common borosilicate substrates n is about 1.52, which makes the shift very close to one third of the glass thickness.
| Filter | Glass thickness | Focus shift |
|---|---|---|
| Astronomik — 1 mm glass | 1.0 mm | 0.3 mm |
| Baader — 2 mm glass | 2.0 mm | 0.7 mm |
| Chroma — 3 mm glass | 3.0 mm | 1.0 mm |
| ZWO 1.25" / 31 mm / 36 mm — 2 mm glass | 2.0 mm | 0.7 mm |
Because focus moves further out, the distance from your corrector to your sensor must GROW by that amount. A 2 mm filter needs roughly 0.7 mm of extra spacing, not 0.7 mm less.
Why the direction confuses people
The filter physically occupies space in the train, so it feels like it should take up part of the distance you were going to fill with metal. It does occupy that space — but it also pushes focus outward by more than nothing, and the two effects do not cancel. The glass is not a spacer with the same optical length as its mechanical length.
When it matters and when it does not
- A 1 mm filter shifts focus about 0.34 mm. That is inside many correctors' tolerance, which is why thin filters are often described as needing no respacing.
- A 2 mm filter shifts about 0.68 mm, which is outside a typical 0.5 mm tolerance. That one you should account for.
- Swapping between filters of the SAME thickness changes nothing. This is what parfocal means mechanically, and it is why manufacturers who keep one substrate thickness across a range are doing you a favour.
- Swapping a 2 mm filter for a 1 mm one moves your focal plane by about 0.34 mm — enough to matter, and easy to forget when you change brands.
One practical note: the figure that matters is the GLASS thickness, not the mounted cell. A 2 mm filter in a 5 mm cell shifts focus by 0.68 mm, and the cell is metal that displaces nothing optically.