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Par focal really only means "same thickness". While Baaders are indeed the same thickness spec. , you will however find there are small batch variations from one production run to another.
Convergent light cones (i.e fast optics) exaggerate filter thickness variations.
If you are using a refractor, spherochromatism will show focal differences from the Red through Blue of the spectrum.
The degree to which your objective is corrected will hence determine whether or not you still need to re-focus.
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