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Parfocality Limitations (1 Viewer)


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A zoom objective lens or eyepiece is considered parfocal if its focal plane remains constant at all focal lengths, allowing a change of magnification without altering focus.
Some sets of astronomical eyepieces of different focal lengths are also claimed to be parfocal.

In a review of the 85 mm Zeiss Harpia the reviewer was puzzled by the fact that it sometimes appeared parfocal across its range and sometimes not, and then went on inadvertantly to supply a possible explanation in which he stated that the depth of field at 65x was very shallow.

The usual use of a zoom function is to acquire and focus on an object at the lower magnification and then zoom in for more detail. However, the depth of field at 22x is about nine times that at 65x, so there is a distinct possibility that when focussed at 22x the object will no longer be sharply focussed at 65x.

I performed a quick experiment at home with my Kowa 883 and 25-60 zoom eyepiece viewing small print at 10 m. Surprisingly focus remained constant from 25x up to 60x repeatedly. However, this was not a very exacting test for the scope and under these conditions the reduced DoF of the scope was probably mitigated by the increased DoF of my own eye going from an exit pupil of 3,5 mm down to 1,5 mm.

The prerequisite for parfocality though is that the focal planes of objective and eyepiece are coincident. A near-sighted observer would however place the focal plane of the objective inside the focal plane of the eyepiece and a far-sighted observer would place it outside. There is no way that the eyepiece can compensate for this and when I removed my glasses and focussed at 25x (I have a +1,75 dioptre correction on my viewing eye) the print was illegible at 60x.

The reviewer mentioned above wears glasses, so perhaps his prescription was out of date or he wears varifocals (see Looksharp's comments here Eyeglasses - a few unqualified remarks). The motto is, if you wear glasses, keep them on when using a scope with a zoom function.

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