ATS65 re-focus (1 Viewer)

[nzwild]

I have an ATS65 (2020). Great scope and I use the 25-50 eyepiece. When I zoom from 30 to 50 I need to re-focus to maximise detail. Other spotting scopes I have used have not needed a refocus when zooming.
The scope works well and is CA free and its a minor annoyance. Image is sharp at 50x once tweaked.
Any idea if this would be an eyepiece or scope issue? Anyone else noticed the same issue?
Just curious.

 

[PatR]

I have an ATS65 (2020). Great scope and I use the 25-50 eyepiece. When I zoom from 30 to 50 I need to re-focus to maximise detail. Other spotting scopes I have used have not needed a refocus when zooming.
The scope works well and is CA free and its a minor annoyance. Image is sharp at 50x once tweaked.
Any idea if this would be an eyepiece or scope issue? Anyone else noticed the same issue?
Just curious.

Unless your eyepiece is parafocal, and I doubt that it is, it will require focusing as you zoom in or out. It is perfectly normal.

 

[nzwild]

Unless your eyepiece is parafocal, and I doubt that it is, it will require focusing as you zoom in or out. It is perfectly normal.

That makes sense, Thanks

 

[Tringa45]

The depth of field at 50x is only 1/4 that at 25x, so the chances are that an image which appears to be sharply focussed at 25x in a parfocal eyepiece would no longer be perfectly sharp when zoomed to 50x.
Parfocality would require that the focal plane of the eyepiece, regardless of magnification (focal length) is always coincident with the image plane of the objective.
However, you have probably noticed that different observers require different focus settings for the same object. The best that a parfocal eyepiece can be expected to do is to remain sharp when reducing magnification. It cannot compensate for near- or farsightedness.

John

PS:- The 25-60x TE-11WZ on my Kowa 883 is parfocal but the parfocality goes out of the window when I remove my glasses.