Hi .... After reading some of the comments about the light cut off saying this is not a true 80mm scope, my question is which eyepiece of the three XW eyepieces would make this scope be more of a 80mm scope based on the three different field stops quoted from Pentax's website ...... XW10 - Field Stop(12.4mm) XW14 - Field Stop(17.6mm) or the XW20 - Field Stop (24.0mm) Which one is the best one to use to compensate for the light reduction ? I know this might sound like a silly question & I know how the exit pupil relates to the objective size of the scope but Iam not sure about the field stops. Could one of you tech guys answer this for me ? .... gwen
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Pentax Pf-80A ED Light Restrictions What XW Eyepieces ? (1 Viewer)
- Thread starter gcole
- Start date
RJM
Don't Worry, Be Happy!
The eyepeice fieldstop is not the issue with this scope. The vignetting is caused intenally and changes with the focuser position. The clear aperture ranged from ~77mm at infinity to ~65mm at near focus in my sample. For most practical uses between 40m-100m the clear aperture was ~70mm.
RJM ... So when you say clear aperture, you mean you get the visual effect of the 80mm objective going from 77mm to as low as 65mm ? & when you say 40-100m thats the distance of the object you focus on. Anything out further than 40-100m pushes the scope back toward the 80mm clear aperture of the objective lens .... gwen
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RJM
Don't Worry, Be Happy!
Gwen, I thought my post was rather straight forward. The focusing mechanism impinges on the incoming light cone, the amount depends on the direction of focus. The effect is the same if you covered the objective lens with "donut" masks with center holes of varying size. The scopes effective aperture is reduced in all scenarios and is eyepieces independent.
There is some benefits to this for the visual user, as any aberations in the lens are probably minimized. The downside is some loss of light that is probably not such a big deal for visual use but is more critical for digiscopers. Main reason I sold my sample.
There is some benefits to this for the visual user, as any aberations in the lens are probably minimized. The downside is some loss of light that is probably not such a big deal for visual use but is more critical for digiscopers. Main reason I sold my sample.
