ATM [Fwd: BOUNCE atm@shore.net: Non-member submission from [epabcc@assf00.epa.ericsson.se (Bratislav Curcic)]]

[Mel Bartels <mbartels@efn.org>]

> Subject: Re: ATM EP distortion
> X-Sun-Charset: US-ASCII
>
> Nils wrote :
>
> > Nice of them - but the trouble is they seem to have forgotten or
> > hurried past the case of *astronomical* eyepieces which have a finite
> > object distance but infinite image distance. They simply don't fit any
> > of the cases 1..3
>
> Actually they do. If you think in terms of eyepiece as a loupe, when
> you present your eyepiece with a nice square grid at field stop (your
> Nagler or Huygens won't work, but you can still do it by using a camera
> lens that is free from distortion :-) to form the virtual image at
> focal plane of an eyepiece) , the angles and straight lines seen
> through the other end should be preserved. Kind of reversed camera
> lens.
>
> > The good news is that we have, from now on, at least 4 entirely
> > different definitions of "distortionless", and thus have even greater
> > freedom to choose the one appropriate to a given situation.
>
> I always assumed that distortion is defined as departure from
> rectilinear; that is, straight lines stay straight and 90 degree is
> preserved (no pincushion or barrel distortion). That is what Ernst Abbe
> tackled with his orthoscopic design, and that is how MOST books show
> distortion ("barrel" or "pincushion").
>
> > One interesting thought (I have forgotten where I saw it) is that
> > lowered astigmatism can be had for the cost of increasing distortion
> > (by either criteria), and I think a good example of this is the
> > Panoptic design, which has considerable distortion even of type 4.
>
> Not necessarily, as Nagler for example is sharper than Panoptic for given
> angle, yet has less distortion. New Radians are (supposedly) quite distortion
> free, as many people complained about Panotics' barrel distortion.
>
> > But what prompted me to enter the thread was that I believe that the
> > curving of lines of terrestrial objects is often taken as a sign of
> > eyepiece distortion. As I have tried to explain, I do not agree.
>
> _If_ we agree that distortion is defined as "rectilinear distortion"
> (i.e. square grid has to stay square and preserve 90 degrees), curving
> lines DO indicate presence of distortion. Main problem is that when
> we're talking about huge AFOVs (60+ degrees) it is very difficult to
> define what should we REALLY see as 'distortion free'. When presented
> with a picture done with an extreme wide angle lens (more than 90
> degrees), if there is no straight lines in the pic, most people will
> complain about the 'stretching' at extreme angles. On the other hand,
> same people will complain if you take the pic of object WITH straight
> lines (buildings etc.) using a fisheye lens (constant angle mag. as
> opposed to image height). Yup, our mind is a strange beast.
>
> But in any case, when talking about moderate angle eyepieces (40 deg or
> so), I expect lines to stay straight. That is how good ("real")
> orthoscopics work; and they are 'distortion free' by definition.
>
> Bratislav



--
Clear skies, Mel Bartels
http://www.efn.org/~mbartels