[photo-3d] Viewer Review - 2 - Viewing

["Dr. George A. Themelis" <drt-3d@xxxxxxx>]

Continuing with the Ekeren 5XR review.

---  CHAPTER 2: VIEWING

As with most Ekeren viewers the viewing part is quite 
simple.  It consists of two lenses, a plastic box to 
hold them, a slot for the slide and a plastic diffuser 
in the back for "available light" illumination.

The lenses are fully-coated, 30mm in diameter.  After 
taking the front apart I knew (from their length and 
optical quality) that they have to be double achromats 
(two achromatic lenses per eyepiece).  I confirmed with 
Jacob that this is indeed the case.  This is the same
design found in the deWijs viewers and IMO it is the
best available design in stereo viewers today.

The image shows no distortions or aberrations all the 
way to the edge.  Field of view and eye relief are both 
very good.  Even though the design is such that my 
eyes cannot get very close to the lenses (I estimate 
that they are about 1 inch away from the lenses) I am 
still able to see the widest RBT stereo mount (33mm wide).  
By pulling my eyes one inch further from the closest 
viewing position (simulating "worse case" eye glass 
wearers) I can still see 7p slides without any blockage.

The effective focal length of the lenses (which determines 
the magnification) is 47mm and to my eyes it is the same 
size as the Realist red button which has a reported 44mm FL.  
That's plenty of magnification for me.

The lenses are joined together for ganged focusing via a 
screw type mechanism.  To my knowledge, this is the only 
Ekeren viewer with ganged focusing (the rest have 
individually-focusing eyepieces).  In my opinion, ganged 
focusing is a better design in terms of convenience and 
ease of use.  The length of travel for focusing is quite 
long, which is nice for those who want to use the viewer 
without their glasses on.

There is no interocular adjustment.  In principle, this 
adjustment is not needed if the lenses are large enough 
and centered at the infinity (maximum) separation of the 
stereo slide.  However, as I am slowly discovering, if 
the image is mounted so that the infinity separation is 
larger than the maximum standard value then my eyes 
cannot diverge to fuse the infinity points, resulting 
in double images (other people have no problem with this).  

I have the tendency to increase the interocular spacing 
of the lenses (if an interocular adjustment is available), 
resulting in viewing with a slight convergence.  Either 
by habit or "by design" this appears to be the most 
comfortable setting for my eyes.  When faced with a viewer 
with no interocular adjustment, like this one, I just hope 
that the lenses are properly centered, which is the case 
here.  Whenever I see a slide that gives me double images 
at infinity then I know that it is mounted with too wide 
of an infinity separation.  That's a warning if this is 
one of my images (especially dangerous in larger-screen 
stereo projection) so there is an advantage in not having 
interocular adjustment in a viewer (promotes good mounting
skills).

The slide slot is wide enough to accept the thickest 
possible stereo slides.  There is no retaining clip to 
keep the slide flushed to one side.  After viewing slides 
mounted in all different mounts, I don't think this 
function is necessary in this viewer.

Overall, nothing earth-shaking in this "optical part" of 
the viewer, other than the superb optical quality and 
convenient ganged focusing in a simple (almost 
indestructible) lightweight housing.

To be continued....

George Themelis