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One interesting observation is how beefy the Optolyth eyepieces are compared to the Nikon. Unfortunately, the eyepieces on the Optolyth scopes are not removable, so I could not inspect them on both sides, but the lenses looked quite a bit bigger than those on the Nikon, and a bit bigger than the ones in the Pentax as well. I’ll refer to this difference between eyepieces later again when I touch on eye relief.
I think I’ll skip the description of the packaging the scopes come in, since noone (including me) particularly cares what the box looks like. I will however note that the manual included a plot representative of the spectral profile of Optolyth’s Cerulin-Plus coatings. Since to get the spectral profiles of other coatings I typically have to dig through a bunch of patents (and even then most are hard to get), I really appreciated that. The plot below shows the reflectivity of one layer of Cerulin-Plus coating. An interesting aspect of this profile is the double-dipped shape. Apparently, the coatings were optimized for maximum transmission in the blue region (~420nm) and green-yellow region (~530-580nm). Whenever you see a company optimize their coatings for the blue colors, you know they were thinking about low light:
Here is another snapshot from the manual that shows the specs for the scopes:
There are two 25×70 models, I looked at the one leftmost in the table (BGA/WW). The XS model is almost identical except the eyepiece can be stored inside the spotter body for transport. I have never seen the 30×80 model. I do not think it is currently imported into US.
The 25×70 spotter has a field of view of 50m at 1000m (150ft at 1000 yards for the metrically challenged).
For comparison the field of view of the other spotters mentioned at the same magnification (25x) is as follows:
My observations at the range confirm the differences in the fields of view. One thing to note, however, is that all this field of view can be pretty hard to use if the eyepiece does not have enough eyerelief to show the full field of view:
I have tried to use all four scopes with glasses and without glasses (I normally wear contacts). The three scopes with variable eyepieces have adjustable plastic eyecups, while the 25×70 has a folding rubber eyecup. With both Optolyth scopes I was comfortably able to get the full field of view with both glasses and contacts (even at 45x with the variable Optolyth). With the Pentax XF eyepiece that I was using (Pentax offers a bunch of higher end XW eyepieces with 20mm of eyerelief) I was able to get the full field of view up until about 40x when wearing glasses.
Same for the Nikon: after about 35-40x, I could not get a full field of view while wearing glasses. Moreover, even with glasses out of the picture, the eyepiece of the Nikon made it very difficult to use, since eye position had to be exactly right and the slightest tremor would “kidney bean” the image.
Pentax, although it has less eyerelief on paper, turned out to be a fair bit more forgiving for eye position than the Nikon (that has been my main source of dissatisfaction with Nikon spotters: excellent image quality, but hard to utilize because of the eyepiece design). Here is where I come back to the comparative size of the eyepieces: Nikon eyepiece looks minuscule compared to the Optolyth eyepieces. Pentax is somewhere in between. Higher end Pentax eyepieces are also a bit bulkier. It is just easier to make a nice long eye relief eyepiece when you have more real estate to work with (Nikon must have come to the same conclusion since at SHOT 2009 the eyepiece on their new EDG spotter was positively humongous compared to the one on the Fieldscope).
This is a bit off topic, but here is where a choice of an eyepiece makes a big difference. For example, with my Pentax spotter, field of view with XW20 eyepiece is 61m (that is at 19.5x) vs 37m for the variable XF at 20x. Moreover, that field of view comes with longer eye relief. Looking at some more Pentax eyepieces, the fixed 28x XW eyepiece has wider field of view than the XF eyepiece at 20x.
Before I talk about optics, here are a couple of comments on weight:
Collapsible Optolyth spotters are fairly light: 20×75 is about 35 oz and 15-45×80 is about 49 oz. For comparison, the 65mm Pentax with 20-60x XF eyepiece is about 42oz and the 82mm Fieldscope is about 56oz. If you are looking for the largest objective lens in the lightest and most compact package, 15-45×80 Optolyth has got to be near the top of the list.
Now, let’s talk a little about optics. In a nutshell, I wish that the Optolyth spotters had ED glass in them. I have already talked about field of view and eye relief, which are very good. Depth of field is exceptional as well. Image quality is generally quite good on the Optolyth collapsible spotters, except for one thing: for the same money or less you can buy a Pacific Rim spotter with ED glass, that will be sharper. With spotters you really see what Extra Low Dispersion glass does for you. On a sunny day, looking at sharply defined objects, you can definitely see chromatic aberration on Optolyth scopes. Even the appreciably smaller 65mm Pentax could easily hang with the larger German scopes. Much larger and heavier Nikon delivered superior image quality.
As the light sets, Optolyth scopes did much better, comparatively speaking. They brought more light to the eye than Pentax and Nikon and their configurations are generally more conducive to low light, especially the 15-45×80 when set to 15-18x. Another point I have to make is that with their wider fields of view, Optolyth spotters tend to collect a little more light than their narrower FOV brethren. Quite simply, you are looking at more of the scene with the Optolyth, so you get more reflected light in. With dimmer lighting and, ultimately, bluer overall color gamut, the chromatic fringing so visible in the middle of the day faded away.