Optolyth is not as well known in the US as it should be. Their older style Alpin series porroprism binoculars were always some of my favorites...the 8x30s held the Mid-sized Reference Standard for several years until the introduction of the Leica Ultras...and I am anxious to test the redesigned Alpins.
Optolyth is one of the more adventuresome European optics houses. They were the first German maker to use fluorite elements (that I am aware of , at any rate), and now, with the introduction of a 100mm fluorite scope they are breaking more new ground.
This is a massive scope. They have managed to keep it quite compact. It is not markedly longer than most 80mm scopes, actually a bit shorter than some, but there is no disguising that 100mm of glass (and fluorite) out front. The body trumpets out to accommodate the objective and the prisms have to be (for good optical reasons) considerably oversize as well.
The body is Optolyth olive...in two contrasting shades like the Kowa, but with the lighter olive being considerably darker. It too is a rubberized finish and appears ready to stand up to any amount of abuse in the field. Once again, the body is waterproof, with a clear, multicoated window to seal the eyepiece end.
On a tripod, the scope is something to behold...and to hold on to. This is not the rig for a light weight field tripod. Bogans or better only need apply. The tripod mount is far forward on the scope body to balance the heavy objective. This places the eyepiece considerably behind the tripod head. It takes some getting used to...but is actually an advantage if you are using a video head with a long panning handle.
To provide wide range focus, Optolyth has given the TBG 100 two controls. There is a helical focus ring, fairly narrow, where the body of the scope meets the prism housing, for rapid changes of focus from close in to far out ("N" and "ƒ" as marked on the control). The ring is not intended for fine focus ...just for getting the right range, and it is really too narrow and too stiff, to use for anything else. The actual focusing is done with a conventional focus knob on the front of the prism housing, and is extremely smooth and precise. I found that I could easily reach the focus range ring with my left hand and the focus knob with my right.
Optically, the TBG 100 gives excellent views at lower powers; bright, as you would expect, with fine detail and excellent contrast. The eyepieces provided were obviously designed for Optolyth's smaller (80mm) scope, and the powers marked are not anything like the powers you actually get on the 100mm. Low power on the zoom (marked 20x) is nearer 30x when compared to the image scale of other scopes...and high power (marked 60x) appears closer to 75x. It should be noted that the zoom is definitely a traditional zoom, with a large zoom artifact. The field shrinks and the view recedes radically as you zoom up in power. The extra power of these eyepieces on the 100mm scope, which might be nice in some situations, is more than offset by the correspondingly narrower field of views.
The highest power view is somewhat disappointing. It lacks the crisp details I would have expected, and is not outstandingly bright. Switching to a single power eyepiece yields similar results... though detail is somewhat better. I suspect, at this aperture, at the 75-80 power provided by the eyepieces on this scope, we are running up against atmospheric effects. 100mm simply cuts a bigger cone out of dirty, unstable air than 80mm (or 60mm) does...thus limiting the highest power views. Now that I live in Maine next to the ocean, it is difficult to find cool, clear, dry, still air (or will be until well into winter, and then it won't be dry), so I may not be seeing everything this scope can give at the highest powers.
If you back the zoom off to a more reasonable 60 power (actual power on this scope, set, for my testing, by matching image scale with comparison scopes...in the field you would just crank it back a little), the view improves dramatically. Now you see the detail. The resolution and brightness of the TBG 100 is everything you could ask...equaling and exceeding the Nikon Fieldscope 78ED (the current BVD Reference Standard) and the Kowa TSN824 tested elsewhere in this issue. The zoom even has a good field of view and acceptable eye relief at this power. Switching to a fixed 30-40 power eyepiece (which would be 40-50x on this scope) should provide stunning views. (For NEED Test comparison charts for the Optolyth take a look at NEED Test Revisited.)
Bottom line: while Optolyth could clearly improve the TBG 100s performance by redesigning the eyepieces to better match the focal ratio of the objective (and providing a less zoom like zoom), I have to give them credit for attempting the biggest and the best. They have clear title to the biggest ...and, if you work within the limitations of the available eyepieces, they have one of the best as well. The scope is just slightly too large and too heavy to displace the much more compact Nikon Fieldscope as the Reference Standard, but there is not doubt that on a clear, calm day the TBG 100 will deliver everything the eye can see, at any power and any distance. That is high praise indeed. Clearly a Product of Special Merit.