The C90 has had a following among birders for quite a number of years now. I tested the model with special coatings. It comes equipped with a "hybrid" 45° diagonal (it adapts the .975 inch back of the C90 to use standard 1.25 inch eyepieces), the same high quality 30mm Ultima eyepiece as the C5, and the same just adequate finder scope. If you have not looked though a C90 lately, the addition of special antireflective coatings and the change to the better eyepiece have made an incredible difference in the image you see. If you add the necessary adapters to use a standard 1.25 inch star diagonal, the C90 provides exceptional performance in a very small package.

This is a field scope, not an astronomical scope adapted for field use. Focus is controlled, camera lens style, by a large rubberized ring that rotates the whole front of the scope. While this arrangement is quite a bit faster than the knob at the back of the other Cats, it is not nearly as precise. You sometimes have difficulty hitting the exact focus point. The scope is also available in a completely armored model with a heavy rubber casing for field durability. It is quite compact, less than 9 inches in length and only 4 inches in diameter, and weighs in at about 3 and a quarter pounds.

Optically, the special coatings model is among the best spotting scopes I have tested. Although not up to the level of either the Questar or the C5, and not quite up to the level of the Tele Vue Pronto among the refractors, the C90 provides bright, contrasty, high resolution views (tested resolution was 1.66 arc seconds, or about a third of a second off theoretical limits) at medium powers (the supplied eyepiece provides 33X), and will easily reach 100X with an eyepiece like the long eye-relief 10mm LV.

The C90 will cost you somewhere in the neighborhood of $500, and for that money you simply can not buy a better medium power view of the bird. Both the Kowa TS614 and the Nikon FieldScope IIED come close to (or even, under some conditions, equal) the C90's optical quality (slightly lower resolution, but higher brightness and contrast), and offer lower powers and wider fields, but both cost more on the street. All of the 77 80mm ED or Fluorite scopes are considerably more expensive, and would not give you a significantly better view than the C90. Among the refractors, only the Tele Vue Pronto would exceed the C90s performance and then you would have to be willing to pay somewhat more and carry a much heavier and bulkier scope out into the field (and a correspondingly larger and heavier tripod to go with it). All in all, the C90 with special coatings makes a very attractive scope for birding, and, quite possibly, is the best buy of any spotting scope currently on the market.