SIZE DOES MATTER:

OPTICRON ES 100 GA ED

The maximum resolution and brightness a telescope can provide are directly proportional to the diameter of its objective. Quality factors in turn determine how close to this maximum performance a given scope comes. This means that of two scopes of otherwise equal quality, the one with a larger objective will always outperform the smaller one. However, since enlarging the objective also increases the scope's price, weight and size, and since it is harder to attain high quality in a larger instrument, birding scopes have almost exclusively been in the 60-85mm range. Therefore, one of the most interesting new products to hit the market recently is the Opticron ES 100 that boasts a 100 mm ED objective. Prior to this, only Optolyth has dared to offer a birding scope this big, but unfortunately its quality was not on the level of smaller scopes. My interest in the new Opticron was heightened by a review on the Norwegian Kikkertspesialisten web site, where the ES 100 was rated above all the quality models we are familiar with here. The Finnish importer dared to order three ES 100's, and I tested the unit that looked best in a quick screening. Since it is more difficult to produce a high-quality 100mm scope than smaller ones, I would have preferred to try out a larger number to get some idea of how even their quality is. However, it is encouraging that all three in this batch were okay in this regard.

Opticron is a relative newcomer to the Finnish market. The company is based in England, and carries a large range of binoculars, telescopes and accessories that are manufactured to their specifications in Japan and the Far East. In Britain and the United States they have a large market share especially in the more affordable segment. Present offerings include about a dozen different scopes. One of these is the ES 80 ED on which the ES 100 design is based. This scope is made in Japan using the same body as the Falco Kite/Kamakura 80 ED we tested earlier, but according to Opticron, with different optics and eyepieces. Eyepiece selection available for the ES series is exceptionally wide at 13 different models. The list above shows the long eye-relief HDF models which all feature twistable eyecups.

I tested the Opticron ES 100 with resolution targets against my reference telescope and in the field with the same Swarovski ATS 80 HD we have had in the last two telescope tests. The weather was both bright and sunny, as well as grey and foggy winter weather and one starry night with good visibility, so I was able to test for backlight properties in bright sunlight. I only had the zoom eyepiece for testing. In the shop, I briefly tried the 42x wide-angle. It seemed very good and easy to view with, and offers those who wear glasses reasonable eye-relief.

Technical properties and usability

The ES 100 is rainproof but not nitrogen filled. The body seems to be made mostly of polycarbonate like the Kowa scopes, and features an impressively large retractable lens hood, which slides in and out a little too readily. The exceptionally large and solid tripod foot is made of metal alloy, and features multiple threaded holes allowing for twin-screw mounting for added security. The massive objective lens makes the scope very front-heavy unless the video head allows for ample front-to-back adjustment. The prism housing is partly rubberised, but the rest of the body is very slippery to handle, and I would consider the optional stay-on case a virtual necessity with this scope. The case is well designed and exceptionally well padded, so it also offers improved shock protection. The case fits so tightly around the objective lens, however, that opening and closing it, as well as handling the lens hood is difficult with the case on. The focusing knob is comfortably large and moves smoothly and precisely, and focusing is easy - even with gloves on. The eyepiece is offset to the side, so aiming at flying birds without a "cable-tie sight" or similar device is hard. The press-on eyepiece cover is not very deep and should be tethered to the scope to prevent loss. The objective cover attaches with a bayonet and sits inside the lens hood, though a press-on cover over the lens hood would be better and easier to use. The zoom eyepiece attaches with a threaded mounting ring, which is fast enough to use and holds the eyepiece securely even while zooming. Opticron's extensive range of scopes means that the zoom has a veritable jungle of different magnification markings, with none of them (as yet, at least) corresponding to the actual magnifications obtained through the ES 100. The guarantee is for 30 years. The Finnish importer, Foto Fennica, produces its own quick-to-use digiscoping adapter for Opticron. In principle, the large light-gathering power of the ES 100 is a distinct advantage in digiscoping, but on the other hand the smallest available magnifications are 27x with the zoom and 30x with a wide angle. Usually in digiscoping the preferred scope magnification tends to be 20x. Opticron does offer a specialty eyepiece for digiscoping which gives 16.25x with the ES 100 and maximises the light output of the scope, but this was not available in Finland and is not really suitable for general use.

While testing the ES 100 I observed a new and surprising phenomenon: when sunlight hit the scope body, its image quickly deteriorated, becoming soft and low in contrast. Initially I thought that heat haze had suddenly increased, but the image in the reference scope was unchanged. It turned out that the Opticron's dark grey polycarbonate body absorbs thermal radiation effectively while having very low heat conductivity, whereby unsymmetrical thermal expansion of the body apparently messes up the optical collimation. Indeed, I estimated the temperature difference between the sunny and shady sides of the scope body at over 30 C°, whereas a metal-bodied scope alongside the Opticron showed no discernible temperature difference. Protecting the Opticron's body from direct sunlight with white fabric or the scope's case solved the problem. Later, the Dutch optics tester Jan Meijerink and I experimented with a number of metal-bodied (Zeiss, Nikon and Swarovski) and composite-bodied (Opticron and Kowa) scopes. With unevenly applied heat, the resolution of composite-bodied scopes fell by up to 50%, whereas with metal-bodied scopes the deterioration was 0-15%. Thus, with composite-bodied scopes, a stay-on case not only protects them from nicks and bumps, but also improves their image quality!

Optical performance

Both with test targets and in the field, the resolution of the ES 100 was first-class, equalling the best Zeiss and Nikon specimens I have tested thus far. The 80x maximum magnification is perfectly usable and, under good viewingconditions, a clear advantage over scopes only reaching 60x. I saw the rings of Saturn, their Cassini division and Jupiter's belt formations better with the Opticron than I have with other birding scopes before, and with the very high magnifications attainable with a booster, the image held up better. Contrast in the ES 100 is very good, a bit better than in the Zeiss but not quite on the level of the Nikon or the Swarovski. In brightness, during daylight the Opticron is slightly behind the brightness champion Zeiss Diascope 85 at comparable magnifications, but as twilight deepens it catches up and even inches slightly ahead. The Opticron's larger objective gathers more light than the Zeiss, but Zeiss, Nikon and Swarovski utilise higher-quality prism systems and better multi-coatings whereby their light transmission is better. I did twilight tests in the field on a partly cloudy summer night against the Zeiss. I could not discern a significant brightness difference between the two scopes, but if there had been something critical to identify I would have chosen the Opticron since even in such low light its higher magnifications were a noticeable advantage. The colour balance of the ES 100 is quite neutral. It has a slight yellowish bias, but significantly less than the Zeiss. Colours are vivid but not quite as brilliant as in the Nikon ED 82. Black and white are also very well conveyed, but there is some visible residual chromatic aberration. In the amount of chromatic aberration the ES 100 is pretty similar to the Swarovski ATS 80 HD, but in this respect the Zeiss and Nikon are markedly better. With respect to flare and backlight properties the ES 100 is perhaps the best scope I have tested. There are hardly any reflections visible no matter where you point the scope with respect to the sun, and also in the night under the stars, the sky was exceptionally black and the stars unusually clear points without stray light. In general ease of viewing, the ES 100 is also very good. Eye placement is not difficult, viewing is untiring and focusing was easy, fast and precise. Image quality near the edge of the field is nearly flawless at small magnifications and very good even at high magnifications. I measured the zoom's eye-relief as 16mm @ 27x, 10mm @ 40x, 9mm @ 54x and 13mm @ 80x. At both extremes of the range there is ample eye-relief for spectacle use, but in the middle of the range things get tight. The true field of view of this zoom is a lot narrower (1.4-0.63°) than most other scopes have, and subjective fields are also mediocre at best. The viewing experience thus becomes rather unimpressive with a narrow, tube-like image. The Opticron zoom is not par focal, meaning that refocusing is always necessary after adjusting the zoom. However, since even with the best zooms it is practically always necessary to refocus after zooming from low to high magnifications, I did not find this shortcoming particularly bothersome.

Conclusion

Perhaps a little surprisingly, the Opticron ES 100 GA ED turned out to be a very good scope indeed. It is doubtful whether any other birding scope on the market today would let you identify birds as far away or in as low light as the ES 100. For those with an interest in astronomy it is also better suited than other birding scopes. The only significant image quality shortcoming is the zoom's narrow field of view. However, the scope is large and heavy, and also needs to be mounted on a very sturdy tripod and a video head with a mounting enabling enough front-to-back adjustment to balance it. During the testing I had it mounted on a Berlebach 3042 ash wood tripod with a Manfrotto 501 video head. This combination was stable enough and easy to use, but with such a setup the total weight of the rig approaches 8 kg. Thus, while the ES 100 offers top-notch image quality, it is best suited to birders who do not have to carry their telescopes much.

Specifications:
Length (measured with zoom eyepiece) c. 56 cm
Weight - ” - c. 2,500 g
Closest focusing distance c. 7 m
Eyepiece fields of view:
27 - 80x HDF zoom 1.4-0.64°
30x WW 1.9°
42x WW 1.55°
58x WW 1.2°
67x WW 1.0°
107x WA 0.68°
(the zoom's field was measured, the others are calculated from manufacturer's data)

Kimmo Absetz