Bushnell Infinity 8.5x45 and 10.5x45 binoculars

I’ve always thought that large objectives look wrong on roof-prism binoculars, almost at odds with the compactness that the roof design sets out to achieve. Bushnell, however, appears to have got the balance just about right with its two new Infinity models which, the manufacturer claims, are the most versatile binoculars it has ever built.

Without making the leap to 50 mm objectives, the 45 mm lenses go the extra mile in an attempt to increase that all-important light-gathering capacity for both models over and above the more traditional 42 mm that has been the industry standard in recent years. There is a corresponding increase in magnification, too, with 8.5x and, more interestingly, 10.5x replacing the conventional 8x and 10x featured by most contemporary binoculars.

So what does all this bring to the optical performance party? Theoretically, it adds up to achieving a higher magnification with no corresponding loss of brightness. A quick exit pupil comparison calculation (objective lens diameter divided by magnification) yields ‘brightness factors’ of 5.29 and 4.28 for the two Infinity models, compared with 5.25 and 4.20 for traditional 8x and 10x42 mm models in general. Of course, this is a very crude method of determining light-gathering capacity, and many other factors, such as glass quality and coatings, will come into play when comparisons are made across a spectrum of models from different manufacturers. Nevertheless, it’s a good starting point for judging binoculars that are priced well below top-of-the-range models, as these are.

In terms of handling they have a solid, almost chunky, feel, and at the same time appear well balanced. The open-bridge styling and magnesium alloy barrels combine to reduce weight which, at just over 700 g for both models, falls well within acceptance levels – especially when the extra material needed to accommodate the larger objectives is taken into consideration.

The eye-cups are coated in the same rubber as the barrels. Twisting out, they click-stop loosely in three positions above the base setting, with those of the 8.5x model offering greater eye relief (19.5 mm) than those of the 10.5x binocular (15.5 mm).

All lenses are coated with Bushnell’s ‘Rainguard’, formulated to facilitate cleaning and scatter water droplets. In the rain I found it worked to a degree, insomuch as water droplets pearled on the lens surface without running or smearing, but they would not easily be shaken off.

The broad central focusing wheel, with its widely spaced rubber contact lines, turns smoothly and freely. It appeared to show some initial play or looseness each time my fingers made contact with it, but this did not affect focusing. Pulling out the wheel operates the dioptre for single-eye focusing of the right ocular, while pushing it back locks the setting against an incremental +/- scale at the inner base of the wheel. This was easy to use and worked well for the 8.5x model, but the dioptre mechanics on the 10.5x instrument I tested appeared too loosely ratcheted, with the scale hardly moving from the ‘+’ end, rendering the single-eye focusing more difficult to achieve.

Approximately 1.6 turns of the focusing wheel takes the image from close focus to infinity, and I measured the former at 1.8 m for both models – pleasingly, a shade under the manufacturer’s figure of 2 m. The depth of field was acceptable for both models. The field of view also appeared fine for both – although it was obviously wider (at 107 m) in the lower magnification binocular – and falls within the expected range for binoculars within the same class and price bracket.

Although the image generally remained sharp, it began to lose definition at the field edges (about 5 per cent of the periphery) and there was a noticeable difference between the two models in curvature of field in this area: it was at a low level in the 10.5x binocular, but was rather more evident in the 8.5x model. Chromatic aberration was evident across the whole field, but in most instances it was acceptable and did not detract from general viewing enjoyment.

Putting the binoculars to the test on Durham’s recent Wilson’s Phalarope was as good as it gets during any optics assessment. Watching the bird partly against the light resulted in no obvious glare or image distortion and I found the overall image for both models very bright, with reasonably good contrast and an accurate reflection of reality in terms of colour.

In low light conditions just before dusk, both models performed well, but there was a detectable difference in image brightness between them with, as expected, the lower magnification binocular returning the brighter image.

While both models have their pros and cons, on balance I found the 8.5x the more desirable of the two.