Monday, November 25, 2013

The right tool for the job. Part 4: Dawn, dusk and December

Dusk hunter. Barn owls are just one of the many treats that are seen in the twilight.

The last of 7x42s. Leica Ultravid HD.
The twilight hours at dawn and dusk can be some of the most productive for nature watching. Wildlife of crepuscular habit may only be seen now, whilst at dusk nocturnal animals become active and the creatures of the daylight seek shelter and safety for the night ahead. In the depths of winter in Britain half light can persist nearly all day in overcast weather. These times present challenges to the observer and brightness of image becomes the most important factor in choosing optics for this situation. As ever, the competent all-rounder the 8x42 will not disgrace itself, but other specifications can offer a little more. If size and weight are no object then an 8x56 or a 7x50 will deliver the largest exit pupil, and therefore brightest image, that the human eye can accommodate. These are often not a practical option due to their bulk and so a good alternative is a 7x42. This specification is becoming increasingly hard to come by, Leica now being the only top draw brand producing a 7x42 in their flagship line-up.

If you require a bit more magnification then there is no getting around the fact that they need to be bigger. To achieve nearly the same level of brightness as an 8x42 with a 10x binocular you need a 50mm lens.

The same principles apply with telescopes, with generally the bigger the objective lens, the brighter the image. One partial exception to this rule is with the new Swarovski ATX range, in the case of which the 85mm version can achieve a brighter image at minimum magnification than the 95mm can. This is because the 85mm zoom range starts at 25x, where as the 95mm starts at 30x. Obviously the 95mm will then be the brighter of the two above this magnification, but the 85mm has the greatest potential brightness.

Swarovski ATX 65,85 and 95mm objective modules
In theory, on most scopes a fixed magnification eyepiece can deliver not only a wider field of view than a zoom at any given magnification, but also a marginally brighter image. This is because, as a rule, fixed wide angle eyepieces have fewer lenses in them and so fewer glass surfaces at which light can be lost. The best combination for a bright image is a large objective lens and lower magnification, say 20x-30x and, as ever, good quality glass.

Exit pupil and brightness explained.

The brightness of a binocular or telescope can be calculated by dividing the objective lens diameter by the magnification (e.g. 42mm/8 = 5.25mm or 50/10 = 5mm). The figure obtained is known as the exit pupil which is the diameter of the disk of light projected by the optics. The larger this is, the more your pupil can dilate in low light.