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