Q: I am in the market for new binoculars and trying to decide between 8×30 and 8x42mm. The former would probably be a bit lighter, but would there be much difference in their poor-light performance?
Errol King
A: For high-contrast viewing, your binocular needs to transmit as much light as it can. This calls for a relatively large exit pupil and a high twilight factor.
Exit pupil refers to the diameter of the circle of light available to your eyes when you look through a binocular.
This measurement is determined by dividing the diameter of the objective lens in millimetres by the amount of magnification. An 8x30mm glass has a 3.75mm exit pupil against 5.25mm for an 8x42mm binocular.
In low light, the pupils of your eyes dilate to about 5mm — larger than the exit pupil of an 8×30. The 8x30mm optic may be alright for daylight viewing, but it won’t deliver all the light your eyes can utilise at dawn and dusk.
The twilight factor is more difficult to calculate. It’s found by multiplying the magnification by the objective lens diameter and finding the square root of the product. Theoretically, the higher the number, the better the binocular should perform in poor light conditions.
But magnification also matters. While 8×42 binoculars have a larger exit pupil (5.25mm) their 18.33 twilight performance isn’t as good as the 20.5 rating for the 10x42mm glasses with an exit pupil of only 4.2mm.
Theoretically then, these numbers provide a guide to performance, but in truth performance depends heavily on the quality of the lenses.
The multiple components in a viewing system must be firmly and accurately aligned. The glass must be high quality and lenses precisely ground and multi-coated to increase light transmission and eliminate unwanted reflection. The mechanical components must be accurately aligned and adjust smoothly.
How do you determine binocular quality? Price used to be a good indication, but in recent years optics from Pacific Rim countries using Ohara glass have improved optically, mechanically and in quality to rival more expensive European binoculars and are less expensive. I suggest you check them out.
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