19
Feb
12

Bin it or not? Part 3

The next of the articles on Binoculars starts with Twighlight Factor:

Twighlight factor is supposedly a measurement of the performance of binoculars in low light. It is calculated as the square root of the sum of the aperture and magnification. So on a pair of 10X40’s the twighlight factor is 20 (10 x 40 = 400 Square root of 400 = 20) For a pair of 7×35’s the twighlight factor would be 15.65. The higher the Twighlight factor the better the resolution in low light conditions … reputedly. You will often see this as one of the listings in binocular specifications however the reflective coating and quality of the optical internal structure have a greater effect.

Reflective Coating: This is the coating placed on the lenses to firstly optimise light transmission by decreasing the light reflected from the objective lens and increasing the brightness of the image. Good coatings can double the amount of light passing into the binoculars.

When light enters a set of prisms in binoculars it gets split up into its coloured components; red, orange, yellow, green etc. It’s termed to be ‘out of phase’. In simple terms ‘Phase Correction’ is the second job of lens coatings and avoids you seeing coloured halo’s around bright objects.

The ability of some binoculars to focus on close subjects means they are ideal for dragonfly and butterfly observation. If you have to walk away from something to get a good view it can get very frustrating. Check the minimum focus distance. If you can focus on your feet you have an ideal pair for watching insects.

Eyecups can be adjusted down or up. Some have rubber ones that fold down or they may twist or push pull into place. Generally if you wear glasses, to get correct eye relief. put them down. Otherwise have them up. To be honest there is room for personal preference here; do whatever you feel is the most comfortable. This also goes for the general feel of your binoculars. They must feel comfortable in your hands; not too big not too small. When you try them out in the shop spend some time handling them and make sure you visit on a dull day – don’t try them out on the brightest day of the year!

Close Focus – essential for seeing detail at close range like on this Common Lizard

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