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Pentax VD 4x20Although they may not be a top recommendation, we've included these binoculars for their cleverness and novelty factor.★★½

I asked this question in a hurry. We were at the airport waiting to fly to Peru, on a once in a lifetime trip was planned to the last detail.

Stylish, small and lightweight, the compact Nikon Aculon T02 8x21 offers limited — yet pleasing — views of planets, the moon and star fields.

This is one of the weaknesses you have to accept when choosing a compact binocular. In poor light, you will not perceive as bright an image as a larger binocular with the same or similar magnification. Rubber-coated, non-slip, and water-resistant, these Pentax binoculars use an unusual reverse Porro prism construction. Most compact binoculars use a roof prism construction - whilst alternative, larger models usually use the Porro prism design.

Although they are not the lightest on the market, weighing in at 12.9 oz (345 g), they feel more substantial and less toy-like than others. You'd be hard-pressed to find something more compact. Binoculars with 8x40, 8x42, 8x70, 8x32, or even 8x21 all offer the same magnification. They will all magnify the image by eight times. Likewise, 10x42 will magnify ten times, 12x50 will magnify twelve times. Binocular magnification is sometimes referred to as power, I will just use the term magnification. You will notice the “x” alongside the “8” in my example, this belongs with the number. Meaning “8x” or eight times magnification. Hopefully, I have been able to demonstrate that even within the subset of compact binoculars, the combination of the lens size and magnification has a big effect on its performance and making the right choice for your particular needs is important. This can get very technical and if you are interested, read BBR’s complete guide to the exit pupil.

Tiny lenses (21mm / 22mm) are for those requiring the smallest, most lightweight instrument possible, but be aware this comes at the expense of image brightness and potentially image quality as well. Their size obviously plays a large part in determining the overall dimensions of the instrument and larger lenses generally weigh and cost more to make. However their size also has a big effect on the image quality, brightness and low light performance of the device. Image Quality & Brightness However and as you probably know, the pupils in your eyes expand in low light conditions, allowing them to take in more light. Their maximum diameter varies from person to person and with age, but in general, we take 7mm as being a good rule of thumb. Thus if you want a binocular with an excellent low light performance, you need a large exit pupil of around 7mm. To achieve this you either need extremely large lenses or a very low power, or a combination of both. So, for example, a popular low light binocular is a 7x50 and has a 7.1mm exit pupil (50÷7). If the magnification is underwhelming, you can convert to telescope mode for 16x magnification. Bear in mind that this results in a very narrow field of view of just 2.6 degrees, so the usefulness of this is questionable. We expect it to give a worthwhile view of the moon but not much else in the night sky.