Optics by use
For birdwatching brightness and clarity are paramount. A magnification between 7x and 10x is recommended as this will give a clear and stable image with enough power to see detail. 8x is the most popular as this gives the best mix of power, brightness and field of view.
Large objective lenses will let in more light but maybe too big and heavy to carry. A good compromise would be around 40mm – 45mm, as these should be comfortable to handle yet offer good performance in low light. A lightweight solution that is easy to carry and compact enough to fit into a pocket would be 8×21, 8×25 or 8×30. These should perform well in daylight conditions.
If you plan on birdwatching from a longer distance, a spotting scope with a magnification of around 25x (most have a zoom function with variable magnification) may be more suitable. This would need mounting on a good, sturdy tripod.
Walking / Hiking / Climbing
Size and weight are critical factors for this application. Between 7x and 10x magnification would be ideal, for most daylight uses, 20 to 34mm diameter objective lenses are fine, look for 8×25, 10×26, 10×34 or similar.
Ruggedness is vital, look for a model with rubber armour for added protection and you may wish to consider a waterproof model.
If you want binoculars or a monocular for general use, rather than a specific hobby, 7x to 10x magnification would be best.
This will give adequate magnification with a more stable image.
Binoculars with High-magnification and large-lenses are generally heavier so bear this in mind if you will be using them for long periods of time.
Consider the weather conditions they will be used in. For rainy or damp weather, waterproofing is a must.
Rubber armoured cases will give extra protection against everyday knocks and bumps.
To view detail from a distance a higher magnification is required. For handheld use, 12×50 or 12×60 is about the maximum size you can use without a tripod. If you go higher than this the instrument would become too heavy and require a sturdy tripod to keep the image stable.
With a good quality tripod, 15×70, 20×80 or even a scope with a magnification of 20+ would be suitable for viewing detail at a distance.
If you have a room or balcony with a great view then you could even consider something from the larger models available such as 25×100 or 20/40×100.
Theatre & Concert
For a theatre or concert, a compact binocular would be best as they will easily fit into a pocket or handbag, they should also be lightweight as you may be holding them for long periods.
Most stages are well lit so a large objective lens is not required.
Some binoculars come with a small built-in light which can be useful to help find your seat number or to illuminate reading material without disturbing people sat nearby.
With marine use, stability can be an issue so choose a model with low magnification. In some circumstances low light can also be a hinderance so a larger objective lens will give a brighter image. 7×50 would be ideal, for a little more magnification choose a high quality 8×40 or 10×50.
Waterproofing is essential and it is also prudent to use a flotation strap incase the binocular goes overboard!
Some models come with a built in compass which may prove a useful accessory.
The priority for most sports fans would be to select a binocular with a wide ‘field of view’ allowing you to take in more of the action and quickly locate a moving subject. Stability could be an issue but usually, as a spectator you are not too far away so a lower magnification should suffice.
If the binocular is going to be exposed to the elements then it is important to choose a waterproof model.
For aviation, you will require a higher magnification to see any detail.
Many aircraft spotters use 20×50 or 20×60 with a tripod or other support.
For handheld use, 12x magnification is around the maximum you can go before image stability becomes an issue.
Anti-UV lenses can be helpful in hazy conditions to increase sharpness.