The Urban Birder

David Lindo — The Urban Birder (R. F. Spencer)

How do I choose my urban birding optics? What do all the numbers mean?

All binoculars are defined by two key numbers such as 8x42. The ‘8’ is the magnification and means a bird appears to be 1/8th of its actual distance away. The other number is the diameter of the objective lens (the large end of the bins) in mm.

There are two basic types of binocular — porro prism and roof prism. The simple optical design of the ‘traditional shaped’ porro means they tend to deliver better optical performance for your money compared to roof prism binos. Roofs tend to be more compact but typically more expensive.

The development of the now popular roof prism design provides some urban birder friendly features though — internal focusing allows for waterproofing; long eye-relief eyepieces give a full field of view with or without glasses; close focus makes them useful for watching butterflies and other insects.

What magnification does an urban birder need?

First, bear in mind that as magnification goes up, the apparent image quality will reduce for a given objective lens size. Images appear less sharp and have a lower relative brightness and poorer colour contrast. Depth of field is shallower meaning you have to work the focus wheel more and any shaking of your hands is amplified. That means I recommend a 7x or 8x for general urban birding with binoculars. If I'm on top of Tower 42 looking for distant raptors, I'd need to reach for my 'scope and get 20x or more magnification on a nice stable tripod.

What objective lens size?

The bigger the objective lens, the more light there is coming in. A 50mm lens will admit 2.5x the light of a 30mm lens. For daytime use, a 7x42 or 8x42 bino will deliver a pool of light to your eye that's large enough to cover your iris so you get a full field of view.

What if I'm a glasses wearer?

Many binoculars provide the full field of view when wearing glasses by having either fold down or twist type eyecups. As a general rule the longer the eyerelief, the better the instrument will be for spectacle wearers so try to find something with at least 15mm eyerelief.

If I'm urban birding all day, I don't want a big, heavy pair of bins round my neck.

Small, lightweight binoculars that can be taken anywhere are a far better tool for the urban birder — you'll be more inclined to carry them all day, every day. And that means you're more likely to add those all important ticks to your list.

What about 'scopes? How do I choose one of those?

Again, there are a few key numbers to look for. Most manufacturers put the objective lens size somewhere in the name of the product. For example, my Opticron 'scope is an HR 66 — it uses a 66mm objective lens. Other typical 'scope sizes are 50mm, 60mm and 80mm. Once you go above 100mm, the 'scope starts to get big and heavy. This is fine if you're planning a long sea watching session sat in one place but is not such a good pick for a walking tour around the city streets.

Eyepieces are usually defined by their magnification and whether they are wide angle. There are also zoom eyepieces, which offer a range of magnifications. The zoom on my HR 66 goes from 18 to 54x for example. As well as my zoom eyepiece, I have a 20x wide angle which has a great field of view. For general daytime urban birding, good magnifications are between 20x and 30x for a 60mm scope, 25 to 35x for a 66mm and 25 to 40x for an 80mm.

What's this term ED that I see everywhere?

When light gets shoved through the various prisms and lenses in a 'scope, the different colours that make up the image can get ‘split’ and this results in colour fringing and lower contrast. Using ED glass in the objective lens, these effects can be reduced but this type of glass is more expensive. It's worth bearing in mind that at magnifications of 30x or lower, the benefits of ED glass are hardly noticeable.

My best advice is to try out your chosen optics before you buy them. There's no substitute for handling them and looking through them because the numbers don't tell you the whole story. You can choose a pair of shoes based on your foot size but not every pair is going to be comfortable. And you need different types of shoe for doing different things. The same is true for optics — a 100mm 'scope with its zoom set to 60x is not going to help you watch blue tits that are 10 feet from the kitchen window on your garden feeders!

— Pete Gamby, Opticron