Tuesday, October 23, 2007

Optics testing

Scopes have become an essential part of the equipment of most birders nowadays, from the beginner to the expert, and one great benefit of the constant improvement in optical technology is that the smaller scopes (objective lenses of under 70mm) can often do a job that would once have been the preserve of the big boys.

They offer lightness and stowability, attractive to anyone whose birdwatching involves covering a lot of ground, on foot or by bike, or who simply doesn’t want to haul a heavier piece of equipment round all day.

For the November issue of Bird Watching, we tested six scopes from four manufacturers, covering a wide price range, and offer a guide to the performance of all. In the end, though, there’s no substitute for trying them out thoroughly yourself before you buy. Every birder’s preferences will be different, based on factors as diverse as the sort of birdwatching they do, their eyesight and physique, the scopes they’re used to, and of course their budget, and all the scopes in the test had plenty to commend them.

With that in mind, we offer a guide below to the features we looked at and that you should also test, and tips for buying.
For our test, we asked major manufacturers for up to two examples each of their 60-70mm objective lens scopes, plus a 30x (or equivalent) eyepiece. All were tested alongside each other at the Anglian Water Birdwatching Centre, Rutland Water, on a single day in September. A resolution chart provided by Minox was used.

Here are the optical criteria we tested each of the scopes on – you should be looking at all these when buying. While optical quality is always likely to be a high priority, remember that you need to settle on a scope that is also easy and pleasurable to use.

FIELD OF VIEW: The wider the field, the easier you’ll find it to observe flocks of birds, or birds in flight. It is best tested by focusing on a man-made object, where the relative width of field of different scopes can be measured.

RESOLUTION: Focus each scope on the same object – a particular part of a bird, or a branch maybe – and look at the difference in detail. If at all possible, test this in low light and deep shade, to separate the very best from the rest of the field. We used a resolution chart, but placed particular emphasis on in-the-field testing.

BRIGHTNESS: All the scopes did well for brightness, impressive given their relatively small objective lenses. A smaller scope would suffer by comparison with a large one in poor light, but factors such as the type of glass used also make a big difference.

COLOUR CAST: This is not a fault – some models will have a slight blue or yellow tinge to the image. The former produces a brighter, cleaner image, with slightly reduced contrast, while the latter produces better contrast but slightly reduced colour.
COLOUR FRINGING: This is the appearance of a faint blue or yellow ‘halo’ around the object you are viewing. It is best tested by viewing an object against a pale background. All scopes are likely to suffer from it at least a little, especially towards the edge of images, but you only need to worry if it becomes distracting.

If you can, ask other birdwatchers if you can try their scopes, and take note of any features you like or dislike.
Before you go to buy, make a shortlist of models that you’re interested in, and give your dealer a call to check that they’re available.

Make sure the shop you buy from has good viewing facilities, so you can test the scopes yourself. Some retailers hold field days at reserves and birdwatching fairs.

Compare all the models on your shortlist thoroughly before trying any suggested by the shopkeeper.

If the shopkeeper gets too technical for you, don’t be afraid to ask for clarification. If he is particularly enthusiastic about a certain model, remember that profit margins vary, and that it might be one that earns him more money.
Try to compare only two models at a time, three at most.

Take your time about comparing. Don’t be hurried (good retailers will be happy to give you as long as you need), and make notes as you compare – these will be invaluable if you decide to come back later after thinking things over.
Take all factors into account – a superb image is all very well, but the scope also needs to be one that you’re comfortable using.

Test the actual scope you’re buying before taking it home.

Check that the box carries an approved importers mark – buying a ‘grey’ import can cause problems if anything goes wrong.

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