Whenever I field test binoculars, I do the lamppost check. Believe it or not, such a simple thing like a lamp post can reveal a lot about the optical quality of binoculars.
First test is to centre the lamp post in he centre of view and focus sharply. What you are looking for here is chromatic aberration (false colour) on the edge of the post. This will be a blue or green. With some budget binoculars this will be quite a distraction. With mid priced optics it will be very minimal. Then with high end ED glass it will not be visible at all. So how is this a benefit ?. With less chromatic aberration the image will be much sharper and brighter, plus any contrasting colours on animals will be sharp and well defined.
Next, without adjusting the focus move the lamp post to the edge of view. By doing this you can do three more tests shown below
- Check for false colour as above
- Check if it is in focus. Due to the construction on lenses the edge of field focus may be slightly out when compared to the centre of field. This is quite common, but will be less evident in higher end binoculars. You will often hear the term flat field mentioned in some reviews. Binoculars with a flat field will give better even focus to the edge. 7×50 binoculars give very flat fields.
- Image curvature. In the centre of view the lamp post will be a straight line. However if you place on the edge of view to will notice it bend (image curvature). Once again, with more expensive binoculars this will be less pronounced, leading to an overall better image
Buy great quality binoculars online here