Optical equipment is designed to fulfill a set of criteria that best suits some applications over others.
Understanding the specifications is the best way to choose the perfect optical equipment for your own needs. This article has been written based upon my 30 years experience in optics in ‘real life’ applications.
This article deals specifically to daytime optics, though much of this information will also apply to night vision optics as well.
Magnification
This is the magnified increase in size that an object looks when viewed through the device. A 4x device will make an object appear 4 times larger, or if the object is 100 metres away, a 4x optical device will make the object appear as if it was only 25 metres away.
The advantage of increasing magnification is an apparent increase in object size or ‘closeness’, but there are also disadvantages. As the magnification increases:
1. The image appears to shake more if hand held
2. The image appears darker (assuming all other specifications remain the same)
3. The image quality reduces (assuming all other specifications remain the same)
4. The field-of-view reduces (assuming all other specifications remain the same)
5. The exit pupil (diameter of light entering the eye) reduces as well as the eye relief (the distance between the eye and the rear ocular)
Item 1 can be solved by using a rest or a tripod.
Items 2 and 3 can be reduced by increasing the size of the objective lens, which also increases the weight and size of the instrument.
Item 4 and 5 can be improved by a design change in the optics, which usually incurs extra cost.
In practical terms, if you need to view objects at a long distance, in bright conditions, a high magnification becomes the paramount specification. If you need to scan a wide field of view or require optimum twilight performance, a low magnification is beneficial.
If a small size and low weight are your main criteria, then a small objective lens will limit magnification and twilight performance. Spotting scopes tend to be the first choice instrument for use when walking great distances, because the reduction in weight of a single optical channel allows the user to increase the objective lens size thus allowing for a greater magnification and twilight performance. Whilst binoculars are better for distance perception and are easier for viewing for longer periods, but are heavier to carry.
