Binoculars – A Primer
Have you ever wondered what is the difference between a telescope and a spotting scope?
Both are designed to watch distant things. But while telescopes (they come in different designs and capabilities) are primarily aimed at watching the night skies, spotting scopes are aimed at terrestrial viewing. In case of the latter, a prism assembly makes sure the viewer sees an erect image of the subject. Telescopes have their existence sorted out. Celestial objects need not be upright/ erect. Telescopes are corrected for gathering maximum light in a dark sky and often a bit ungainly and usually setup at one spot. That doesn’t mean one can’t use a telescope for viewing terrestrial subjects. One can and one does. Only that the view might not be the best. Similarly, there is no reason why a spotting scope cannot be used to take an immersive view of the moon or planets. But then again, it might not be the best view.
Spotting scopes have two typical use cases. One is to watch birds from a distance and the other is long range shooting. The magnifications offered by a spotting scope is much higher than most binoculars. But the price to be paid is in stability. A spotting scope needs a tripod, preferably a sturdy one and a very good ball head and again preferably a fluid head. Make no mistake the proper tripod is a life line for using a scope. A telescope on the other hand is preferably placed at an appropriate spot and put on a mount that accommodates the earth’s rotation - preferable but not essential. Spotting scopes are of two basic designs. A straight scope and an angled. A straight scope is often preferred by shooters/ snipers as it is easier to instinctively find the subject/ target. But the angled scope can be easier when sharing the scope and need not
be extended to the eye height. Smaller straight spotting scopes with 27-65mm objectives are often used by birders but angled scopes are just as popular.
The best objective glass (spotting scopes vary in objective size 77mm, 80mm, 85mm and 100mm) helps to create an aberration minimised, sharp and contrasty image inside the scope. The prism assembly inverts an inverted image to make it erect.
So how do you see it?
Bare eyes… well if you can take your eyes near the prism; a magnifying glass - a possibility.
It is the eyepiece that makes the view. The width of field and overall image quality including sharpness and brightness are functions of the eyepiece.
Most spotting scope manufacturers make their own eyepieces. A sensible step is to have proprietary eyepieces otherwise they would lose money. Usually a zoom eyepiece is offered - 20x-60x odd magnification. Note here the same eyepiece will offer a lower magnification on a smaller scope. Using a zoom eyepiece you spot the subject and then zoom in to see its details. As you zoom in (in most cases the zoom mechanism is a part of the eyepiece) the image becomes bigger, the field darker or let’s say less bright and narrower. Often spotting scopes are not totally parfocal and need minor focus adjustments. The focusing mechanism is usually on the scope barrel - either as a large ring (or two separate rings for coarse and fine adjustments) or a small focusing knob on the barrel. Back to eyepieces. Invariably, fixed focal length eyepieces offer brighter, sharper, wider views than a zoom eyepiece. But the convenience of a zoom eyepiece is always to be considered. So what exactly is an eyepiece? It is a type of lens that is attached to a variety of optical devices such as telescopes, spotting scopes and microscopes. It is named because it is usually the lens that is closest to the eye when someone looks through the device.
The objective lens or mirror of a scope collects light and brings it to focus creating an image. The eyepiece is placed near the focal point of the objective to magnify this image. The amount of magnification depends on the focal length of the eyepiece.
The focal length of an eyepiece is the distance from the principal plane of the eyepiece where parallel rays of light converge to a single point. When in use, the focal length of an eyepiece, combined with the focal length of the telescope, spotting scope or microscope objective, to which it is attached, determines the magnification. It is usually expressed in millimetres when referring to the eyepiece alone. When interchanging a set of eyepieces on a single instrument, however, some users prefer to identify each eyepiece by the magnification produced. So the Pentax zoom eyepiece can be called 8-24mm or 20x-60x (the latter on an 80mm scope).
While simple and inexpensive Plossl
eyepieces have a decent 52 degrees field of view,
complex eye pieces have anything between 70-110 degrees field of view.
Coupled with the fact that fixed focal length eyepieces are brighter, sharper than zooms, make them great for terrestrial viewing even.
The 1.25-inch eyepieces are most popular as they provide a balance between width of field, eye relief and quality of corrections. This is why many spotting scopes take 1.25-inch eyepieces through an adapter or native. Moreover, the eye relief offered is extremely generous.
Spotting scopes are tools for digiscopers. Digiscoping is a technique wherein a DSLR with lens or DSLR without lens or a fixed focal length point and shoot camera is attached to a spotting scope. The camera takes an image of the image formed inside the scope. When done properly, surprisingly good results are obtainable, at least for stationary birds (subjects).
eyepieces have a decent 52 degrees field of view,
complex eye pieces have anything between 70-110 degrees field of view.
Coupled with the fact that fixed focal length eyepieces are brighter, sharper than zooms, make them great for terrestrial viewing even.
The 1.25-inch eyepieces are most popular as they provide a balance between width of field, eye relief and quality of corrections. This is why many spotting scopes take 1.25-inch eyepieces through an adapter or native. Moreover, the eye relief offered is extremely generous.
Spotting scopes are tools for digiscopers. Digiscoping is a technique wherein a DSLR with lens or DSLR without lens or a fixed focal length point and shoot camera is attached to a spotting scope. The camera takes an image of the image formed inside the scope. When done properly, surprisingly good results are obtainable, at least for stationary birds (subjects).
