Both optical systems represent an optimum ratio between magnification, depth of field and linear field of view. The choice depends on what Optical characteristics are preferred specialist for a more accurate and predictable result. In the market there is increasing spectacle optics, where you can adjust the working distance to the observed object, the magnification the stereo and the individual, and can and do not regulate, and to trust completely immutable factory settings. Of course, glasses should have a tool and light weight, not more than 98 grams., quickly and easily fixed into position on the head, and also easily be displayed in the off position. Donald Gordon is likely to increase your knowledge. In the market many models who have a working distance of the object is not regulated, instead of glass – easy transparent plastic and vulnerable, visual stereo, respectively, is set consistently, the average temperature in the hospital, under the nose there is a soft lining and a convenient carrying strap, Velcro, which leads to visual discomfort and, in essence – money. When selecting optics, remember that the more opportunities there for adjustment, the greater its value and functional adaptation of the above view of the user to work with a small strain the eye muscles. The errors in the settings lead to premature fatigue of eye muscles, pain in the shoulder and occipital areas. In my 10 years of experience and increasing the assembly of the spectacle optics to order, many do not experience difficulty when using it, if the lenses of glass, it is possible to adjust the stereo visual individual, and it weighs less than 100g. Ryder: the source for more info.