With apertures of only 10 mm (Carl Zeiss Optical Inc Monocular (5×10 T MiniQuick)), 12 mm (Carl Zeiss Optical Inc Monocular (4×12 T Monocular))and 18 mm (Carl Zeiss Optical Inc Monocular (6×18 T Monocular))these monoculars from Zeiss are really not bigger than a fountain-pen, but will blow your breath away with what they can do, even though they are only a few inches long and weigh only between 0.81 oz to 2.05 ounces.
Overall there’s not much to choose between the three monoculars as far as quality is concerned. They are all from Zeiss and that means quality. All three of them are also sealed against water spray, but not fogproof (nitrogen filled), as is the case with the Zeiss 10x25T and Zeiss 8x20T monoculars.
The differences in magnification and objective lens diameter (aperture) do account for some technical differences, which might become important when finally deciding which one to buy.
Bigger aperture always means more light entering the instrument, which eventually translates into brighter images, as long as the magnification does not go up too much. The 5x10T has an exit pupil diameter of only 2 mm, but the 4x12T and 8x18T 3 mm, which is not much and will restrict the clearness of images in dim light. However, in bright daylight this will not be a problem. Having bigger apertures the twilight factor of the 6x18T is higher (10.4) than the other 2 (7 and 6.9).
In one feature the 4x12T is way ahead of the other two: Field of view (FOV). A number of factors determine the FOV of a binocular and monocular, among which magnification. Having a magnification of only 4x, the 4x12T boasts a FOV of 540 feet at 1000 yards, which is really exceptional. See if you can find any instrument close to this. This is very handy for locating an object and following moving objects. The FOV of the other two is 300 feet/1000 yards (5x10T) and 360 feet/1000 yards (6x18T), which are not to be sniffed at in any case.
Another feature in which the 4x12T and 6x18T excel, is close focus distance. The 5x10T has a close focus distance of 13.12 feet, but the 4x12T focuses as close as 1.48 feet (17.76 inches), and the 6x18T 0.98 feet (11.76 inches). This is so close, that these two could be used to study small insects at close range.
In terms of overall size these three are all very small: The 5x10T is 4.45 inches long, the 6x18T 3.35 inches and the 4x12T only 2.4 inches. However, in terms of weight the 5x10T weighs only 0.81 oz, as compared to the 1.59 oz of the 4x12T and the 2.05 oz of the 6x18T.
Pricing: This could still be a deciding factor, since the 5x10T is virtually half the price of the other two. It sells for $143.32, whereas the 4x12T goes for $269.99 and the 6x18T $299.99. Whether the huge difference in price is justified by the benefits of really close focus, more power and FOV, well as less length you will have to decide for yourself.