The Zeiss 10×25 monocular and Zeiss 8×20 monocular represent the top-of-the-range products from Zeiss. Both are excellent instruments and worthy of the high accolades they have been receiving. There are some minor differences, which will be pointed out.
Both have fully multicoated lenses, which improves light transmission, but the 10×25 has phase correction coatings as well, which ensures truer colours. On the other hand the 8×20 has AquaDura® Lens Coating on the outer glass surfaces as well, which repels water and residue on outer glass surfaces.
Objective lens opening and magnification (power):
The Zeiss 10×25 have both a higher magnification and bigger objective lens opening than the Zeiss 8×20. The bigger lens captures more light but the higher magnification restricts the exit pupil to such an extent that this advantage cannot be utilized to the maximum. Had the magnification been 8x as well, then the benefits would have been considerable. However, in the end the wider aperture does make a difference.
The higher magnification of the 10×25 makes a big difference, but makes the instrument more difficult to use as well. You have to have a very steady hand, since all movement is amplified – something that goes with higher power. On top of this higher power usually means narrower field of view as well (see below), which makes finding an object and following a moving object more difficult.
The higher magnification does have advantages as well.
Both the Zeiss 10×25 Monocular and the Zeiss 8×20 Monocular have sturdy housing, resistant to corrosion. On top of this, the 8×20 has rubber armoring as well for a secure, non-slip grip. Both have an adjustable eyecup, which can twist up and down for comfortable viewing even with eyeglasses.
In both cases O-rings have been used to prevent moisture, dust and debris from getting inside the instruments, but in the case of the 8×20 the barrel have been filled with nitrogen as well to prevent fogging from inside. So the 8×20 is fogproof as well.
Other technical feature:
Field of view (FOV):
Usually the FOV diminishes as the power increases and this is what has happened with the Zeiss 10×25: Its FOV is 285 feet/1000 yards, which is not as good as the 331 feet/1000 yards of the Zeiss 8×20.
Close focus distance:
As in the case of the FOV, the close focus distance of the 10×25 is not as good as that of the 8×20: Only 16.4 feet, whereas the 8×20 can manage 6 feet. With the 8×20 you get an additional special close focus lens as well, which brings this distance down to an incredible 10-14 inches. So you can actually use this as a field microscope for viewing very small creatures like bugs.
10×25: 16 mm; 8×20: 15 mm.
Weight and length:
The Zeiss 10×25 is actually lighter than the Zeiss 8×20: Only an incredible 3.1 ounces, to the 4 ounces of the 8×20 (4.5 ounces with the close focus lens), but the 10×25 is slightly longer (4.6 inches) than the 8×20 (4 inches).