42/10 = 4.200 mm exit pupil
50/12 = 4.167mm exit pupil
Because these two optics have almost the same exit pupil it comes down to glass, coatings and twilight factor. The Zeiss has the edge in glass and coatings but the Leica has a better twilight performance (also known as twilight factor). Twilight Performance/Factor is not widely accepted but here is how it works.
Twilight Performance - During daylight hours the magnification will be the principal factor in image resolution. At night, when your pupil is dilated, objective size is the controlling factor. In twilight conditions both of these factors affect resolution. The twilight performance compares the binoculars performance under these conditions. A higher twilight performance indicates that the binocular will resolve images better under dim light conditions.
Calculate the twilight performance of a binocular this way:
1) Multiply the magnification by the aperture 2) Find the square root of this product
IE:
Leica 12x50
12 x 50 = 600
Square root of 600 = 24.49489742783178
Leica 12x50 has a Twilight Performance of 24.5
Zeiss 10x42
10 x 42 = 420
Square root of 420= 20.493901531919196
Zeiss 10x42 has a Twilight Performance of 20.5
It's kind of hard to digest and there are many more factors to consider (like distance and target), but in most cases the Zeiss will present a brighter image with less resolution under twilight conditions.
If you had both of these binoculars side by side sitting in a deer stand looking deer 100 yards away here is what would happen.
When it is first getting dark you will be able to count the deer with both binoculars but only able to count points with the Leica. As it starts to get really darker you will not be able to see the deer with the naked eye or the Leica but will be able to see them with the Zeiss enough to count them but you can't tell bucks from does.
Because a spotting scope has both extreme magnification and an extreme objective lens it very well in twilight, add in the variable magnification of a spotting scope and you can alter the exit pupil on demand.
To really hunt mulies and elk successfully you really need to be using a high quality spotting scope and a 10x42 or 8x42 binocular.
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