Really depends on the scope, as it can be the combination of a number of things like the objective size. The trend is the bigger the both, the better.
Here's the science bit...
Magnification when quoted, as far as I am led to believe, and not so far contradicted, is a number derived from the focal length of the objective lens cluster divided by the ocular (eye piece). The higher the focal length, the narrower the field of view (the width of what you can see left, right, up and down) and the shallower the depth of field (the amount of stuff you can see behind and in front of the target, in focus... a deep depth of field allows you to say see targets say 10yds either side in focus as well, a shallow one perhaps 2-5 yds).
So it's entirely possible to have a 'high' mag scope which doesn't differentiate between targets behind or in front as well as another high mag scope which does... all with the same 'mag'.
Focal lengths of lens clusters are rarely ever published, so it's difficult to know. But certainly comparing a few 50x scopes there's a different field of view between them and a different feeling between them.
Had an interesting chat with the Khales technical expert in Germany, which confirmed quite a few theories, however I didn't get any actual specific facts to relate or compare to. Once the basic behaviour of the lens focusing is established there are other factors as to how it performs with a wheel, as the actual focusing mechanism can be changed to make it slower, faster, wider gaps etc.
So I don't think there's an actual number per se to cling to, but going bigger is the general trend on objective and zoom, but scopes with small specifications may pip others for other reasons.
Try before you buy... but i'd say 32x seems to be the lowest that is used, but many are using 40 or 50 in the mainstream.
Interesting conversations with Calps & Tool in Germany about how they rangefind, suggesting good reasons why some like some scopes and some like others, due different ways of using them. The trick is to find one that matches how you like to work, being objective in making sure that's the key reason for using that scope and not something else.
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