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Step one: decode the two numbers
A pair marked 8x42 magnifies the view eight times and has 42-millimeter front lenses. Magnification makes distant subjects larger; the lens size governs how much light enters. Higher magnification narrows the scene and amplifies hand shake, so a 10x pair feels twitchier than an 8x pair. Before comparing models, write down the subject you watch most, how far away it usually is and whether you hold the binoculars for minutes or hours. That short note gives every later number a purpose and stops you chasing the biggest figure on the shelf.
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Step two: read the field of view figure
Field of view is often listed as feet at one thousand yards, or as degrees. Suppose an 8x pair shows 420 feet at that distance and a 10x pair shows 330 feet. The extra magnification costs ninety feet of width. For a warbler moving through branches, the wider view makes the bird quicker to find. For a distant hillside animal that sits still, the narrower, closer view may be worth it. Compare the two figures side by side for the same subject instead of judging magnification alone.
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Step three: test the choice against your habits
Now apply the note from step one. A birder in woodland who scans quickly might favor the 8x pair with its wider field, steadier image and easier tracking. A hunter glassing open country from a fixed position could accept 10x and the smaller field. Then check practical limits: weight on a long walk, eye relief if you wear glasses, and close focus distance for nearby subjects. If you can try both pairs, view the same distant object and note which one you find easier to hold on target.