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The second number in the binocular’s rating. Objective diameter is essentially a measurement of how much light can be gathered by the objective lens. A greater objective diameter will typically produce a brighter, sharper image compared to a smaller objective diameter. In other words, 10x42 binoculars are more likely to produce a brighter image than 10x26 binoculars. However, other factors also come into play, including lens and prism quality.