If I could have only one lens it would be the 23mm. You can use it for pretty much everything, indoors and out. But if I had just the 35mm and I were given the choice of adding either the 23mm or the 16mm to my collection, I'd choose the 16mm. Together with the 35mm, those two lenses give you a huge amount of scope and variety, whereas the 35 and the 23 have a bigger area of overlap. Outdoors, head-and-shoulders portraits etc - that's your 35. Indoors, tight spaces, close-up action, fast-moving pets, landscapes - that's your 16.
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