Yes, i agree with both the previous posts. It really depends inn what kind of photos you are taking as to what lens you need, but the 50mm is very versatile.
For Senior portraits, any prime (non-zoomable) lens 50mm or more will do great. Use aperture priority (AV) mode and pick a very low F number. The 50 f/1.8 is a great choice if you're on a budget. Also, the guys at Schiller's are extremely knowledgeable and will show you tons of lenses that would be great for senior portraits.
Nifty Fifty 50mm 1.8 its only $120 or the 40mm micro 2.8
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