How many lens threads can I make in a span of 7 days? I love my tamron 28-75 but I want something sharper. Seems like the L is my best bet. I found a used 28-70 that I can get for 650-700.. Is it worth it to just grab that or am I better off waiting for a used 24-70? I've got a 24 2.8 prime by teh way.