I own a D5100 and am slowly buying lens currently own a 50mm 1.8f. I understand since I have a DX body it will cropped but I am hoping to one day upgrade my lens not anytime soon but eventually. I would like to purchase a 24-70 2.8f lens looking at Sigma or Tamron. I've also considered the 28-75mm but I am thinking with my camera body it maybe to cropped. I plan on taking photos in natural light and portraits. Can you please give me your honest opinion on this lens on my camera body and if you would go for which brand and size. If you think maybe a different range is better as well. I really appreciate your help.