Sorry, he shoots a Subal housing, not Ike
Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.
Benefits of registering include
I already know what you are going to say....get them both! But if you had to choose one to buy first as your "everyday" lens, which would you pick
You can use the 17-70 on a 5D but there will be problems. Lenses for APS-C size senors are designed to focus the image onto the smaller sensor. When you use them on a full frame sensor you get vignetting because the image produced can be smaller than the sensor. With a zoom lens like the 17-70 it might work fine at one end of the zoom range but have vignetting at the other end.Though I could have sworn I saw some guys on fredmiranda using it on a 5D.
Gudge:The Sigma 17-70 is designed for digital SLR cameras featuring APS-C size image sensors and it isn't suitable for the full frame sensor 5D. Go with the Sigma 50 macro.