Portraits, especially headshots, I have to stop down to get both eyes reliably in focus to 3.2 from 2.0 on my 135 when the subject turns their head off camera axis. If you are shooting more than one person, you may need to stop down from the 4. However, I am not familiar with this lens and since poster above indicates the lens IQ isn't that of the 24-70 you have to ask yourself will you or your clients see the difference. Test geeks quibble over a tiny difference here or there. Your subjects probably like cell phone images so I doubt if they can tell the difference. Focus speed might be a question with pets who move about. Check the lens performance on that. But try a wedding indoors, and that extra stop just might be the difference between a useable image and a noisy one. I'm not familiar with the 750 high iso but it should be darn good. I am shooting an 850 and am amazed at the dynamic range and iso performance. I wonder if you used that lens and picked up an 85 1.8 or 1.4 for the shallow dof if that would be a good combination for much of your work.