Hello, if you can only afford 1 lens the best pro level, "multi-talented" lens for Canon is the 24-105 L series zoom. It is the best value lens in the Canon pro L series, yet still about $900 depending on what country you are in. If you feel that the price is prohibitive, OR that you can afford an even better lens I can give you more detailed answers over the phone. Feel free to schedule a chat anytime. Here is my portfolio: www.hemmingshouse.com

Depending on the amount of space you have to do the shoot I would suggest one of two lenses and I will explain why I suggest each.

If you have plenty of room, I suggest the Canon 70-200mm 2.8L lens. It is the best option. When you are taking photos of people, you want to avoid distortion. Shooting too wide and standing too close to your subject can produce this. You can kind of fix it in post, but why? Getting the camera further away from the subject is a better option. The 70-200 is such a sharp lens.

If you don't have the room, get the 24-70 2.8L. It's a great lens that I use almost every day. I use it when I am going to be closer to the subject. I almost never use it in the studio unless I am doing a group shot.

Depending on what your budget for a lens is, I would purchase one of the two. If you are not sure about committing to the cost, rent a lens from Borrowlenses.com or Lensprotogo (I am not affiliated with either of these companies, however I have been a customer of both before).

If you need help learning about getting better photos out of your camera, check out my course on Udemy.com: https://www.udemy.com/ditch-auto-start-shooting-in-manual/

If you're looking for a sharp portrait lens, an 85mm prime lens (f1.4) would be an excellent choice. It's incredibly sharp and fast, but you will also get the depth of field you want while looking accurate/real.

I can't add to what Mark, Jerad, and Brandon have suggested. All give great advice. My suggestion would be to follow Jerad's second bit of advice and rent several lenses. In my practice there are some lenses I use all the time, and some I use very, very rarely. Those latter are the ones I rent because I need their specific qualities, but don't have the capital to own them outright.