I have 3 different Canon lens 50mm f-1.8, 24-105mm f-4, 70-200mm f-2.8, and I ran an exercise to see what lens and f spot together would give me the shallowest depth of field Iwould likedto shot at. To keep it simple I shot each lens at the greatest focal length and at the smallest aperture.
50mm at f-1.8
105mm at f-4
200mm at f-2.8
After looking at the 3 images I realized that I really like the 24-105mm lens at 105mm at f-4 the best. I feel that with the 50mm at 1.8 the back ground is too defused and it's harder to recognize what's back there as well as you have to get really close to your subject to fill the frame, and for the 200mm at 2.8 the length of the lens gives you the same problem with back ground defusing as the 50mm and it keeps you to far from you subject and you are not able to make that connection as easy. With the 24-105mm I like how the lens length at 105mm and the f-stop at f-4 give you the right amount of defusing of the back ground and the length is also just right not too far from your subject or not to close to be in their personal space.