Some time ago there had been a discussion on PAD about the common technique of focusing on the near eye and letting the far eye go out of focus. It was suggested that you cannot get both eyes in focus when using f-2 or f-2.8 because the depth of field is too shallow. At those f-stops you just had to accept an unfocused rear eye. It was suggested that you could get both eyes in focus at f-4 but then you would lose background bokeh. Studio portraits are often taken at f-5.6 or f-8 but available light outdoor portraits usually need to be taken with a lower f-stop such as f-2, f2.8 or f4. I suggested there should be an f-stop which allows you to achieve both eyes in focus and background bokeh as long as the distance between the subject and background is great enough.
I finally was able to get a short time with my model in fading light this evening so I quickly took about 10 shots just off my driveway with a pine tree in the background about 15 feet behind my subject to produce some out of focus background bokeh. I used a D600 and a 85mm f1.8 D lens. Picture Controls was set to Portrait. I shot at f1.8, f2, f2.8. I did not have time to shoot at f-4 before I lost the light. Here are my results along with crops of the eyes from each photo. I will subjectively grade the focus, or lack thereof, on the rear eye as "acceptable" or "not optimal."
Now we are going to turn the face to 45 degrees from the camera. The two eyes will be different distances from the camera. If we focus on the near eye that rear eye my fall outside the DOF of a shallow f-stop. That is our problem. This was shot at f1.8
Here are the eyes.
I judge this acceptable. This is not so much attributed to the difference between f1.8 and f2 as much as it is attributable, I think, to the those narrow D600 focus points I have complained about, which in this case are small enough to allow me to focus on the tear duct of the near eye rather than on the entire near eye. This helps shift the DOF rearward a bit. Since 1/3 of the DOF is in front of the focus point that is enough to take in the entire near eye. The 2/3rds of the DOF behind the focus point is enough to acceptably sharpen up the rear eye.
Now a word about bokeh. Look at the bokeh in the portraits taken at f-2.8. I say that is acceptable bokeh as long as you can obtain a background about 15 feet from your subject. So I would say f-2.8 works, both for eye focus and for background bokeh. But of course everyone has their own preferences and that is fine. Shoot what you will, as you will and your photo may be the prize winner. I am just making a technical argument as to how you can balance eye sharpness with bokeh, not an artistic one.
What about spending all that extra money (like a thousand dollars extra) to get a f1.4 85mm lens faster than f2.8 if f2.8 is a good balance between eye sharpness and bokeh? I wish I had Nikon's new 85mm f1.4 lens so I could shoot it back to back with my old 85mm 2.8D and see the difference. I hear the bokeh it produces is great. Perhaps someone here has an 85mm f1.4 and could add some photos from it to this thread showing how you can get both eyes in acceptable focus at f1.4 and get wonderful bokeh with that lens. I doubt it can be done unless the subject's face is parallel to the camera. If you want both eyes in acceptable focus and the subject's face is at 45 degrees I think you are going to have to shoot your expensive f1.4 85mm lens at f2.8 anyway.