A little more visual
If possible, get them in the same plane.
If not, split the difference and take several different aperture settings with complimentary setting in ISO and Shutter-Speed to maintain your EV (Exposure Value).
The COF (Center Of Focus) should be on the rear person's eyes ... and then choose the aperture setting that gives you your DOF (Depth of Field). Please remember to consider that this diagram is a "relative" depiction ... and distances vary with focal length and distance to subject.
For instance: Using a 70mm lens ... and the COF is 10 ft. away
@ f/2.8, the DOF is 0.69 ft ... or ~7.5-inches
@ f/4, the DOF is 0.97 ft or ~12-inches
@ f/8, the DOF is 1.96 ft or ~24-inches
The Focal Plane Shift requires you to position the camera so that both subject's pair of eyes are in the focal plane or parallel with the camera's body. Doing this reduces the impact the DOF will have in the shot.
I hope this helps you visualize the scenario.
Try and make use of the information provided by the Online DOF Calculator (<- click here)
Last edited by DonSchap; 05-01-2009 at 08:52 PM.
- BFA, Digital Photography
A Photographer Is Forever
Look, I did not create the optical laws of the Universe ... I simply learned to deal with them.
Remember: It is usually the GLASS, not the camera (except for moving to Full Frame), that gives you the most improvement in your photography.