Group shots and radial distance

[Field of 33]

Lot of words here for a hopefully simple solution...

Let say you're taking a wedding type pictures, or any pictures where a group is standing in a line. Obviously, if you focus on the person in the center, the distance between the camera and the center person is less than the distance between the camera and the people at the ends of the line. For instance, let's say the distance between the camera and the center of the line is 10' and the line of people is 6' from center to end. The distance between the camera and the person on the end would be ~11'-8".

My question is do I just need to ensure that my DOF is greater than this 1'-8" (50.7 cm, for my metric friends) difference and all will be well? Is it better to have the DOF significantly greater than this difference, say greater than 4', or would a DOF of 2' be "good enough"?

Since depth of field is based on aperature and focal length, I'm assuming that zoom lens and primes handle differences in radial distance equally well? Please correct me if my assumption is incorrect.

Thanks!

[Jon]

If they're in a straight line and parallel to the camera, you're overcomplicating things. Remember, the center person at 10 ft. is in focus on the center of the sensor. The end people are way out on the edges of the sensor, so they're also still in focus. Basically, you're looking at congruent triangles since the sensor's flat, not curved. So anything on a plane parallel to the sensor and running through the point you focused on will be in focus.

[Doug Pardee]

What Jon said. With the occasional rare exception, still photography lenses are designed to have a flat focus field, not a curved one.

A complication, however, is that any on-camera flash has an effect that depends on the square of the radial distance. So it's not possible to simultaneously get even flash effect and even focus.

By the way, the flat focus field is why recomposing after focusing can be a problem at short range with shallow depth of field.