Perfectly Even Copy Lighting - Page 9
The appeal of this technique is that it's not always possible to get your lighting as even as you may desire. It is also possible that there is a certain unevenness being introduced by your optical system. All these factors will be taken into account with this technique since both images are shot through your camera system.
With large format cameras, the wide angle lenses often have a certain amount of falloff into the corners. Circular graduated neutral density filters can be purchased to correct for this effect, but they are very expensive. Using the technique described in this tutorial, you can shoot an evenly lit sheet of white paper and capture a reference frame of the falloff inherent in your wide angle lens.
Shooting the white reference under direct sunlight may provide more even lighting than can be achieved in the studio. If you decide to shoot this reference using artificial light, there can be no uneven sections on the white paper due to lighting. If you regularly use camera movements, then you will need to shoot additional white references to approximate the final position of the camera.