# Does shift based perpsective correction have any effect on focus?

Consider a photographer with an SLR camera and a PC lens attached in its default, unshifted position, standing on the ground at the base of a columned building whose height is X times that of the person and is located Y feet away from him.

The photographer composes a frame with the center of the building in the center of the frame with some empty space above it, to do so they have to pitch the camera slightly upwards which creates a keystone effect with the columns appearing to converge vertically towards a point above the frame.

Next the shooter maintains the camera orientation levels the camera and adjusts the shift of the PC lens until the columns appear parallel in the image.

Is there any change in focus at the bottom of the columns and the focus at the top of the columns before and after the shift ?

## 2 Answers

The shift will not change the convergence of vertical lines. The verticals converge because the lens isn't horizontal. The purpose of the shift is to allow you to frame the building while maintaining the lens axis horizontal.

If you focus to the middle of of the building, and then make the camera horizontal, then the focus has changed: in the worst case scenario, if you are right next to the wall, you focus at half the building height, but when you take the picture you are just a few inches away. Of course for a real shot it will depend how far you are and how high is the building.

• You are right, edited to make the described scenario actually possible – SirPeepsalot Jan 8 at 12:27
• It would also depend on how well corrected for field curvature the lens is, since shift movements move the part of the image circle centered on the sensor towards the edge of the lens' image circle. – Michael C Jan 8 at 12:41
• Well it's a PC lens so that's what it was designed to do. – SirPeepsalot Jan 8 at 12:56

Possibly. That is because when the lens is shifted it is using the periphery of the objective lens and not the center. The actual result would depend on the focus field curvature of the lens. It is typically only specialized macro lenses that are designed to be flat field.

That said, I have not noticed any such effect using Nikkor PC-E lenses, but I also tend to use them tilted more than shifted.