Certified Professional Photographer (CPP) Practice Exam

When you use extension tubes, what happens to the principal plane relative to the focal plane?

1. It moves closer

2. It stays the same

3. It moves away

4. It becomes irrelevant

The correct answer is: It moves away

When using extension tubes, the principal plane of a lens system moves away from the focal plane. Extension tubes are used to increase the distance between the lens and the image sensor. This increase in distance shifts the lens's principal plane because the focal length of the lens effectively becomes longer due to this added distance. As a result, the focus point moves further away from the lens, necessitating adjustments to achieve sharp focus. An understanding of principal planes and their relationship to focal planes is crucial for macro photography, where extension tubes are commonly used to achieve greater magnification. This is why knowing how the position of the principal plane is altered when adding extension tubes is important as it directly affects the way the photographer must compose and focus their images.