What is the correct formula to compute the new focal length using a crop factor?

• A. The crop conversion factor is unrelated to focal length.
• B. The crop conversion factor adds to the focal length.
• C. The crop conversion factor divides the focal length.
• D. The crop conversion factor multiplies the focal length to give the new focal length.

Answer: (D)

Crop factor shows how much smaller the sensor is compared with a full-frame sensor, and it tells you how the lens’s field of view changes relative to that standard. To find the equivalent focal length on a full-frame camera, you multiply the actual focal length by the crop factor. That gives the new focal length that would produce the same field of view on full-frame.

For example, with a 50 mm lens on a sensor with a 1.5x crop factor, the equivalent focal length is 50 × 1.5 = 75 mm. The actual focal length stays the same; the crop factor just scales it to an equivalent value for comparison.

The other ideas don’t fit because the crop factor is a multiplier, not something you add or divide, and it isn’t unrelated to focal length. It directly converts the lens’s focal length into its equivalent on a different sensor size.