From which component do you read the larger prism component when checking for prism?

• A. Reticle scale inside the instrument
• B. External prism compensating device
• C. Frame bridge
• D. Vertex distance gauge

Answer: (B)

When checking prism, you determine how much prism a lens is introducing by neutralizing it with a compensating prism. The external prism compensating device is the part used to add or subtract prism until the image aligns, and the amount you read on that device directly reflects the prism magnitude in the lens. The internal reticle can show changes, but it doesn’t isolate the larger prism component as reliably, because it reflects the instrument’s optics and alignment. The frame bridge and vertex distance gauge aren’t used to measure prism power—they relate to frame fit and lens position, not prism magnitude.