The plane
of focus

How to read it

• Rays. Each subject sends a cone of light into the lens. The lens bends it back into a cone that closes to a point.
• Point or disc. If that point falls exactly on the sensor, the subject is sharp. If it falls in front of or behind the sensor, the cone is cut early or late and lands as a disc. That disc is blur.
• Plane of focus. The one distance in front of the lens whose rays land as points.
• Internal focusing. None of these lenses change length when focusing. Turning the focus ring drives linear motors that slide the focus groups inside the barrel; that changes where the light converges, and the plane of focus moves with it.
• Focal length. Longer focal lengths see a narrower slice of the world and, at the same distance and ƒ-number, give a much thinner sharp zone. The live view is rendered at true scale, so switching lenses changes framing and blur exactly as it would on a camera.
• Aperture. Stopping down (ƒ/16) makes every cone thinner, so the discs shrink and the sharp zone grows.

Drag the ribbed focus ring on the lens, or use the controls. Orbit with the mouse, scroll to zoom. Distances are measured from the sensor, like the scale on a real lens. On the bench they are compressed into the miniature scene; the blur discs on the image plane are drawn 10× larger so you can see them.