Lens Basics

A lens decides how a slice of the world is cut and projected onto the sensor. If exposure is about "how much light," the lens is about "the shape light arrives in."

Four things a lens decides at once

• Width of view — Focal length
• Sensor coverage — Crop factor
• Background blur — Depth of field
• Sense of space — Distance to subject

These don't operate independently — they push on each other. Going telephoto narrows the angle and shallows the depth at the same time; the same focal length on a smaller sensor gives a different result.

Children

• focal-length — What mm really means, why 50mm became standard
• crop-factor — Smaller sensors and 35mm equivalence
• depth-of-field — The four variables (aperture, focal length, distance, sensor)
• perspective-compression — Zooming is not the same as walking closer

One-line principles

F-number is a ratio. Same F-number = same brightness across focal lengths. Longer mm = shallower depth. Counterintuitive but true. Perspective is set by distance. Not by the lens — by your feet. Crop only narrows the angle; it doesn't make the lens a real telephoto.

Related

• aperture — Aperture sits at the intersection of exposure and DoF
• exposure-triangle — Parent of the exposure side
• camera-and-video — Topic hub