A camera is a light collecting device that uses a converging lens to form a real image on a sensor or film. Light from each point on an object spreads out, passes through the lens, and is bent back toward a matching point in the image plane. The image is inverted because rays from the top of the object cross the optical axis before reaching the sensor.
Understanding this process connects optics, geometry, and everyday technology.
Key Facts
- Thin lens equation: 1/f = 1/do + 1/di, where f is focal length, do is object distance, and di is image distance.
- Magnification: m = hi/ho = -di/do, where the negative sign means the real image is inverted.
- For a distant object, do is very large, so di is approximately f and the image forms near the focal plane.
- Focusing is done by changing the lens to sensor distance so the image distance di matches the sensor position.
- A smaller aperture lets in less light but increases depth of field, so more distances appear acceptably sharp.
- Exposure depends mainly on aperture area, shutter time, and sensor sensitivity: more light is captured with a wider aperture or longer shutter time.
Vocabulary
- Converging lens
- A lens that bends parallel incoming light rays toward a common focal point.
- Real image
- An image formed where light rays actually meet, so it can be projected onto a screen, film, or sensor.
- Focal length
- The distance from a lens to its focal point when incoming rays are parallel to the optical axis.
- Aperture
- The adjustable opening in a camera that controls how much light passes through the lens.
- Depth of field
- The range of object distances that appear acceptably sharp in an image.
Common Mistakes to Avoid
- Treating the sensor as if it can be placed anywhere behind the lens is wrong because a sharp image forms only at the image distance required by the lens equation.
- Forgetting that a real camera image is inverted is wrong because rays from the top and bottom of the object cross after passing through a converging lens.
- Assuming zooming and focusing are the same is wrong because zooming changes focal length and image size, while focusing adjusts the lens position to place the image on the sensor.
- Thinking a smaller aperture always makes a better photo is wrong because it increases depth of field but reduces light, which may require a longer exposure or higher sensor sensitivity.
Practice Questions
- 1 A camera lens has focal length 50 mm. An object is 2.0 m from the lens. Use 1/f = 1/do + 1/di to find the image distance di in millimeters.
- 2 A 35 mm tall object is photographed with do = 1.5 m and di = 52 mm. Find the magnification and the image height.
- 3 A photographer closes the aperture to take a landscape photo with both nearby flowers and distant mountains sharp. Explain how this changes depth of field and exposure, and state one adjustment that could compensate for the reduced light.