The Optics of the Camera Obscura or Why pinhole cameras are not cutting-edge

Electromagnetic Radiation Light Photons

These three are the same… Light pure energy Electromagnetic Waves energy-carrying waves emitted by vibrating electrons Photons particles of light

EM Radiation Travels as a Wave c = 3 x 108 m/s

The Visible Spectrum Light waves extend in wavelength from about 400 to 700 nanometers.

Transparent Materials Transparent - the term applied to materials through which light can pass in straight lines.

Opaque Materials Opaque - the term applied to materials that absorb light.

Are clouds transparent or opaque to visible light? Answer: opaque Are clouds transparent or opaque to ultraviolet light? Answer: almost transparent

Special Things About a Light Wave • It does not need a medium through which to travel • It travels with its highest velocity in a vacuum • Its highest velocity is the speed of light, c, equal to 300,000 km/sec • The frequency (or wavelength) of the wave determines whether we call it radio, infrared, visible, ultraviolet, X-ray or gamma-ray.

Camera Obscura "When images of illuminated objects ... penetrate through a small hole into a very dark room ... you will see [on the opposite wall] these objects in their proper form and color, reduced in size ... in a reversed position, owing to the intersection of the rays". Da Vinci Slide credit: David Jacobs

Jetty at Margate England, 1898. http://brightbytes.com/cosite/collection2.html (Jack and Beverly Wilgus) Slide credit: David Jacobs

Pinhole Cameras Pinhole camera - box with a small hole in it Image is upside down, but not mirrored left-to-right Question: Why does a mirror reverse left-to-right but not top-to-bottom? The point to make here is that each point on the image plane sees light from only one direction, the one that passes through the pinhole.

Pinhole and the Perspective Projection Is an image being formed on the screen? YES! But, not a “clear” one. (x,y) screen scene image plane effective focal length, f’ optical axis y x z pinhole

Magnification y z x d f’ optical axis Pinhole d’ planar scene B d f’ optical axis z A Pinhole A’ d’ x planar scene image plane B’ From perspective projection: Magnification:

Properties of Projection Points project to points Lines project to lines Planes project to the whole or half image Angles are not preserved Degenerate cases Line through focal point projects to a point. Plane through focal point projects to line

Distant Objects are Smaller Size is inversely proportional to distance. Note that B’ and C’ labels should be switched.

Problems with Pinholes Pinhole size (aperture) must be “very small” to obtain a clear image. However, as pinhole size is made smaller, less light is received by image plane. If pinhole is comparable to wavelength of incoming light, DIFFRACTION effects blur the image! Sharpest image is obtained when: pinhole diameter Example: If f’ = 50mm, = 600nm (red), d = 0.36mm

The Reason for Lenses