Light, vision, and photography

Waves

• An electro-magnetic (EM) wave is a signal carrying energy through space/time, with a physical value at each space-time point given by the strength of its electrical and magnetic fields. These fields oscillate perpendicular to each other, and perpendicular to the path of the wave (EM waves are transverse).
• EM waves travel through a vacuum (no medium)
• EM waves travel at a fixed speed c = approximately 3 x 10^8 (300 million) meters/second, or 186,000 miles/sec (compare to sound at roughly 340.29 meters/sec, or 768 mph- the difference enables us to estimate the remoteness of a lightening flash.)
• Like periodic sound waves, periodic EM waves are characterized by their:
  • Period: T
  • Frequency: f = 1/T
  • Wavelength: L = c*T = v/f, i.e. c = f*L, f=v/L (higher frequency implies lower wavelength)
  • Amplitude: A
  • Power: proportional to A^2 , where A is the root mean square amplitude, as for sound.
  • Here is an animation, showing EM wave propagation. Also [1].
• Like sound waves, EM waves can be analyzed as the sum of sine and cosine waves at different frequencies and amplitudes, using the Fourier theorem
• Light comprises a frequency range within the EM spectrum in the visible range: 750 - 390 nanometers (billionths of a meter), i.e. frequencies in the range 400 to 790 terahertz (tera = trillion = 10^12), corresponding to colors ROYGBIV (red-orange-yellow-green-blue-indigo-violet)
  • Waves (transverse, longitudinal)(1D, 2D, 3D).
  • Inverse square law: the spread of wave energy is proportional to the square of the distance from the source. If the power (energy per unit time) is constant, then intensity (power per unit area) is inversely proportional to the square of the distance. What this means: a flashlight's intensity at 8 feet is only a quarter its intensity at 4 feet.
  • Reflection: a wave will bounce off a reflective boundary, partially (glass) or fully (mirror).
  • Diffraction: while light shadows appear relatively sharp (compared to sound shadows), a light wave can travel around an obstacle significantly smaller than its wavelength (see periodic waves, below), and will spread after passing through a hole.
  • Refraction: a wave changes direction when the medium changes.
  • Light waves are tranverse traveling electromagnetic waves through space (no medium) in 3D.

Terms for light and optics

• Watt: unit of power, 1 joule/second
  • a joule is the energy required to exert a force of 1 newton over one meter (recall that a Pascale is a unit of pressure: the force of 1 newton spread over 1 sq meter)
  • a newton is the force required to accelerate a kilogram 1 m/s each second
• Radiant flux: measure of power in EM radiation
• Luminous flux: measure of power of light source, as perceived by the eye (= radiant flux adjusted for wavelength)
• Lumen: unit of luminous flux
• Luminous intensity: measure of power in EM radiation per solid angle unit
• Candela: unit of luminous intensity
• Lux: measure of luminous intensity = one lumen per square meter

Analog (traditional) photography

theory

• light
• pinhole camera
  • pinhole
  • film
• SLR camera
  • lens and its focal length
  • f-stop
  • focal plane
  • depth of field
  • shutter speed
• film development (chemistry) and printing

camera types

• SLR 35 mm. Mirror shows you what the camera sees. Makes camera bulky. Use behind the lens built-in light meter for accuracy. Interchangeable lenses, manual settings allow maximal control.
• Rangefinder, or "point and shoot". Quiet, light, tiny, inexpensive, typically fully automated (though settings may allow some manual functions). But rangefinder is not accurate (parallax error), meter is not behind lens, lens is usually not interchangeable, lens is small and poorer quality, can’t apply filters. Come in 35mm and other film sizes. Good for speed.
• Twin-lens reflex. Like rangefinder but accepts larger format film. Older style.
• View. See exactly what you will photo on ground glass. Insert film plate, and take picture. Simplest, oldest type of camera. Can make large negatives. 4x5 is common.

Digital photography

Terms

• Sampling and quantization error
• Codecs
• Pixel and megapixel (sampling rate)
• Bit depth (sample size)

Camera types

• DSLR: digital version of SLR (above): provides maximum control, higher quality optics, AD capture, processing, interchangeable lenses, BUT more weight, bulk, cost
• point and shoot: digital version of rangefinder (above): provides less quality and control, BUT less weight, bulk, cost
• smaller cameras installed in laptops, tablets, and phones: minimal quality and control, BUT more readily available

Note that digital cameras have blurred the lines between photography and videography (and audiography), so you may be able to get away with a smaller number of multipurpose devices....however specialized devices will always have an edge: with less to do, they do it better.

File formats

As for audio (mp3, ogg, wav, etc.) there are a multitude of image file formats. These differ according to:

• the codec (the way the image is digitally encoded as pixels
  • color depth per pixel (8 bit pixel = 256 colors; 24 bit pixel = 14 million colors, or "truecolor")
• compression scheme
• whether compression is lossy or lossless
• raster or vector: whether a map of pixels or set of vectors is stored. Normally real-world images are not stored as vectors, which are used for computer graphics

Here are some of the more common ones:

• tiff, Tagged Image File Format: may be uncompressed (~ wav) or compressed, lossy or lossless
• jpg, Joint Photographic Experts Group: compressed, usually lossy (~ mp3)
• PNG, portable network graphics - a lossless, open-source compression scheme
• GIF: graphics interchange format. 8 bit palette provides limited color range (256). Often used also for animations.
• BMP: Windows bitmap format, an uncompressed pixel representation
• SVG: a common and open vector graphics standard

Software

• Adobe Photoshop: commercial
• The Gimp: open source equivalent. Note that you'll need to install and run X11 on Mac OS X before starting the Gimp.