question: How does exposure allow a camera to see into space?
comment: The reason we can't see stuff on the night sky as well as we would like to is not that they're invisible, it's just that there's so little of their light reaching us that they're indistinguishable from the backdrop. Long exposure on camera sort of gathers up all that dim light and sums it up - with long enough exposure and stable enough camera, you end up with a much clearer view of the night sky (or other cool effects, like tracks of the paths the stars take in the sky for really, really, really long exposure). Edit: I accidentally a word. I blame low blood caffeine content.
passages: Section::::Light sources.:Electron-beam exposure.
   BULLET::::- "E" is the irradiance of the surface, measured in ;
  
  BULLET::::- "t" is the exposure duration, measured in s.
  
  Section::::Definitions.:Luminous exposure.
  
  Luminous exposure of a "surface", denoted "H" ("v" for "visual", to avoid confusion with radiometric quantities) and measured in , is given by
  
  where
  
  BULLET::::- "E" is the illuminance of the surface, measured in lx;
  
  BULLET::::- "t" is the exposure duration, measured in s.
   Exposure (photography)
  
  In photography, exposure is the amount of light per unit area (the image plane illuminance times the exposure time) reaching a photographic film or electronic image sensor, as determined by shutter speed, lens aperture and scene luminance. Exposure is measured in lux seconds, and can be computed from exposure value (EV) and scene luminance in a specified region.
   Section::::Calculation.:Advanced approach.
   Negative/Print film tends to bias for exposing for the shadow areas (film dislikes being starved of light), with digital favouring exposure for highlights. See latitude below.
  
  Section::::Latitude.
   Section::::Automation.
  
  The first camera to feature automatic exposure was the selenium light meter-equipped, fully automatic Super Kodak Six-20 pack of 1938, but its extremely high price (for the time) of $225 ($ in present terms) kept it from achieving any degree of success. By the 1960s, however, low-cost electronic components were commonplace and cameras equipped with light meters and automatic exposure systems became increasingly widespread.
   Exposure compensation is particularly useful in combination with auto-exposure mode, as it allows the photographer to "bias" the exposure level without resorting to full manual exposure and losing the flexibility of auto exposure. On low-end video camcorders, exposure compensation may be the only manual exposure control available.
  
  Section::::Exposure control.
   Any scene of photographic interest contains elements of different luminance; consequently, the “exposure” actually is many different exposures. The exposure time is the same for all elements, but the image illuminance varies with the luminance of each subject element.
  
  Exposure is often determined using a reflected-light exposure meter. The earliest meters measured overall average luminance; meter calibration was established to give satisfactory exposures for typical outdoor scenes. However, if the part of a scene that is metered includes large areas of unusually high or low reflectance, or unusually large areas of highlight or shadow, the “effective” average reflectance
   With a few exceptions, astronomical photography employs long exposures since both film and digital imaging devices can accumulate and light photons over long periods of time. The amount of light hitting the film or detector is also increased by increasing the diameter of the primary optics (the objective) being used. Urban areas produce light pollution so equipment and observatories doing astronomical imaging are often located in remote locations to allow long exposures without the film or detectors being swamped with stray light.
   Special films are used for the long exposures required by astrophotography.
  
  Section::::Decline.
   Frequently, cameras are made from upcycled aluminum or tin cans which can be made light tight and are generally weather proof for the time needed to make the exposure. Apertures usually range in fractions of a millimeter
   For example, using ISO 400 film and setting the camera for EV 11 allows shooting night sports at a light level of EV = 9, in agreement with the example done the other way around above. An online calculator that implemented this calculation was available at dpreview.com.
  
  Section::::Setting EV on a camera.
  
  On most cameras, there is no direct way to transfer an EV to camera settings; however, a few cameras, such as some Voigtländer and Braun models or the Kodak Pony II shown in the photo, allowed direct setting of exposure value.
   Section::::History.:Secondary emission.:The race towards a practical electronic television camera.
   The absolute value for the exposure time required depends on how sensitive to light the medium being used is (measured by the film speed, or, for digital media, by the quantum efficiency). Early photography used media that had very low light sensitivity, and so exposure times had to be long even for very bright shots. As technology has improved, so has the sensitivity through film cameras and digital cameras.
   Section::::Physics.:Exposure control.:Metering.
  
  Measurement of exposure so that highlights and shadows are exposed according to the photographer's wishes. Many modern cameras meter and set exposure automatically. Before automatic exposure, correct exposure was accomplished with the use of a separate light metering device or by the photographer's knowledge and experience of gauging correct settings. To translate the amount of light into a usable aperture and shutter speed, the meter needs to adjust for the sensitivity of the film or sensor to light. This is done by setting the "film speed" or ISO sensitivity into the meter.
  
  Section::::Physics.:Exposure control.:Exposure and rendering.
   A more technical approach recognises that a photographic film (or sensor) has a physically limited useful exposure range, sometimes called its dynamic range. If, for any part of the photograph, the actual exposure is outside this range, the film cannot record it accurately. In a very simple model, for example, out-of-range values would be recorded as "black" (underexposed) or "white" (overexposed) rather than the precisely graduated shades of colour and tone required to describe "detail". Therefore, the purpose of exposure adjustment (and/or lighting adjustment) is to control the physical amount of light from the subject that is allowed to fall on the film, so that 'significant' areas of shadow and highlight detail do not exceed the film's useful exposure range. This ensures that no 'significant' information is lost during capture.
   Section::::Scanners.
  
  Modern scanners are steppers that increase the length of the area exposed in each shot (the exposure field) by moving the reticle stage and wafer stage in opposite directions to each other during the exposure. Instead of exposing the entire field at once, the exposure is made through an "exposure slit" that is as wide as the exposure field, but only a fraction of its length (such as a 9x25 mm slit for a 35x25 mm field). The image from the exposure slit is scanned across the exposure area.
   Science uses image technology that has derived from the design of the Pin Hole camera. X-Ray machines are similar in design to Pin Hole cameras with high-grade filters and laser radiation.
  
  Photography has become universal in recording events and data in science and engineering, and at crime scenes or accident scenes. The method has been much extended by using other wavelengths, such as infrared photography and ultraviolet photography, as well as spectroscopy. Those methods were first used in the Victorian era and improved much further since that time.
   Nine years later, in 1882, French scientist Étienne-Jules Marey invented a chronophotographic gun, which was capable of taking 12 consecutive frames a second, recording all the frames of the same picture.
   The Zone System is another method of determining exposure and development combinations to achieve a greater tonality range over conventional methods by varying the contrast of the film to fit the print contrast capability. Digital cameras can achieve similar results (high dynamic range) by combining several different exposures (varying shutter or diaphragm) made in quick succession.
  
  Today, most cameras automatically determine the correct exposure at the time of taking a photograph by using a built-in light meter, or multiple point meters interpreted by a built-in computer, see metering mode.
   Camera Obscura one of the first techniques that lead to creating photos. It could create an image on a wall or piece of paper.  Joseph Niepce was a French inventor that took the first photo in the 1827 that required 8 hours of exposure. In 1839, Louis Daguerre introduced the daguerreotype that reduced exposure time to about thirty minutes. As the years progressed so did photography techniques, including creating better image quality, adding color to an imagine and reduced exposure time.
  
  Section::::Types of mass communication.:Photography.:Photography Today.
   The image-plane illuminance is directly proportional to the area of the aperture, and hence inversely proportional to the square of the lens "f"-number; thus
  
  for constant lighting conditions, the exposure is constant as long as the ratio "t"/"N" is constant. If, for example, the "f"-number is changed, an equivalent exposure time can be determined from
   In photographic jargon, "an exposure" is a single shutter cycle. For example: a long exposure refers to a single, protracted shutter cycle to capture enough low-intensity light, whereas a multiple exposure involves a series of relatively brief shutter cycles; effectively layering a series of photographs in one image. For the same film speed, the accumulated "photometric exposure" ("H") should be similar in both cases.
  
  Section::::Definitions.
  
  Section::::Definitions.:Radiant exposure.
  
  Radiant exposure of a "surface",
  
  denoted "H" ("e" for "energetic", to avoid confusion with photometric quantities) and measured in , is given by
  
  where
   Section::::Relationship of EV to lighting conditions.
  
  "Correct" exposure is obtained when the f-number and exposure time match those “recommended" for given lighting conditions
  
  and ISO speed; the relationship is given by the exposure equation prescribed by ISO 2720:1974:
  
  where
  
  BULLET::::- "N" is the relative aperture (f-number)
  
  BULLET::::- "t" is the exposure time ("shutter speed") in seconds
  
  BULLET::::- "L" is the average scene luminance
  
  BULLET::::- "S" is the ISO arithmetic speed
  
  BULLET::::- "K" is the reflected-light meter calibration constant
  
  Applied to the right-hand side of the exposure equation, exposure value is
   After the exposure, the pixel is read out and the following stages measure the signals S1 and S2. As the length of the light pulse is defined, the distance can be calculated with the formula:
  
  In the example, the signals have the following values: S1 = 0.66 and S2 = 0.33. The distance is therefore:
  