Photography Basics… A film camera works because certain silver salts (silver halides) react to exposure to light energy by turning black… silver halide, in the form of approximately micron-sized crystals or "grains," is suspended in gelatin. The gelatin emulsion is coated as a thin film on glass plates or flexible sheets of plastic or paper. Areas exposed to light turn black (a negative)… the developing process washes off the unexposed silver halide crystals and chemically ‘fixes’ the exposed silver halides (so they don’t change any more) The printing process reverses the negative to a positive print

Safety Film… The original base medium to hold the silver halide gelatin emulsion was glass The next major innovation was cellulose nitrate (the original movie film base)…. Cellulose nitrate and TNT have a lot of common chemistry … explosive stuff. A more stable plastic base was developed by Kodak still today you can see “KODAK Safety Film” stenciled on photographs.

UV Filter Minus blue filter (e.g. Wratten #12)

Generalized cross section of color film. Note the structure of the film which is designed with an understanding of the relative NRG content of various wavelengths…

Filters for aerial photography Blue scatter is always present in aerial photography (long atmospheric path length) UV or Haze filter is critical For enhanced panchromatic or CIR photography a yellow (- blue) filter is necessary

Color stereopair-w haze filter

CIR stereopair with yellow filter

The polarizing filter: a very useful accessory

The polarizing filter cuts out one axis of light. A polarizing filter allows the camera to ‘see’ beneath the surface of still water (just like the proper sunglasses assist a fisherman seeing beneath the water) A polarizing filter deepens and ‘purifies’ the blue of the sky, improves color saturation and enhances contrast in the clouds A polarizing filter will however make rainbows disappear! And may make plastic windows less than transparent.

Post processing to remove atmospheric scatter.... Photoshop has an auto contrast or auto settings conversion... When applied to an aerial photograph it can dramatically enhance the image (to the point of being too much... So use with caution and keep the unaltered image as a backup/reference)

The camera has to perform several functions… Light proof container to store and secure film A means to control the direction and duration of light exposure A means to focus light onto the film plane

The ‘camera obscura’ or pinhole camera The simplest camera… no lens (even film is optional!)

A more advanced camera Focus is provided by sliding the lens assembly (the fabric bellows facilitate the movement of the lens) Light is controlled with a shutter diaphragm

A stereo camera Two lenses capture parallel scenes

A modern aerial mapping camera A modern aerial mapping camera

Aperture:The size of the hole in the lens also called F-stop The shutter speed determines how long the hole will be open, measured in fractions of a second. Both the shutter speed and f stop are critical to correct exposure. For example: The data sheet for Kodak 2443 CIR film suggests an f stop of 5.6 and a shutter speed of 1/500 sec Basic Photography Principles:

Bigger # indicates a smaller hole. e.g. f 22 Is much smaller than f 2.8

Changing the aperture changes the depth of field of a photograph… a ‘deep’ depth of field is highly useful for aerial photography… use as small an f stop as practical given the motion of the aircraft.

Camera Systems Most common are frame cameras (single exposure/single frame) Video, panorama, strip, multiband (IMHO, digital systems offer the best development platform for aerial ‘photography’) Must maintain constant spatial relationship between focal plane (film) and lens CFL (calibrated focal length lens)… certified focal length by National Bureau of Standards (f) Film flattening… if less than 4”X5” format and no requirement for precise photogrammetry… direct tension Most mapping cameras have a vacuum platen

Film format 35mm (diagonal measurement) 70mm 5”X5” 9”X9” Examples…. The larger the format, the more information can be stored on each frame… e.g. 35mm has just ¼ the area of a 70mm frame.

The Digital revolution in Image Acquisition In a digital camera the silver halide emulsion on the film is replaced by Charged Couple Devices (CCD’s) The CCD measures the intensity of incoming light and converts this signal into a digital number The number indicates how bright to make the resulting image (usually an 8 bit binary number)

Detector head from a 3 CCD camera. Red, Green and Blue each band has a designated CCD. Most cameras currently have a single CCD array. Light is separated into bands by an internal filter array.

CCD’s are not capable of distinguishing different wavelengths of light. So a "mosaic" pattern of color filters, a color filter array (CFA), is placed on top of the sensor to filter out the red, green, and blue light falling onto it. This is the most common pattern for color filters. Software ‘connects the dots’ to make smooth full color pictures.

Foeven Sensor

Irregardless of the format of the color sensing capability… the ‘footprint’ of a CCD array is much smaller even than a conventional 35mm image. Film has a higher resolution than digital images (for now)

Advantages / Disadvantages Film Digital