Exploring image mediums, usually either analog or digital. Film consists of a transparent material (usu. plastic but sometimes glass or paper), with a coating of an emulsion of grains (globules) of photo-sensitive (light-sensitive) chemicals (such as silver halide) in an inert substance (like gelatin). The grains are will change with light until they are fixed during the process of developing the film.

Developed film may be either positive or negative. When lit up and examined closely, developed positive film has all the colors and shades look as they did in real life. In contrast, developed negative has all the colors and shades inverted.

Film Speed

The film speed refers to how quickly the emulsion reacts to light and its most common unit of measure is called an ISO number. Usually a faster film has larger grains but this has improved in recent years. To freeze high speed motion (EG: a runner), or to photograph in low light conditions, then a faster film is desired. In bright light conditions, a slower film speed is desired.

The most common unit of measure for film speed is called an ISO number (via International Standards Organization 5800:1987). The ISO number is arithmetic. When the ISO number doubles, the film is twice as sensitive to light. EG: In bright sunshine the correct exposure for a 200-speed film is 1/250th of a second at f16, and for a 400-speed film it is 1/500th at f16. Usually 100 ISO is fine bright sunlight and 800 is fine for normal indoor lighting.

Here are common film speeds: 25, 40, 50, 64, 80, 100, 125, 160, 200, 250, 320, 400, 500, 640, 800, 1000, 1250, and 1600.

The other unit of measure for film speed is logarithmic, where a change of 3 either doubles or halves the film's sensitivity. EG:

ISO 400 = 27°
ISO 800 = 30°

Other entities have set film speeds such as the ASA (American Standards Association), which became ANSI (American National Standards Institute), and the German DIN (Deutsche Industrie Norm). Thankfully they've given up and only ISO counts anymore.

See also Kodak's fine FAQ on film speed.

Film Latitude

The film latitude (aka dynamic range, contrast) refers to the ability to capture a range of shades of darkness and tints of brightness. Human can dynamically cover a large latitude because the pupil enlarges and shrinks as needed, we can shade our eyes, and our eyes can look at different parts of a scene and adjust the pupils just for that portion.

In contrast a photo is static in that the scene it capture may be bright, dark, or have a range of both dark and light. If we assume that the scene has a mix of light and dark, and the film has low latitude, then these problems might occur:

Positive film has less latitude than negative film.

The most common tricks for adjusting for the dynamic range within a shot are as follows:

Film Format

The film format refers to the captures size of each photo taken. Larger film formats allow higher quality larger enlargements but are much bulkier (unless your camera is digital). See Film format [W] for more film formats.

Common film formats include:

Here are some discontinued film formats.

Digital "film format" is largely a matter of the resolution of the original capture. Note that with digital cameras, you can upgrade your film format with a much smaller increase in physical bulk in comparison to physical film. Obviously for the same amount of memory, you will be able to take fewer pictures at a higher resolution. So you should either take lower resolution pictures or get more memory. Often you will crop you digital photos so you will often want a higher resolution.

The table below covers 4:3 images for common screen resolutions and beyond.

    @ 72 ppi @ 150 ppi @300 ppi
w (pix) h (pix) MP w (in) h (in) w (in) h (in) w (in) h (in)
640 480 0.31 8.9 6.7 4.3 3.2 2.1 1.6
800 600 0.48 11.1 8.3 5.3 4.0 2.7 2.0
1024 768 0.79 14.2 10.7 6.8 5.1 3.4 2.6
1152 864 1.00 16.0 12.0 7.7 5.8 3.8 2.9
1200 900 1.08 16.7 12.5 8.0 6.0 4.0 3.0
1280 960 1.23 17.8 13.3 8.5 6.4 4.3 3.2
1400 1050 1.47 19.4 14.6 9.3 7.0 4.7 3.5
1600 1200 1.92 22.2 16.7 10.7 8.0 5.3 4.0
1920 1440 2.76 26.7 20.0 12.8 9.6 6.4 4.8
2000 1500 3.00 27.8 20.8 13.3 10.0 6.7 5.0
2400 1800 4.32 33.3 25.0 16.0 12.0 8.0 6.0
2800 2100 5.88 38.9 29.2 18.7 14.0 9.3 7.0
3200 2400 7.68 44.4 33.3 21.3 16.0 10.7 8.0
3600 2700 9.72 50.0 37.5 24.0 18.0 12.0 9.0

Resolution quality:

300 ppi (pixels per inch) is the highest resolution that the naked eye will need at an average length, thus 300 ppi is the highest effective resolution that a printer will need. However, since most printers needs to fit in 1-6 colors of ink per pixel, printers may actually need 300-1800 dpi (dots per inch). The ppi should vary with viewing distance. EG: Close inspection with a device might need 600 ppi, while a bill board viewed from far away may be fine at 100 ppi.

If you frequently crop your photos, i.e. just use some fraction of the photo, then you may need a higher digital resolution.

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