Photo Paper Sizes

Standard American photo paper sizes:
4"x6", 5"x7", 8"x10", also 8"x12", 12"x18", 16"x20", 16"x24", 20"x24", 20"x30".

Standard European photo paper sizes: 9x13 cm, 10x15 cm, 13x18 cm.

European Passport photo size: 5,12 cm x 3,84 cm (2" x 1.5").

Photo prints are based on the 35 mm photo film format:

Format: 35
Width: 36mm
Height: 24mm
Area: 864 sq mm
Diagonal: 43.27mm

This standard ratio for 35mm negatives corresponds directly to print sizes of 4" x 6" and 8" x 12" and 20" x 30", and is fairly close to 5" x 7" (1.4:1) and 3.5" x 5" (1.43:1).

Digital photo prints:
The aspect ratio of most digital camera pictures is not proportionate to the size of these standard photograph print sizes.
Most digital cameras reproduce pictures in the aspect ratio of 3:4.
Some print services introduce now photo prints in the 3:4 aspect ratio. It would be for instance a 4.5" x 6" paper print (11cm x 15cm).

Most document imaging resolution measures are in pixels (PICture ELement) per inch (or per mm - millimeter), and are commonly referred to as dpi (dots per inch) or dpmm (dots per mm). Most motion picture and still-photographic resolution measures are in pixels per image. This is most commonly seen in the 525 lines of NTSC (National Television System Committee), 625 lines for PAL (Phase Alternating Line) and SECAM (Sequential Couleur Avec Memoire or Sequential Colour with Memory), resolution of television images.

In pixels per image the horizontal resolution is given first. If the horizontal dimension is larger than the vertical dimension in pixels, the image or display is said to be landscape, if the horizontal is smaller, the image or display is said to be portrait.

dpi: dots per inch
ppi: pixels per inch
lpi: lines per inch

dpi and ppi, lpi: For digital images, we are dealing with pixels and for printing we are dealing with dots and lines.

300 dpi are needed to produce a photo-realistic image.

Suggested Pixels for different print sizes:

Photo Print Size Suggested Pixels
3" x 5" 1024 x 767
4" x 6" 1280 x 960
8" x 10" 2400x3000
16" x 20" 4800x6000

The Kodak PhotoCD family of resolutions (based on a 2 x 3 portrait aspect ratio)
A Kodak PhotoCD contains five resolution of each image: 1/16 Base through 16 Base. A Kodak Pro PhotoCD contains the five resolutions for each image found on a PhotoCD plus a sixth 64 Base resolution.

1/16 base (thumbnail, index print on CD cover) = 128 x 192
1/4 base (largest Kodak size that is smaller than 480 x 640 for display on TV) = 256 x 384
1 base = 512 x 768
4 base (largest Kodak size that is smaller than 1920 x 1152 for HDTV) = 1024 x 1536
16 base (captures all the resolution on most 35 mm film images) = 2048 x 3072
64 base (captures all the resolution for most film formats larger than 35 mm) = 4096 x 6144

Digital cameras expose images on a CCD (Charged Capacitance Device) or a CMOS (Complementary Metal-Oxide Semiconductor). This acts actually as the camera's digital medium.
CCD chips are more common but some cameras are using the CMOS chip due to its inexpensive manufacturing process and lower power consumption. The CMOS chip does not perform as well as the CCD chips in low light. Once exposed, the picture is stored onto some type storage medium. The resolution is measured in pixels.