Quality of Image Prints

 

Optimal quality of image prints can be achieved, when, for a chosen print size, the printed image has the minimum required base image resolution, and the printing resolution of the printer is adequate.

Definitions of the related parameters, and the required values of these parameters, are presented and discussed below.

ppi  (pixels per inch)

ppi is the number of pixels in each inch of the image, horizontally and vertically, at the current printing size of the image.
For example, 360 ppi can be understood in two dimensions as 360 x 360 pixels per square inch.

dpi  (dots per inch)

dpi  is the number of droplets of ink sprayed per inch of the image in one particular direction during the printing process. Frequently, the horizontal resolution and vertical resolution of the image print are not equal. In high quality photographic printing, the common resolutions are 1440 dpi horizontally, and 720 dpi vertically. This means that in a square inch, the image is 1440 dots wide and 720 dots tall.

dpi is limited by the type of printer to be used for printing the image.

In high quality photographic printing, dpi is always going to be a larger number than ppi.

Base Image Resolution  (determined by the value of ppi)

Base Image Resolution is the image’s resolution before it is artificially increased (interpolated).
For high-quality image prints, required minimum value of the base image resolution depends on the print size. Optimal minimum value of the base image resolution is a function of the print size. Smaller prints will be viewed from a closer distance, and, therefore, will require greater image resolution, larger prints will most often be viewed from a greater distance, and, therefore, will require lower image resolution..

 

Table 1 shows, for common print formats, the (subjective) acceptable minimum base image resolutions (MBIR), and the minimum required sizes of the corresponding digital images.

Table 1

Print Format			Common Usage	MBIR	Digital Image Size
Name	Size (width x height)			ppi	pixels
A6 (ISO-216)	105 mm x 148 mm	4.13” x 5.83”	books, postcards	360	1487 x 2099 ≈ 3.12 x 106
A5 (ISO-216)	148 mm x 210 mm	5.83” x 8.27”	books	300	1749 x 2481 ≈ 4.34 x 106
A4 (ISO-216)	210 mm x 297 mm	8.27” x 11.70”	letters, magazines, catalogues	240	1985 x 2808 ≈ 5.57 x 106
A3 (ISO-216)	297 mm x 420 mm	11.70” x 16.54”	posters, newspapers	200	2340 x 3308 ≈ 7.74 x 106
A2 (ISO-216)	420 mm x 594 mm	16.54” x 23.40”	posters	180	2977 x 4212 ≈ 12.54 x 106
A1 (ISO-216)	594 mm x 841 mm	23.40” x 33.11”	posters	160	3744 x 5298 ≈ 19.84 x 106
A0 (ISO-216)	841 mm x 1189 mm	33.11” x  46.81”	posters	140	4635 x 6553 ≈ 30.37 x 106
4R	102 mm x 152 mm	4” x 6”	photographs	360	1440 x 2160 ≈ 3.11 x 106
5R	127 mm x 178 mm	5” x 7"	photographs	300	1500 x 2100 ≈ 3.15 x 106
8R	203 mm x 254 mm	8” x 10"	photographs	240	1920 x 2400 ≈ 4.61 x 106
	203 mm x 305 mm	8” x 12”	photographs	240	1920 x 2880 ≈ 4.67 x 106
ANSI – A  ( ≈ A4 ) Letter	216 mm x 279 mm	8½” x 11”	posters	240	2040 x 2640 ≈ 5.39 x 106
Legal	216 mm x 356 mm	8½” x 14”		240	2040 x 3360 ≈ 6.85 x 106
14R		11” x 14"	photographs	200	2200 x 2800 ≈ 6.16 x 106
ANSI – B  ( ≈ A3 ) Tabloid	279 mm x 432 mm	11” x 17”	posters	200	2200 x 3400 ≈ 7.48 x 106
	305 mm x 457 mm	12” x 18”		200	2400 x 3600 ≈  8.64 x 106
	330 mm x 483 mm	13” x 19”		200	2600 x 3800 ≈ 9.88 x 106
	406 mm x 508 mm	16” x 20"		180	2880 x 3600 ≈ 10.37 x 106
	406 mm x 619 mm	16” x 24”		180	2880 x 4320 ≈ 12.44 x 106
ANSI – C  ( ≈ A2 )	432 mm x 559 mm	17” x 22”	posters	180	3060 x 3960 ≈ 12.12 x 106
Broadsheet	457 mm x 610 mm	18” x 24”	posters	160	2880 x 3840 ≈ 11.06 x 106
	508 mm x 610 mm	20” x 24"		140	2800 x 3360 ≈ 9.41 x 106
	508 mm x 762 mm	20” x 30”		140	2800 x 4200 ≈ 11.76 x 106
ANSI – D  ( ≈ A1 )	559 mm x 864 mm	22” x 34”	posters	140	3080 x 4760 ≈ 14.66 x 106
	610 mm x 914 mm	24” x 36”	posters	140	3360 x 5040 ≈ 16.93 x 106
	689 mm x 991 mm	27” x 39”	posters	140	3780 x 5460 ≈ 20.64 x 106
ANSI – E  ( ≈ A0 )	864 mm x 1118 mm	34” x 44”	posters	140	4760 x 6160 ≈ 29.32 x 106

 

Table 2 shows, for common print formats, the minimum required sizes of the corresponding digital images, when the value of the minimum base image resolution (MBIR) is assumed to be equal 300 ppi.

Table 2

Print Format			Common Usage	Digital Image Size
Name	Size (width x height)			pixels
A6 (ISO-216)	105 mm x 148 mm	4.13” x 5.83”	books, postcards	1239 x 1749 ≈ 2.17 x 106
A5 (ISO-216)	148 mm x 210 mm	5.83” x 8.27”	books	1749 x 2481 ≈ 4.34 x 106
A4 (ISO-216)	210 mm x 297 mm	8.27” x 11.70”	letters, magazines, catalogues	2481 x 3510 ≈ 8.71 x 106
A3 (ISO-216)	297 mm x 420 mm	11.70” x 16.54”	posters, newspapers	3510 x 4962 ≈ 17.42 x 106
A2 (ISO-216)	420 mm x 594 mm	16.54” x 23.40”	posters	4962 x 7020 ≈ 34.83 x 106
A1 (ISO-216)	594 mm x 841 mm	23.40” x 33.11”	posters	7020 x 9933 ≈ 69.73 x 106
A0 (ISO-216)	841 mm x 1189 mm	33.11” x  46.81”	posters	9933 x 14043 ≈ 139.49 x 106
4R	102 mm x 152 mm	4” x 6”	photographs	1200 x 1800 ≈ 2.16 x 106
5R	127 mm x 178 mm	5” x 7"	photographs	1500 x 2100 ≈ 3.15 x 106
8R	203 mm x 254 mm	8” x 10"	photographs	2400 x 3000 ≈ 7.20 x 106
	203 mm x 305 mm	8” x 12”	photographs	2400 x 3600 ≈ 8.64 x 106
ANSI – A  ( ≈ A4 ) Letter	216 mm x 279 mm	8½” x 11”	posters	2550 x 3300 ≈ 8.42 x 106
Legal	216 mm x 356 mm	8½” x 14”		2550 x 4200 ≈ 10.71 x 106
14R		11” x 14"	photographs	3300 x 4200 ≈ 13.86 x 106
ANSI – B  ( ≈ A3 ) Tabloid	279 mm x 432 mm	11” x 17”	posters	3300 x 5100 ≈ 16.83 x 106
	305 mm x 457 mm	12” x 18”		3600 x 5400 ≈  19.44 x 106
	330 mm x 483 mm	13” x 19”		3900 x 5700 ≈ 22.23 x 106
	406 mm x 508 mm	16” x 20"		4800 x 6000 ≈ 28.8 x 106
	406 mm x 619 mm	16” x 24”		4800 x 7200 ≈ 34.56 x 106
ANSI – C  ( ≈ A2 )	432 mm x 559 mm	17” x 22”	posters	5100 x 6600 ≈  33.66 x 106
Broadsheet	457 mm x 610 mm	18” x 24”	posters	5400 x 7200 ≈ 38.88 x 106
	508 mm x 610 mm	20” x 24"		6000 x 7200 ≈ 43.20 x 106
	508 mm x 762 mm	20” x 30”		6000 x 9000 ≈ 54.00 x 106
ANSI – D  ( ≈ A1 )	559 mm x 864 mm	22” x 34”	posters	6600 x 10200 ≈ 67.32 x 106
	610 mm x 914 mm	24” x 36”	posters	7200 x 10800 ≈ 77.76 x 106
	689 mm x 991 mm	27” x 39”	posters	8100 x 11700 ≈ 94.77 x 106
ANSI – E  ( ≈ A0 )	864 mm x 1118 mm	34” x 44”	posters	10200 x 13200 ≈ 134.64 x 106

Printing Resolution (Output Resolution) (determined by the value of dpi)

Printing Resolution is the number of droplets of ink sprayed per inch of the image (dpi) in one particular direction, during the printing process. In most cases, the horizontal printing resolution will be larger than the vertical printing resolution

Assuming that the available inkjet printer has the maximum printing resolution 2880 dpi (horizontally) x 1440 dpi (vertically), one can determine the minimum required (horizontal) printing resolution of the printed image, as a function of the viewing distance of the observer from the print, which will satisfy the observer.

Prints which will be viewed from larger distance need smaller printing resolution.

A table of viewing distance versus the required minimum (horizontal) printing resolution of the printed image which will satisfy the observer

 

Viewing Distance	Minimum (Horizontal) Printing Resolution
3 ft. and closer	1440 dpi
6 ft. and closer	720 dpi
Farther than 6 ft.	360 dpi

 

Printing resolution of 2880 dpi (horizontally) is, in most cases, too high and unnecessary. It is justified only if one would like to have exceptional shadow details in the print. For reproducing a photograph with a base resolution of around 300 ppi, in which one wants to see amazing details, the printing resolution of 2880 dpi (horizontally) could be justified. For reproducing artwork, the printing resolution of 1440 dpi (horizontally) is always adequate.

For printing a retail store poster, which will not be scrutinized closely, the printing resolution of 720 dpi (or even 360 dpi) (horizontally) is adequate.

Image Resizing

In practical terms, the size of the original image can be increased up to four times, through interpolation, if the original image has an acceptable base image resolution. In other words, a good 8” x 10” image can look fairly good after increasing its size to 16” x 20”, through interpolation.

Resizing programs create extra pixels from existing pixels, using some special mathematical algorithms. Better known algorithms include: Nearest Neighbor Algorithm, Bilinear Algorithm, Bicubic Algorithm, and Fractal Algorithms.