FAQ

Pixels and MegaPixels

moshebaram@gmail.com (Administrator) — Sat, 16 Aug 2008 21:46:17 +0000

Pixels and MegaPixels

Pixel:

Literally a "Pixel" is a combination of the words "Pictorial Elements".
Pixels represent blocks of color arranged in a grid. By looking at the grid of pixels from a sufficient distance we are able to see (visualize) the image.

A printer pixel is the smallest dot a printer can produce. The density or the amount of pixels per inch of picture output determine the picture quality. The more pixels we have in an inch (linear) the higher the image quality.

A screen pixel is the is the smallest area that can be illuminated independently on a monitor.

Megapixel:

1 megapixel = 1,000,000 pixels

In relating to digital photography and other uses, when people use the term 'Megapixel' they mean a figure that is around 1,000,000 pixels. For instance: some 4 megapixels digital cameras can produce pictures which are 2272 x 1704 pixels = 3,871,488 pixels ~ 3.87 megapixels.
In the same manner for 2 megapixel - 1600 x 1200 pixels = 1,920,000 pixels = 1.92 megapixels.
These values vary from one camera to another.

Examples:

1. If you scan at 300 PPI a 6 x 4 inch photo.
The number of pixels your picture file will have is:
(300 x 300) x (6 x 4) = 90,000 x 24 = 2,160,000 pixels = 2.16 megapixel

DPI

moshebaram@gmail.com (Administrator) — Sat, 16 Aug 2008 21:45:17 +0000

DPI

An abbreviation for dots per inch. Refers to the resolution at which a device, such as a monitor or printer, can display text and graphics. Monitors are usually 100 dpi or less, and laser printers are 300 dpi or higher. An image printed on a laser printer looks sharper than the same image on a monitor.

PPI

moshebaram@gmail.com (Administrator) — Sat, 16 Aug 2008 21:43:59 +0000

PPI

PPI = Pixels Per Inch (the number of pixels per linear inch in the image).

Example:

If a resolution of a photo is 300 PPI it means that in every Sq. Inch of the photo there are 300 x 300 pixels thus 90,000 pixels.

Printing:

If you don't resample your image, changing the PPI value will change the size of the printed image in the following manner:
Increasing the PPI value will result in decreasing of the print size.
Decreasing the PPI value will result in increasing of the print size.

If you do resample your image, changing the PPI value will maintain the print size but will change the number of pixels in your image (and thus the file size) in the following manner:
Increasing the PPI will result in increasing the number of pixels in the image by generating new pixels artificially - this is not recommended.
Decreasing the PPI value will result in decreasing the number of pixels and therefore in losing data and ultimately losing quality.
Therefore it is not recommended to decrease the PPI unless you intend to reduce picture size.

Example: Lets say you print at 200 PPI so that the print will be an 8 X 12 inch image. By increasing the PPI value to 400 without resampling first you'll reduce the size of the image to a 4 X 6 inch.

Newspaper scanning

moshebaram@gmail.com (Administrator) — Sat, 16 Aug 2008 21:42:41 +0000

Newspaper Scanning

When scanning a picture from a newspaper we have 2 problems:
1. The printing resolution of a standard newspaper is very low - 80 PPI.
2. Newspapers are printed in an "Offset" technology which means the picture is made of rather big and visible spots.

If you scan a picture from a newspaper without obeying very certain rules the final result will be bad.

In order to get a nice result from a newspaper scan, we recommend:
1. Always to scan at a resolution of 170 PPI if you have a option of 170. If you don't have it you can choose 150 or 200 PPI.
2. In most of the scanning softwares you'll be able to find an option called "Descreen". Activate this option, its purpose is to decrease the effect of the grid caused by the "Offset" printing.

If you can't find the "Descreen" option or if you used it and you can still see the grid, you'll be able to find in most graphic softwares a tool which will take care of this problem. Sometimes you'll be able to fine such a tool under "Moire Pattern Removal".

Screen resolution

moshebaram@gmail.com (Administrator) — Sat, 16 Aug 2008 21:41:21 +0000

Screen Resolution

Higher screen resolution doesn't mean better image quality (or clarity). The quality of a certain image is a constant no matter what the screen resolution is.
The same image is displayed (appears) smaller on a high resolution screen than on a low resolution screen.

For example:
Let's say you have a 17 inch screen (10.2 in. x 13.6 in.). Let's consider 2 options:
Option 1 - the screen resolution is 600 x 800 pixels. In PPI - 600/10.2 (=800/13.6) = ~ 59 PPI.
Option 2 - the screen resolution is 960 x 1280 pixels. In PPI - 960/10.2 (=1280/13.6) = ~ 94 PPI.
Let's say you have a 600 x 800 pixels image.
In the first option the image will take up the entire screen height and width (in full screen).
In the second option it will take up only 600/960 (=800/1280) thus 62.5% of the screen's height and width (0.62 x 0.62 = ~ 40% of the screen area).

Some softwares fit the image to screen or to a window or to a sertain size on the screen but some fit the image resolution to the screen resolution i.e. there is a screen pixel for every image pixel and this results in an enlargement or a reduction of the image on the screen as a function of the screen resolution (and/or as a function of the resolution of the picture itself).

But there is more to it, you might ask: so, why am I seeing an image less clear on a low resolution screen? The explanation for this is simple:
The clarity or sharpness of any image that consist of pixels or any sort of a grid depends on the combination of 2 factors:
1. The density of the grid (PPI).
2. The distance from which we look at the image.
For a human eye for a distance of approx. 1 ft. (~ 30 cm), pixels 1/128 of an inch (~ 0.2 mm) in size will give us a clear and sharp image.
A 600 x 800 pixels screen is not enough for this matter. A 960 x 1280 screen is optimal for a distance of 1 - 2 ft.

More important facts:
1. The farther our distance is from an image the larger the pixels can be (lower resolution or lower PPI) in order for as to see what seem to us as a sharp image. This fact is true for prints of all sizes as well as for computer screens T.V. screens etc.
2. With no relation to the screen size and resolution it is recommended to whach the computer screen from a distance of ~ 2 1/4 ft. - 2 1/2 ft ( ~ 70 - 80 cm).

Printer resolution

moshebaram@gmail.com (Administrator) — Sat, 16 Aug 2008 21:40:25 +0000

Printer resolution

Printer resolution is the quality of the image the printer can produce, a higher resolution results in a sharper, more detailed image. Resolution is measured in dots per inch (dpi). A 300 dpi is acceptable for most documents.

More technically, it is the number of ink dots per inch (dpi) produced by all laser printers, including imagesetters. Most desktop laser printers have a resolution of 600 dpi and imagesetters have a resolution of 1200 dpi or higher.

Number of colors

moshebaram@gmail.com (Administrator) — Sat, 16 Aug 2008 21:39:07 +0000

Number of Colors

Color photos:
Always choose 24-bit color scan (millions of colors or 17 million colors in some scanners).
It is not recommended to choose the "billions of colors" / 36-bit option if you have it. A human eye can never tell the difference, it will be just a waste of disk space.
Never choose the 256 colors (8-bit) option, it is not enough, you will notice the difference.

B&W photos:
For photo preservation and restoration purposes we recommend always to scan in color - B&W photos as well as color photos. It takes a little bit more space on your disk but most of the times you get much better results in terms of authenticity. Many old photos, even though taken in B&W, have their own special hue, which gives them that special authentic look. Restoring an old photo does not necessarily mean losing that look, but rather fix the flaws and preserve that authentic look. Scanning the photo in B&W, might result in losing that look and damage the final result.
However, if you still choose to scan in B&W, always choose the 256 gray shades (8-bit grayscale) option.

Scanning resolution

moshebaram@gmail.com (Administrator) — Sat, 16 Aug 2008 21:36:31 +0000

Scanning Resolution

Since professional printing is at resolution of 300 PPI, the optimal scanning resolution for printing at the same size as the original photo is 300 PPI as well.
In most cases it is unnecessary to scan at a resolution greater than 300 PPI.

However, it is necessary to scan at a resolution greater than 300 PPI when you scan a very small item with purpose to enlarge it.

If you wish to double the original size of your picture (thus 4 times in area), double the resolution of your scanning i.e. scan at 600 PPI.
In the same manner, if you wish your picture to be 4 times the original size (thus 16 times in area) scan at 1200 PPI.

If you do have to scan at higher resolutions such as 600 or 1200 PPI or higher, be aware that the scanning time increases at a non-linear rate.
Therefore your computer must be powerful enough to handle such enormous amount of data.
Furthermore, the greater the resolution is, the more disk space it will occupy, as described in the following table:

File size per area of scanned photo (jpg format)*
24-bit color
Scanning resolution
[PPI] File size
[Kb per sq. inch]
300 55
600 150
1200 630
8-bit grayscale
Scanning resolution
[PPI] File size
[Kb per sq. inch]
300 35
600 110
1200 440

* Appoximated values only, real values may vary depending on the photo being scanned, type of scanner, and more.

Examples:

1. You have a 3 X 4.5 inch color photo that you want to enlarge to a 4 X 6 inch. Although it's not double in size you still need to scan it at 600 PPI instead of 300. The file size will be approximately:
3 x 4.5 x 150 = 2025 Kb ~ 2 Mb

2. You have a 2 X 3 inch B&W photo that you want to enlarge to an 8 X 12 inch. It is 4 times larger which means that you have to scan at 1200 PPI.
The file size in this case will be 2 x 3 x 440 = 2640 Kb ~ 2.6 Mb

For most personal computers it is not recommended to scan any photo at more than 1200 PPI.