The print size, pixel dimensions and image resolution of a picture

People always have an illusion about the print size of a picture saw on the computer screen. They believe that the print size and quality will be same as the one they see on the pc screen. If you are one of them and are planning to print some professional quality pictures, the article below is highly recommended.

The print size of a picture can hardly be the same as the one you see on the screen. The monitor resolution play the tricks. Even for the same picture, the size displayed on the screen is different if the monitors have 2 different resolution. A monitor set to 800×600 will show an 800 pixel wide by 600 pixel tall image as a full screen image. On a monitor that is 1600×1200 the image will only take up 1/4 the screen. You might have thought it would take up half, but it’s actually going to be 1/2 as wide and 1/2 as tall (so 1/2 times 1/2 = 1/4). Long story short, the image will look much smaller on that screen even though the image is the same size.

Well, so how do we know the actual print size of a picture since we can not get this information visually on the screen? To get an answer to this quesiton, we need define the concepts below first.

Pixels, short for “picture elements,” are the building blocks that make up a digital image — the tiny individual dots that a digital camera uses to capture a scene or that a computer uses to display images onscreen. A file’s pixel count (or pixel dimensions) is arrived at by multiplying its pixel height and width (as in 3000 × 2000 pixels). The Pixel Dimensions section tells us how many pixels are in our image.

The easiest way I can explain image resolution is to say that more resolution means an image displays more detail (or is capable of displaying more detail). It is measured by the pixel counts either from the top to the bottom  or from the left to the right inside each inch of the image. The pixel counts inside each inch from the top to the bottom is equal to the pixel counts from the left to the right. In one word, the resolution (or “res,” for short), is the fineness of detail in a document, and is measured in pixels per inch (as in 250 or 300 ppi).

While the print size, also called document size  tells us how large the image will appear on paper if we print it.

For now, we have explained pixel dimensions, document size and image resolution. What is the relationship between these 3 factors?

pixel height = image resolution * document height

pixel width = image resolution * document width

Below is a picture I took of a fower by the river. Let’s open it in the photoshop to have a check.

document-size-pixel-dimensions-printing-size

I’ll go up to the Image menu at the top of the screen and choose Image Size, which brings up the appropriately-named Image Size dialog box:

image-size-photoshop

Let’s do the calculation.

1200 = 72* 16.667

800 = 72*11.111

Bingo, all is correct.

Here the image resolution is only 74 pixels per inch. However, if we want a professional quality print, we need increase the resolution to make the image suitable for printing. 300dpi is standard, sometimes 150 is acceptable but never lower, you may go higher for some situations. Image files with higher resolution (more dpi) will also have a bigger file size because they contain more data. Start with the biggest images you can but when putting images on the web they should be set to 72dpi, it’ll save you a ton of bandwidth and they’ll load faster. Yes, they’ll be smaller than the original but should in most cases be plenty big because of monitor resolution (ppi) sizes.

For more information about the resolution, please check the links below

http://www.peachpit.com/articles/article.aspx?p=1660204

http://www.photoshopessentials.com/essentials/image-quality/

http://www.vsellis.com/understanding-dpi-resolution-and-print-vs-web-images/