Images on Screen and Paper

Images /  23 September 16 / by Dale Brett    
Images on Screen and Paper

Image Sizes, Screen Sizes, and Print Sizes

Image sizes, screen sizes and print sizes can be very confusing; if you have ever sent an image to a printer only for it to come out smaller than you expected, or muddier with lower quality than you wanted, you probably didn't take into account the various resolutions of the devices you were using.

Screen Sizes

You may have heard the word resolution with regard to televisions with numbers such as 1080, or 720 being high definition resolutions; this isn't the physical size of the TV, but the number of dots of light the TV uses to display it's full height when displaying a full screen image. So the resolution is the measure of pixels in the television.

A 1080HD TV has 1,920 dots horizontally and 1,080 dots vertically, which means it has a total of just over 2 million dots of light in total to display full screen images.

Unlike televisions, computer monitors and other device displays, such as tablets and mobile phone displays, all differ in size and resolution.

You may have heard of DPI which stands for dots per inch, in the case of a display device this is the number of dots of light that are used to display a single line of an image that is one inch long.

So a display device that is 20 inches across with a resolution of 1920 x 1080 will have 96 DPI (dots per inch), meaning 96 dots of light are used to display one inch of an image on the display; a larger screen at the same resolution would have a lower DPI, a display device 30 inches across would have 64 DPI.

So what has this got to do with images and printing?


Digital Images

Digital images themselves are made up of collections of pixels; a pixel is the smallest unit of information in a digital image and represents a single colour value.

Pixel

Digital images have a width and height that is measured in pixels; an image may be 500 pixels wide and 500 pixels tall for example, but it's important to remember that this does not translate into units of length in the real world until you attempt to display or print the image.


Display

The size of an image being displayed on a computer, tablet, or mobile display, depends upon how many pixels make up the original image and the resolution of the display itself.

If we were to display the 500x500 image on the 96DPI computer display then the image would appear to be just over 5 inches wide, in this case one dot of light is being used to display one pixel.

On a high resolution display that can show 192DPI the image would appear to be just 2.6 inches wide; the amount of information in the original image has not changed and one pixel is still being displayed by one dot of light on the display but there are more dots per inch so the image can be displayed in a smaller physical space.

The idea of the DPI determining the physical size of the image is important when it comes to printing.


Printing

Digital printing is the process of creating high quality printed media from a digital image source; in order to produce an image on paper, ink is transferred to a paper stock in a particular pattern of dots that makes up the final image.

Like the displays, the higher the number of dots used to make up the image, the better the quality of the printed image.

Print resolutions are measured in dots per inch (DPI), which is the number of dots the printer can print horizontally onto an inch of paper.

e.g. a printer with a resolution of 300 DPI can print a maximum of 300 dots per inch on a piece of paper.

If we were to print the 500 x 500 image using a 300DPI printer at its full size, the image would be 1.66 of inches wide; what appeared to be a reasonably sized image on a computer screen now appears to be quite small.

We could tell the printer to print the image at 5 inches wide, but if we did this the 300DPI printer would need to print 1500 dots instead of only 500, (300 dots x 5 inches), but remember there are only 500 pixels in the original image so the printer must print some of the pixels multiple times or perform pixel averaging which leads to a blocky, low quality image.

In order for an image to print at 300DPI and be 5 inches long at full quality, the image would need to have a pixel resolution of 1500 x 1500.

High quality digital printers have a much higher resolution than device displays ranging from 300DPI to well above 1200DPI which is why images that are being printed need to have a much higher resolution (have more pixels representing the image) in order to print out at larger sizes while retaining image quality.


Digital Camera Images

Digital cameras use the unit "Megapixel" to inform people of the resolution of the camera; one Megapixel is just over one million pixels; a 5 Megapixel camera can capture a maximum image size of 2560 x 1920 pixels.

Printing this image at 300DPI you would obtain an image 8.5 inches wide by 6.4 inches at full quality which would be fine for your family album but not for a glossy magazine spread or a poster.


Image File Quality

Just because an image has a large pixel resolution doesn't mean it will be great quality; I could take the 500 x 500 image and increase its scale in an image editor to be 3000 x 3000, but because there is no new information some pixels will be re-used and the larger image will appear blocky when viewed at full size.

Image files can also be held in different formats, the format is the way the information is held by the computer or device. Different formats produce different file sizes, but remember that the file size has nothing to do with an image's pixel resolution or it's print size.

Smaller file sizes are created by using compression; compression allows the computer to save disk space by using a software algorithm to reduce the amount of raw information that is needed to represent the image.

There are two types of compression: lossless, which retains all of the original image information; and lossy which reduces the quality of the image in order to increase the efficiency of the compression algorithm thereby reducing the file size.

TIFF and PNG are lossless file formats; JPG is a lossy file format and the higher the requested compression on a lossy format, the lower the quality of the image even if the pixel resolution is high.


So what should I do?

The main thing to remember is that if you are producing a printed image from a digital image make sure that the original digital image is the highest quality with the lowest compression you can get so you won't be disappointed with the end result.

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