Pixels and PPI

Pixels not Inches

Most people are aware that their images are made up of pixels and that their camera is perhaps a 3 or a 5 Mega Pixel version or whatever the number is! The pixels are the picture elements in your camera that record the colours. Each one records just a small part of the overall subject.

The most basic fact you must understand is that your digital image is just a number of pixels - it has no physical size; its not 6" x 4" or A4, its just a number of pixels.

When you come to print the image you have to be aware that a certain number of pixels must be present in each inch of that print in order to get good quality. This will be stated as PPI - Pixels per inch. Take a look at the picture below to see how the pixels are mapped to the physical dimensions.

For this image from a 6Mpixel camera - 3000 x 2000 pixels - to get a 6" x 4" print you need to have 500ppi.

One common mistake is that people often look at the image details in Elements and think their camera has made an image of 3000 x 2000 pixels at 72ppi or 180ppi or 240ppi. This is not true and misleading on the part of the camera manufacturer. All you get is pixels. The PPI setting can be ignored. (If you want to know why its there go here)

How many Pixels Per Inch do I need?

So, we've established that to get from a number of pixels to a physical size we need to have a certain number of pixels per inch. What's the magic number - well the optimum value is 300ppi but you may get away with less. The thing to do is try it and see. Viewing distance can make a difference. The further away you are from the print the less detail your eye will register anyway. The lower the number the less sharp your print appears.

Can you have more than 300ppi? Well that depends on your printer. Some printers wont have a problem with higher ppi settings some will. Once again try it and see.

The magic formula is therefore :-

Physical Size = Pixel Width/resolution x Pixel Height/resolution

Rather than have to work this out each time, here's a table that shows typical camera pixel sizes against print sizes you can achieve.

MegaPixels	Data Size (Mbytes)	Print Size
		6" x 4"	7" x 5"	10" x 8"	A4	12"x10"	A3	19" x 13"
3	8.6	381ppi	293ppi	188ppi	181ppi	150ppi	128ppi	116ppi
4	11.4	440ppi	338ppi	217ppi	209ppi	173ppi	148ppi	134ppi
5	14.3	492ppi	378ppi	242ppi	234ppi	194ppi	166ppi	150ppi
6	17.2	508ppi	391ppi	250ppi	242ppi	200ppi	171ppi	154ppi
8	22.9	587ppi	451ppi	289ppi	279ppi	231ppi	198ppi	178ppi
12	34.3	718ppi	553ppi	354ppi	342ppi	283ppi	242ppi	218ppi

	The data size is extremely large and it takes longer to print, but the print quality is no different from the green zone
	The print quality is as finely detailed as the original allows and this is the recommended zone
	The print quality is good but may lack a little detail but depends on the original and viewing distance
	The print quality is alright but may lack some fine detail. Might be alright at longer viewing distances
	The print quality will less begin to be noticeable.

Why is there a PPI figure in the image details?

Along with all the colour numbers recorded by your camera, the image file also contains additional data sometimes referred to as Metadata, EXIF data, IPTC data and XMP data. Metadata is a name which encompasses all the other three but its the EXIF data we'll refer to here.

EXIF data contains information about your camera settings and many other details. If you really want to know you can get a version of the EXIF specification here. Some additional information is also here.

In the case of the PPI settings, within the EXIF data are fields for resolution which is what the PPI refers to. This really stems from the concept of sharing digital images which needed to be reproduced at the same physical size that they were created from. For instance a scanned document. If you scanned the document at a specific resolution i.e. PPI setting, and passed this setting along with the document data it could be recreated at the same physical size.

Digital Camera images don't have a physical size because the sensor is just recording the image on pixels. However, some camera manufacturers put a recommended PPI setting in the EXIF data. The often seen figure of 72PPI is a default setting if nothing else is specified.