How Pixels, Resolution and Ratio affect your print-outs
Tip: If you are changing the size of an image; or need exact output sizes; discuss it with us before you place an order.
What are Pixels and Resolution?
Pixels
What you see as a photo or scan is really nothing more than a whole bunch of tiny coloured squares. Think of a mosaic made of tiles. If you are looking at it from a distance you see the picture, but if you are standing closer to the image you can see the individual tiles instead of the image. To simplify it a bit, the ’tiles’ are called Pixels
Generally the ’tiles’ on the screen are called pixels per inch – PPI, the printer’s output is called dots per inch – DPI.
Resolution
The term “image resolution” means how many of your image’s pixels will fit into each inch of paper when printed.
The picture on the left was scanned at 400 PPI, the one on the right was scanned at 50 PPI to demonstrate the difference. Click on the image to see a larger version.
To put it simply, the more pixels to the inch, the smaller they are and the better the quality of the image. If you try to enlarge a low-quality image you will get a big image that has big pixels! This is why images taken from ordinary web pages don’t enlarge well.
Pixels for different outputs
A Web Page Image needs 72 pixels per inch
Paper or canvas
For Optical Character Recognition (OCR), text only, scan at 250 – 300 pixels per inch
Images to be printed at the same size or smaller need 300 – 400 pixels per inch
Fine Art images or Images to be enlarged need 400 – 600 pixels per inch
Photos needing repair work need to be scanned in at 400 – 600 pixels per inch
What size print will you get?
We can use professional profiles etc. to overcome some difficulties, but this is still a good guide.
To work out what size print can be made from an existing image for a printout at 300 pixels per inch – divide the number of image pixels along each side by 300.
The image is 12000 pixels wide by 800 pixels high
- 1200 pixels wide divided by 300 pixels per inch = 4 inches
- 800 pixels high divided by 300 pixels per inch = 2.667 inches
- the image will be 4 inches by 2.667 inches if printed at 300 ppi (pixels per inch)
If you try to print this image out any larger than this, the quality will be poor.
| Image size in Pixels | Print size (inches) at 300ppi | Print size (inches) at 400ppi |
|---|---|---|
| 640 x 480 | 2.1 x 1.6 | 1.6 x 1.2 |
| 1,024 x 768 | 3.4 x 2.5 | 2.56 x 1.92 |
| 1,504 x 1,000 | 5.0 x 3.3 | 3.76 x 2.5 |
| 2,000 x 1,312 | 6.7 x 4.4 | 5 x 3.28 |
| 2,590 x 1,920 | 8.6 x 6.4 | 6.47 x 4.8 |
| 3,008 x 2,000 | 10.0 x 6.7 | 7.52 x 5 |
| 4,256 x 2,848 | 14.2 x 9.5 | 10.64 x 7.12 |
| 5,782 x 3,946 | 19.3 x 13.2 | 14. 5 x 9.9 |
To put it simply, the more pixels to the inch, the smaller they are and the better the quality of the image.
If you try to enlarge a low-quality image you will get a big image that may show big blocky pixels!
Ratio Explained
‘Aspect Ratio’ is the term used to describe the height to length relationship of an image, or the ‘Proportions’.
A square has an aspect ratio of 1:1. There is one unit of height to every one unit of length.
A 4 x 6 print for example has 4 units of height for every 6 units of length.
This can be simplified by dividing both measurements exactly by the same number – in this case two.
(2 divides into 4 twice and into 6 three times)
A 4 x 6 print has an aspect ratio of 2:3.
If you wish to enlarge an image the Aspect Ratio should remain the same or the image will be distorted.
We will be happy to help you with this, it’s not as confusing as it seems!
A 6 x 4 will enlarge to 12 x 8 because the proportions are the same, (3:2)
Enlarging it to 5 inches by 9 inches causes distortions because the picture ratio is different. (5 and 9 can’t be divided exactly by 3 and 2)
The only way to keep the image the from distorting is to put in extra background – normally equally divided around the sides.
I hope the examples below will make it a bit clearer.
