What is Resolution?
Resolution is the number of pixels, dots, lines or samples per inch contained in an image – the sharpness of an image depends on the resolution. The same pixel resolution would be sharper on a smaller image than it would on a larger one, as the pixels would be spread over a bigger area on the larger alternative.
There are several units of measurement for resolution:
- Pixels per inch (PPI) – this affects the print size of your image and the quality of the output
- Dots per inch (DPI) – this refers to dots per inch for the resolution of the printer itself
- Lines per inch (LPI) – a measurement of printing resolution referring to the halftones built up by physical ink dots to create different tones, this measurement determines how close together the lines are in a halftone grid
- Samples per inch (SPI) – this refers to scanner and digital image resolution. When scanning an image, the scanner takes a sample of portions of the image. The more samples taken per inch, the closer the scan will be to the original image. The higher the resolution, the higher the SPI.
Pixels per Inch
The primary unit of a printing measurement is PPI. It is crucial that the number of PPI’s is sufficient to create a sharp image – if there aren’t enough pixels per inch, the individual pixels may become discernible to the human eye – thus leading to pixelated outputs. With jagged edges and blurring, the image will appear to be of inferior quality.
Although low resolution can be acceptable for digital viewing on monitors and mobile devices, it isn’t suitable for printing.
The printer will need to create a minimum of about 200 dots per inch on the paper – the more dots there are, the higher the quality and the sharper the print. This is because during the printing process, the printer lays down dots in varying coloured ink. If viewed through a magnifying glass, they would appear as dots but when viewed at a normal viewing distance, they merge together to appear as a sharp, clear image – 300 dpi is recommended for premium quality images.
Digital vs Print
The optimal resolution for digital viewing is 72ppi, whereas it’s 300ppi for print. For example, if you created a 10 x 10in image at 72ppi and the same image at 300ppi, you would notice they were totally different sizes on the screen because of the different number of pixels to every inch.
With a 10 x 10in image, the 72ppi image has a maximum of 720 pixels across its whole width, while the 300ppi image has 3,000 pixels across its width. Although the 72ppi version would appear clear enough for digital viewing, a printed version at this resolution would be too blurred.
If an image was accidentally created at 72ppi for printing, it wouldn’t be possible to alter it and generate extra pixels to transform it into 300ppi – you would need to start from scratch and recreate the whole design.