A question I have been asked twice just recently is what resolution is required in order to produce a decent print when printing a photograph. The answer to this is not as simple as most people might initially think it is.
For your basic photographic print to be viewed in an album or by holding the print then the generally recognised minimum resolution is 220 PPI. In fact if you are using Adobe’s Photoshop (any edition or variation of Photoshop) then it will stop the print job and warn you if you start to print a picture and the effective resolution is not at least 220 PPI.
But this resolution of 220 PPI is based on the picture being viewed from the average hand‑held distance of 12 to 15”.
As the viewing distance increases the human eye and brain combination allows for the resolution to decrease while the perceived sharpness and clarity of the picture will remain the same. For example, for the average person when viewed from a distance of 30” the resolution of the image can drop to about 120 PPI yet the picture will appear to be just as sharp as a 220 DPI picture being hand held.
For billboard pictures intended to be viewed from a passing car about 30 to 50 metres away, or further, the PPI will be down around 8 or less. Seriously. Have you ever got up close to a large roadside billboard? The pixels will be about a half the size of a five cent piece (meaning an Australian five cent piece as I have no idea how big a US five cent piece is).
If you search around the Web you will find various formulas for working out the minimum PPI based on the viewing distance. The two that I know are:
- ppi = 3438 / distance-in-inches
- ppi = 1 / ((distance-in-inches * 0.0006) /2)
Both of these give much the same result.
Remember, this is the minimum PPI for the viewing distance. Apart from the file size there are no issues with the PPI being higher than this, but anything lower is going to impact the perceived sharpness of the image.
Personally, for any ‘normal’ hand held picture I print I aim for 300 PPI although I have done comparisons between 220 PPI and 300 PPI for hand-held viewing and it really is very hard to spot the difference.
Points of Interest (maybe):
- PPI refers to the resolution of a raster image and this is not the same as DPI (which refers to printer ink-dot density).
- Images grabbed from a PC screen will be 72 PPI.
- The resolutions mentioned assume that the printing is being done on a photo-quality printer using photo-quality media.
- For raster images going into technical documentation (like screen grabs, etc.,) I aim for at least 150 PPI. This usually works out okay because documentation is printed on laser printers and laser printers are not classed as photo-quality (and normal A4 printer paper is not classed as photo-quality paper).