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The Digital Domain: Image Size and DPI/PPI
The concept of image size and dots per inch (DPI) is one of the most confusing aspects for beginners. We will start by stating a couple of points and then go on to explain them.
- The size of the image on screen has nothing to do with the size the image will print at. They are completely independent.
- The number of dots per inch in an image is just a number that you can set to anything you like.
Dots Per Inch (DPI) / Pixels Per Inch (PPI)
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If you have looked at the properties of a photograph taken with your camera you may have noticed something called resolution. There’s a word we’ve seen before but here it means something slightly different.
In the example shown on the right you will see the horizontal and vertical resolution is set to 72 dpi (dots per inch). This seems to be a common value for cameras to use - it corresponds to the resolution of older computer monitors.
For the purposes of digital cameras, the terms ‘dots per inch’ and ‘pixels per inch’ (PPI) are interchangeable. We will use PPI from now on.
The first important thing to note here is that this number is an arbitrary value stored in the image file by the camera. It could be set to any value you like and does not imply any particular level of quality. So for now, completely ignore it. |
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The best way to think of PPI is that it ties together the images pixel resolution and the physical size of the image using the following equation:
Pixel Resolution = Pixels Per Inch × Physical Size
If one of these is fixed, then changing a second one must also change the third one.
e.g. if you keep the pixel resolution fixed (as is usually the case with digital images), then increasing the physical size will reduce the pixels per inch. If you decide you must increase the dots per inch, then the physical size must decrease.
As the images that come off a digital camera have a fixed pixel resolution, then when you print the image at different sizes, you are changing the PPI (pixels per inch) on the printout (irrespective of that number in the file).
Before we look at an example, here is a table giving an indication of the output quality of different PPIs.
| PPI |
Comment |
| <100 |
Fairly poor quality (but for very large prints, may still be acceptable) |
| around 100 |
Looks good on screen and OK for larger sized prints which are going to be viewed from a distance. |
| around 150 |
OK for putting up on a wall when viewing distance is a few feet away. |
| around 200 |
Close to photographic quality. The make of printer and the print technology (e.g.inkjet, dye sublimation, laser) can have an effect but generally you should aim for a minimum of 200dpi for all your photo prints. |
| around 300 |
Indistinguishable from a lab based photo print (provided the printer is sufficient quality). |
| above 300 |
No noticeable gain in quality over 300 PPI |
Let's look at an example - suppose you have a 6 megapixel image – the pixel resolution is 3000 x 2000. We can rearrange the equation above to give:
Physical Size = Pixel Resolution / PPI.
Therefore picking some arbitrary PPI values …
| PPI |
The print size is ... |
| 72 |
42" x 28" |
| 100 |
30" x 20" |
| 150 |
20" x 13.3" |
| 200 |
15" x 10" |
| 300 |
10" x 6.7" |
Based on our quality guidelines that say you should aim for at least 200 ppi for photographic quality, this means you can gets prints up to 15” x 7.5” which will look really good. Go any bigger than this and the PPI must decrease and hence so will the quality of the print.
Size on Screen / Zoom Level
Zooming in on an image has no affect on the size or quality of your image. Nor does it have any relationship on the print size, which is always set separately (usually in a print setup dialog box or using a print wizard).
The zoom features in your image software are there to allow you to work on specific parts of an image in more detail.
Note that there are times when you should view an image at actual size (sometimes called 1:1, or actual pixels), such as when sharpening an image. The sharpen features will work exactly the same no matter what zoom level you are at, however you need to be at 1:1 to assess whether the amount of sharpening you have applied is correct as you will not see the effect properly if zoomed out to see the whole image.
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