W hat Does Resolution Mean in PI?

Digital images that come from your camera or from a scanner are constructed of pixels. You can think of them as the same as the grain on an ordinary film. Pixels are individual points that make up our picture. The difficulty most of us have is in understanding the relationship between what we have as a digital file and what is printed.

On the picture above you can see that it is a 2272x1704 pixel image. The corresponding resize box in PI looks like this

There are a few areas to notice.

First look at the new image area. It is greyed out because resample is not currently ticked. More of that in a moment. You will see it says  our image is 2272x1704pixels and that the memory used is roughly 11Mb. We have not changed anything yet so this is what the camera took, a picture 2272x1704 pixels.  2272x1704 = 3871488 which is roughly 4 megapixels. So this means our digital photograph does not have a size just a number of pixels.

You will also see that the current resolution is set to 252 pixels per inch. This makes our document 9.02 inches wide by 6.76 high.  If we now change say the height or width or the resolution then the other two parameters will also change because we only have a fixed number of pixels. Below you will see the resolution changed to 300 pixels per inch and the width and height are changed accordingly.

This is because the digital image has no actual size just a total number of pixels.

So document size has nothing to do with what you see on the screen only what you print out. On the screen if you view at 100% you see only the pixels.

So if you wanted to print this image out at 20" wide it would be 15" high but it would have a resolution of only 114 pixels per inch. This will not give enough output quality for a print.

Can we increase print size without affecting resolution? The short answer is yes but the quality will start to be affected. To do this we would now tick the resample box and select a method (normally bicubic for best quality). With the box ticked we can now  adjust the document size independently of the resolution. You will also see the new image area become active. Of course this is because if we increase document size but keep the resolution the same we must be adding in pixels.

Here you can see I have increased the size of the document but kept the resolution the same.

You can now see the file size has gone up to 79Mb and we now have 6000x4500 pixels. This is where the downside comes in. Where have all the extra pixels come from? Well PI has used the bicubic method to guess what those new pixels should look like. The more you do it the worse the guesses become and you loose quality.

It is normally best to increase in small steps. to the size needed.

Finally what do you need for your output. For the screen 72ppi (pixels per inch) is all you need. Any higher just makes the file size biger but there is no difference in how it will look.

For printing on inkjet printers a resolution of 3-400 pixels per inch is normally enough. DO NOT CONFUSE PPI with the dpi (dots per inch) used to classify inkjet printers. The dpi is a measure of how many dots of ink a printer puts down per inch. To achieve good dithering a printer may quote 1200 dpi but if it uses 4 colour system  it only needs 300 ppi (300x4 = 1200) for perfect prints. In short set ppi to 300-400 and set your printer to as high dpi as possible.

So that's  the theory so what you should do in practice. Well the following might be a typical workflow

1. Get your photo into your pc with the highest possible resolution.

2. Goto resize and uncheck resample

3. Crop what you want in the aspect ratio you want.

4. Goto resize and check resample and set the document size to the required output size.

5. Change the pixel per inch to 300-400. If this is a big jump then do it in stages. If you are at say 100 ppi got to 200 and then 300.

5. Print out at the highest printer resolution (measured in dpi) say 1200 dpi

You will find this easier to understand by doing. It is quite hard to explain but easy to understand if you play around with the figures your self.

 Last updated 4th January 2006