The whole subject isn't helped by units of measure being referred to in different ways.

For example, printer manufacturers tend to refer to dots per inch (dpi), while Photoshop refers to pixels per inch.  For the purposes we're talking, the units are interchangeable.

To illustrate the relationship between picture file size, resolution and final print size, I'll use my camera's file size to illustrate how it works.

• A full size file from my camera is 4,256 pixels by 2,832 pixels.
• If I print it at 300 pixels/inch, the resulting print is 360mm by 240mm.
• If I print it at 250 pixels/inch, the resulting print is 432mm by 288mm.

It's generally reckoned that the human eye cannot detect the individual ink drops at 250 pixels/inch, but if you get down to 200 pixels/inch, they may become apparent.

The distance you're going to look at the print from determines the print resolution you need.  Generally, the answer is more than 250 pixels/inch. 

Controlling Resolution and Print Size

The relationship is controlled in Photoshop using 'Image Size'.

The top section of the dialogue box - Pixel Dimensions - indicates the file size in pixels. 

The middle section of the dialogue box - Document Size - indicates the resulting print size. 

If you type a print dimension into the Document Size section, the resulting print resolution wil be displayed. 

Make the print size bigger, the resolution goes down.  Make the print size smaller, the resolution goes up.

What About 72dpi?

The other resolution you often come across is 72 pixels/inch. 

This resolution isn’t used for printing – it would be far too coarse.  It’s used to control file sizes for images that are going to be displayed on a computer monitor.  Almost all computer monitors display at 72 pixels/inch.

Caution

When resizing image in Photoshop, be careful with the use of the ‘Resample’ option.

The Resample option will either create more pixels if you need a bigger file, or throw pixls away if you need a smaller file size.

It’s very easy to either create totally massive files that take an age to save, or conversely, to turn a nice high resolution file into a very low resolution file.

Good practice is to never save resized files.  Do whatever resizing you need AFTER saving your file.