About file sizes

Digital cameras provide file sizes that are measured in pixels.

The file size from a professional camera with a full frame sensor is typically around 5600 x 3740 pixels (= 21 megapixels) – this we call a camera file and the uncompressed file size is 21 MB.

We provide our clients with more manageable file sizes.

Large files are 3600 x 2400 pixels and are compressed to 90% to give you a file size of around 2 MB.

We also supply small files at 1280 x 850 pixels compressed to 80% (file size around 500 Kb).

But first an explanation of dpi

dpi (dots per inch) is simply a term that describes the resolution at which the files are used – the term in fact should be “pixels per inch”(ppi) because “dots per inch”actually refers to print quality i.e. the number of dots of ink per inch. When viewing on a screen (computer or TV) you are actually looking at pixels (not dots of ink).

So the higher the resolution (measured in ppi or dpi) the higher the quality of the image.

Professional photo labs print at 300 dpi (ie high resolution)– so if you divide 3600 pixels by 300 you get a print that is 12 inches (30 cm) wide. Magazines are also printed at around 300 dpi.

Inkjet printers (like the one in your office) print at 150 dpi and newspapers are printed at around 120 dpi. So once again doing the maths – divide 3600 by 150 and you get an image 24 inches (60 cm) wide.

Screen resolution is actually 72 ppi (not dpi) (ie low resolution) – now divide 3600 by 72 and your image will be 50 inches wide.

So a photo file is just X pixels wide by Y pixels high. Then the software that they are viewed in or printed via will determine what dpi or ppi is used.

Even the simplest photo editing software programs will allow you to change the resolution of the files. It seems that most basic editing programs default to 72 ppi and this confuses many people who think they have the wrong file size.  So all you have to do is change the resolution in the program to 300 ppi (or dpi) and you will see the correct finished print size.

And you will also note that High or Low resolution does not refer to the file size.

Why are these measurements in inches? Because the standard has been set by Americans.

We supply to our clients large and small size files.

Large files are at least 3600 x 2400 pixels and are compressed to 90% to give you a file size of around 2 MB.

So divide 3600 by 300 and you get a professional print that is 12 x 8 inch (close enough to A4) or divide by 150 and you can print on your inkjet printer to 24 x 16 inch (A2).

We also supply small files at 1280 x 850 pixels compressed to 80% (file size around 500 Kb).

These files are designed for web use and usually have to be reduced even further in size for that.  The typical large size on Facebook for example is 1280 pixels wide. To put this in perspective a typical (new) widescreen 20” flat panel monitor is 1920 x 1200 pixels.

Do the sums again and divide by 150 and you have a file (1280 x 850 pixels) that will print at 8.5 x 5.7 inches (close enough to A5) on your inkjet printer.

So now lets take our small file (1280 x 850 px) and view that on a 72 ppi screen – the image will now be 18 inches wide.

We edit at full camera resolution and keep those files on file – so if you have any need of the files for large posters or a billboard, we can supply those files.