Tutorial - Resolution and definition of a digital image.

The resolution and the definition of a digital image are two very specific characteristics but which are often misunderstood or confused. Here is a short explanation on the subject.

The definition.

The definition of an image is expressed in pixels: this is simply the number of pixels contained in the image. A camera sensor that produces 4000 pixels on the long side of the image and 3000 on the short side will therefore have a resolution of 4000x3000 = 12 million pixels.

Obviously, file size (in kilobytes) increases with the number of pixels. But this is not necessarily proportional, as most graphic formats, such as JPG, incorporate compression.

The resolution.

Resolution is the number of pixels per inch. It is expressed in dpi, sometimes inppi.

Resolution is therefore only meaningful when the image is printed. There is no point in defining this parameter in the image file settings: the resolution will simply be the consequence of the print size.

For example, if our 24 million pixel image (6000x4000) is printed on 30x20 inch paper, it will be printed with a resolution of 200 dpi (6000 / 30 = 200). If you want to enlarge the format and print on 60x40 inch paper, the resolution will be lower and equal to 100dpi.

For an art print, the size will be imitated to remain around 300dpi. For a standard print, the dimensions can be increased until the resolution is reduced to 150dpi. Note: the larger the print, the lower the resolution you can accept, because large prints can be viewed from a distance.

Please note that we are talking about photo prints here. However, most word processing or page layout programs take into account the resolution of the image files: They use this information to calculate the size of the image on the page. The processing logic is therefore the opposite of that for photo prints:

• For a photo print, we define the desired dimensions: the resolution is a consequence.
• For page layout, the size of the images is calculated from the resolution indicated in the image files.

Practical applications.

Assessment of maximum print size.

From a given image, you can therefore estimate the maximum print size without loss of quality.

For example, let's take an image with 16 million pixels (say 4900 x 3300 pixels) that we want to print with the best possible quality, i.e. 300 dpi. We need to limit the print size to :
4900 / 300 = 16,33 inches, or approximately 42 cm for the long side,
3300 / 300 = 11 inches, or about 28cm for the short side.

Calculating image size for the Internet.

A screen can rarely display more than 1800 pixels on its long side (width). There is therefore no point in downloading larger images. There are even a number of disadvantages:

• File size increases with the number of pixels. Uploading images that are too large slows down loading times.
• Many sites (Facebook, for example) automatically reduce the size of images. But they're not always the best way to process them, so it's best to do the work yourself before sending the image.

However, it should be noted that some sites offer a magnifying glass function that lets you zoom in on part of the image. In this case, a higher resolution image may be useful.

For publication on social networks (Facebook, Instagram or other) the easiest way is to refer to the help pages: the optimal image size is often indicated.

The ratio.

The ratio is the division of the number of pixels on the short side of the image by the number of pixels on the long side of the image. Reflex camera photos have a ratio of 2/3, while mobile phone photos usually have a ratio of 3/4. A square image has a ration of 1.

In all cases, you should choose a print format that respects this ratio. For example, a photo taken with an SLR camera can be printed in 20 x 30 cm (ratio of 2/3). Of course, it is always possible to crop an image before sending it to print, all graphics processing software can perform this operation.