Have you ever printed a photo that you previously downloaded from the Internet (of course, we understand this is illegally downloaded photo, so a theft of copyright and we do not support that) on a paper and found out that the result is completely different than you expected? The printed photo was the same size as a postage stamp or it was printed in the appropriate size, but blunt and chequered. The cause of this problem is resolution.

In fact, it’s unfair to even say that resolution is the cause of this problem. Resolution is not something that only exists to deliberately make our lives miserable. The problem is simply that the point size of many photos on the web is too low for quality print reproduction. What does that mean? In analogue or physical environment, the size of the photograph is given in a length unit of measure, e.g. 13 x 9 cm. However, in digital or computer environment the size is given in points or pixels. Point or pixel is the smallest unit (the basic component) of any bitmap graphics. Bitmap graphics can be photographs, pictures, drawings, logos, etc., which are stored in computer form and consist of individual pixels.

The printed photo was the same size as a postage stamp or it was printed in the appropriate size, but blurred and pixelated.

Instead of mathematical equations we will try to show the effect of resolution on the quality of the image on the example. In a digital environment, a photo that measures 5 x 5 pixels (5 pixels in width and 5 pixels in height) has a total of 25 pixels. A photo of 25 x 25 pixels has a total of 625 pixels, while the one of 500 x 500 pixels has a total of 250.000 pixels. It is therefore a simple multiplication. To make it easier to understand, more pixels mean a better, sharper photo.

Ločljivost, resolucija

Practical example

Let’s be more practical and print a photo on which the orange slice is caught just as it drops into the water on paper. For a better perception, we will set or reduce the size of the photo of an orange on a computer (this can be done with one of many image processing programs) from the original size of 2592 x 2592 pixels to 5 x 5 pixels. We will now print this photo on paper in the size of 10 x 10 cm (now we are going from digital to analogue or physical environment). What’s the result on paper? On paper we see only 25 squares arranged in 5 columns, each column has 5 squares, the size of each square is 2 x 2 cm. Each square on paper represents one pixel (picture below).

Ločljivost, resolucija 5x5 pikslov

Repeat printing, this time with a photo of 25 x 25 pixels. This photo will also be printed in the size of 10 x 10 cm. What is the result now? We printed 625 squares arranged in 25 columns, each column has 25 squares, the size of each square is 0,4 x 0,4 cm. Each square on paper represents one pixel (picture below).

Ločljivost, resolucija 25x25 pikslov

The difference is considerable. In the second example, the size of the square is incomparably smaller. With a smaller square or pixel we can print images with more details. Similarity can be copied with the use of thicker and thinner pens, whereby smaller letters can be written with a thinner pen, while with a thicker pen it is difficult to write smaller letters or they are unreadable. More pixels therefore mean more detail and sharper and clearer picture. Repeat printing with a photo of 500 x 500 pixels (picture below).

Ločljivost, resolucija 500x500 pikslov

What is DPI and what is PPI?

Of course, the recording of resolution is unified and standardized for easier and clear communication and is, when it comes to printing, expressed in a PPI unit (in a digital environment – a computer) or DPI (in an analogue environment – printed on paper). Though often PPI and DPI are identified as equal and both values indicate resolution, they are not exactly the same attributes. The misuse of these terms in their programs is also used by Adobe, Apple, Microsoft, Corel, etc., so users have somehow unified the attribute PPI and DPI. However, for easier understanding we will limit ourselves to PPI only.

PPI is the abbreviation for “pixels per inch”. Pixel means “picture element”, which is the smallest and basic cornerstone of an image in the digital environment. Inch is an American and English unit of length. Last but not least, the number of pixels per inch can be converted into the number of pixels per cm (1 inch = 2,54 cm), but in printing, the unit of inch is used in the print resolution.

Of course, the recording of resolution is unified and standardized for easier and clear communication and is, when it comes to printing, expressed in PPI or DPI unit.

Let’s get back to the examples with a photo of 5 x 5 pixels and a photo of 25 x 25 pixels, both printed in size of 10 x 10 cm. In the first example, the size of each pixel was 2 x 2 cm, and in the second example, the size of each pixel was 0,4 x 0,4 cm. It is immediately clear that the unit of measure can contain a bigger number of smaller pixels than the large ones. According to the calculations which we will not burden ourselves with this time, we get about 1,3 pixel lengths of size 2 x 2 cm and about 6,35 pixel lengths of size 0,4 cm into the length of one inch. This means that the photo with a pixel size of 5 x 5 pixels and a physical size of 10 x 10 cm was printed in a resolution of 1,3 ppi, and the photo with a pixel size of 25 x 25 pixels and a physical size of 10 x 10 cm at a resolution of 6,35 ppi.

Ločljivost, resolucija

Can a photo of 5 x 5 pixels be minimized to increase the final resolution?

Of course it can be, but this reduces the physical size of the photo. If you minimize it to 5 x 5 cm (i.e. to 50%), the size of the pixel is actually reduced to 1 x 1 cm, which means a higher resolution, in this case 2,6 ppi. But the picture is now physically smaller.

In order to facilitate the representation of actual resolution, some of the recommendations that we use in our practice are listed below:

  • mega format – mega graphics on the skyscrapers – 10-50 ppi
  • bigger formats – jumbo posters, billboards, linen cloth, etc. – 50-70 ppi
  • medium format – boards, bigger posters, banners, floor graphics, freight vehicle graphics, etc. – 70-100 ppi
  • smaller format – posters, city light posters, graphics on vehicles, smaller boards, etc. – 100-250 ppi
  • small format – stickers, flyers, business cards, catalogues, etc. – 250-300 ppi

The list shows that larger graphics, which we usually observe from a larger distance, require lower resolution than those that we look at from a closer distance. The reason is simple, because the objects that are farther away from us seem to be smaller. The basic cornerstone of an image or a pixel can thus be larger, as we will see it much smaller from a larger distance. We see larger amount of small pixels, combined in a whole, as an image.

To make it easier to understand, more pixels should mean a better, sharper photo.

Ločljivost, resolucija

Although we could print mega formats in larger resolution, there is no need for this, as this increases the size of the graphic files, thus increasing the time of the computer processing, since such graphical data files have more extensive data. Multiple pixels mean more data your computer needs to process.

Last but not least, the materials for the mega formats we print do not “bear” such high resolution. But this is already a topic for some of the following posts.