Resolution, a matter of distance

Our perception of image detail varies with viewing distance. A print designed to be viewed at a distance of 1 metre may have to make do with a lower resolution than one viewed at 30 cm.

Let us try the following experiment. Take a close look at an old engraving. It is made up of a multitude of fine hatchings. They simulate halftones. As you progressively move away from the work, the eye confuses the strokes at a certain distance. They turn grey. Discontinuous tones become continuous. This phenomenon of perception altered by observation distance stems from the limits of a human being’s visual acuity.

Visual acuity

A child with normal vision can see an object clearly at 10 cm. In young adults, the distance is 25 cm. From age 40 onwards, presbyopia gradually lengthens this punctum proximum, the minimum distance for sharp vision.

Let us go back to the hatching in the engravings. Imagine a succession of black and white lines of equal thickness. If the thickness is 0.5 mm, each pair of white and black lines measures twice the thickness of a line, i.e. 1 mm. We, therefore, have one line pair per millimetre (lp/mm). If the line thickness is reduced to 0.25 mm, 2 lp/mm is reached; at 0.125 mm, 4 lp/mm, and so on.

At a distance of 25 cm, a young adult can barely distinguish the lines of a pattern with a resolution of 7 lp/mm. At a distance of 50 cm, half of this can be distinguished at best. The corresponding resolution drops to 3.5 lp/mm. At 1 m, go to a quarter, i.e. 1.75 lp/mm, etc.

From lines to pixels

The pixel is the smallest element in a digital photograph. The narrowest line in a picture is, therefore, one pixel wide. 1 lp/mm can be converted into pixels/mm, giving 2 pixels/mm (one black pixel and one white pixel). 7 lp/mm corresponds to 14 pixels/mm or 355.6 pixels per inch (PPI). Let us round up to 360 PPI. This value is the native resolution of most Epson printers. There is little need to go beyond that, as the eye would not know the difference. Canon and HP use a native resolution of 300 PPI (equivalent to 6 lp/mm, or 304.8 PPI).

If you would like to know the optimum PPI resolution of an image as a function of viewing distance, here is a relatively simple formula to integrate into a spreadsheet: Useful resolution = (1/(Distance*0.3))*25.4. The distance is in meters. The formula corresponds to a 6.66 lp/mm resolution at 25 cm.

Resolution and digital images

An image is not black and white lines but varying light intensity pixel modulations. A photograph rarely shows patterns corresponding exactly to pairs of lines with a resolution of 6 or 7 lp/mm. What is more, a print is rarely viewed at such close range. It is customary to hold the eye at a distance corresponding to the diagonal of a print. It means 36 cm for an A4 sheet and 51 cm for an A3. With our formula above, this viewing distance allows us down to 235 and 166 PPI, respectively. There is another factor to consider when judging the sharpness of a print.

Resolution and printing

In order to reproduce a photographic rendering with nuanced gradations and no visible halftone, an inkjet printer delivers continuous tones with more than 1000 ink dots per inch (usually expressed in DPI). Canon’s printer drivers offer 1200 or 2400 DPI. With many Epson printers, it is 1440 or 2880 DPI, or even 5760 DPI up to A2 format. Therefore, the printer’s resolution differs from the image it receives. Moreover, it does not use the same units. In the case of a printer, these are ink dots formed by droplets of a few picolitres. In the case of an image, these are pixels. Finally, you should know that with a file resolution lower than the printer’s native resolution, it is systematically resampled at 300 PPI for a Canon printer or 360 PPI for an Epson. This resampling avoids any risk of pixelation or visible screening on the printed image.

Discover the photography courses at Spéos

Spéos offers various training courses ranging from simple one-week photography workshops (initiation and advanced level) to 3-year courses. The long courses to become professional photographers allow you not only to master all the photographic techniques and its vocabulary (blurs, hyperfocus, sharpness zone, depth of field, backlighting, focal length, shutter release, autofocus, wide-angle, rule of thirds, etc.), but also all the stages of shooting and image processing.

Visiting the school allows you to discover the premises, the studios and the equipment, and is undoubtedly the best way to familiarize yourself with your future way of working. This is why, in addition to the open days, Spéos offers throughout the year personalized visits by appointment to come and discover the school with a member of the team.

Text and photos: Philippe Bachelier, teacher of Printing techniques at Spéos