Vive la Résolution!

There is a commonly held misconception concerning image resolution that I frequently encounter. By way of example, for a recently advertised student photographic competition, entrants were requested to submit files at no less than ‘300 dpi’. This is a fallacy and suggests to me a certain lack of understanding of resolution on the part of the competition organisers. Of course their desire to collect image files of sufficient resolution to ensure a usable quality is entirely reasonable, but the specification ‘300 dpi’ is in this context meaningless and serves only to confuse potential participants.

My first issue with the above is a technicality and whilst it may seem a little pedantic, is nonetheless worthy of clarification. I believe that by they mean ‘ppi’, that is ‘pixels per inch’, not ‘dots per inch’. Pixels per inch describes the number of image pixels over a given area of the image, literally one inch of image being defined by ‘x’ number of pixels. Dots per inch describes the density of ink droplets deposited on the paper by the printing technology. They are not the same thing. Indeed, with inkjet printing the dpi of the print is often much higher than the ppi of the file sent to the printer. This source of confusion for the newbie is not helped by the tendency of some scanning software to (incorrectly) use the label ‘dpi’ not ‘ppi’ when defining scan resolution.

However, all this is irrelevant to my key point which is that ppi, (or dpi for that matter), is not a measure of file resolution, it is simply an instruction for printing. The resolution of the image file itself, i.e. whether it is a high resolution or low resolution image, is governed by the quantity of pixels overall. Thus, a file consisting of 800 x 600 pixels might be described as low resolution, only really suitable for screen display, whilst a file of 3000 x 2000 pixels is much higher resolution, containing considerably more data and is more capable of being printed at a large scale to a high quality. It matters not one jot what ‘ppi‘ that the file has been saved in, but it is extremely important to ensure that the file contains sufficient pixels to be displayed without ‘pixelation’.

Considering screen presentation, monitors, as I am sure you appreciate, are lower resolution devices than printers, and as such require significantly less data to appear at an acceptable quality. On many occasions we are asked for images at ’72dpi’, as this is the correct resolution for screen, right? Wrong! For screen display specifying a ppi is even more ridiculous as web browsers completely ignore the ppi value. They will simply display the image pixel for pixel – one image pixel is represented by one screen pixel. If you don’t believe me, try this…

Take an image and open in Photoshop

Go to…

Image > Image Size

Set width to 800 pixels and click ‘OK’

Now open the image size dialogue box again.

This time, uncheck ‘Resample Image’

Set ‘Resolution’ to ‘300 pixels/inch’

Click ‘OK’

Save a copy (File – Save as…)

Finally, open the image size dialogue box a third time and with ‘Resample Image’ still unchecked set Resolution to ’72 pixels/inch’

Click ‘OK’ then save a second copy (File – Save as…)

Now open each of those pictures in a web browser, (I.E.,Chrome, etc., doesn’t matter which), and observe the results.

Guess what, they look exactly the same; same size, same quality. And the reason for this? Thats right, they are the same. They are (in this example) 800 x 600 px.

In the case of the afore mentioned competition, where the purpose is to insure that submitted images are of sufficient quality to be printed and publicly displayed, the same rules apply. Expressing a requirement for a specific ppi without also specifying the dimensions, is quite frankly nonsense. A file that is saved as 3×2 inches at 300 ppi has far fewer pixels (900×600), and therefore less resolution, than a file saved as 30×20 inches at 100 ppi (3000×2000). What we need to know in this context is not a specific ‘ppi’, but the minimum pixel dimensions.

It is also impossible for you to control exactly how big an image appears on another’s screen. If you set your file to be say 6×4 inches, at 72ppi, the chances are that when you open it and display it, either in your browser, or in Photoshop at ‘100%’, it will not be exactly 6×4 inches. This is because your screen is probably higher resolution that 72ppi, so the picture will appear smaller. However, other monitors may be higher or lower resolution than yours, and will display the image at a different actual size accordingly.

So remember, when you are asked to supply files at ‘300 ppi’, or any other ‘ppi’ for that matter, I suggest you respond politely thus: “Ok, what resolution do you require?”

*When an image has insufficient file resolution for the chosen scale/display technology (screen, inkjet print, laser print, etc.), the viewer will be able to discern individual pixels as distinct and thus the illusion of continuous line and tone is lost. Such an image is said to be ‘pixelated’. This explains why an image which looks to be of high quality displayed on a web page will, if printed at a reasonable size, take on the appearance of Lego bricks, as printers require greater file resolution.

** (When preparing images for the web, forget ppi, concentrate on pixel dimensions. A common standard is 800 pixels wide. This should ensure that the image will fit on any screen/device without cropping.

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Duncan Shepherd