Stephen Eastwood|Beauty and Fashion Photographer

DPI/PPI and web display?

What is the relevance of dpi/ppi to images destined for display on monitors and not printed? There's a lot of talk about 72 dpi/ppi optimized web images. Does it have any effect on image quality or file sizes?

DPI is dots per inch and is the term used in printing and inkjet printers who at a point woudl lay specific dots per inch of paper. ppi is pixels per inch and is a term used for display on a screen of some sort, various screens and computers or devices use differing amounts of pixels per inch so a screen with a higher resolution at the same physical size woudl have a different (higher) pixel density per inch of physical space. If you have an image that is 1000pixels by 1000pixels and a display that is 10 inches by 10 inches and has a resolution of 1000x1000 it will show that image as a full screen image, that is a resolution of 100 pixels per inch, many computers have had a screen resolution of 72 or 96 ppi and that standard was around for a long while, it is now changing as screens are getting larger, but overall they stay relatively close though not exact.

Now for dpi that is the dots per inch and as it relates directly to the image file if you had that same 1000x1000pixel image and printed at the norm of 300dpi you could only print that image at 3.33333 inches by 3.33333 inches. If you were to lower the dpi to 200 you could print the same image at 5 inches by 5 inches, and of course you could use software interpolation or even printer interpolation (which is also software based) to increase the pixel count and allow for larger print sizes with less visible disruption, although no new date or resolution is built or added.

Now for display on the web as asked, you can specify a standard of 72dpi, but in truth the actual image size on screen will vary based on the screen resolution set by the owner, if they have a high resolution of say 1280x1040 on a 17inch non widescreen that will show a smaller image in actual dimensions over the same computer using the same 17inch non widescreen with a resolution set to 960x600

The actual answer is no the ppi set when optimizing will have no bearing on final view since the web browser or software program will adjust it to the screen resolution used.

Colorspace will, or should I say may, have an impact on the look of the image on various systems and browsers, and for safety sake on internet display for the mass market (not a targeted market of known viewers) it is best advised to convert all files to sRBG and save them in that colorspace as that is most commonly used online and it happens to be the closest of the existing colorspaces to a 'theoretically generic' uncalibrated monitor. And the majority of webbrowsers pay no attention to colorspace of a file so the converting to that colorspace makes the lack of attention less damaging. *

The final look of any image online unfortunately is not within your control as you should have a calibrated monitor to see the best image available and that is not something that can be controlled, and if any of you have ever gone into a larger TV appliance store and looked at a wall of televisions all showing the same show and noticed the huge variance between them that is exactly what you are dealing with online as most people are not aware of monitor calibration and even if they are a vast majority still do not have calibration hardware and regularly perform monitor calibration so you are always at the mercy of the monitor and settings and adjustments that they have on the given screen. The one thing that may make you feel better is that as off as your images will look on there screen so to will all the other images they see on that screen so you will be just as messed up as everything else they see hmm

* Thankfully corrected by Kevin Connery his work can be found here