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The Essence of

Resolution & DPI


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Moving on a step further from the description of such things as color models and color attributes - let's take a look briefly at the factors underlying the image itself. Resolution and DPI - and how they affect image construction and properties.

Lot of text - Sorry!

Resolution -

Very briefly, your machine and graphics card performance
along with your monitor screen size, will determine at what resolution level you can operate. Although an old standard was 640 pixels (dots) x 480 pixels (dots) on a 14" screen, many of us now are lucky enough to have a 15" monitor or larger, and a graphics capability of at least 800 pixels x 600 pixels. More likely now 1024 x 768 and upwards! I am now running 1600 x 1200 on a 21".

To gauge what this does for you - let us say that with the lowest figure, you may be hard pressed to see all of a page in your word processor, without zooming out. At 800 x 600, you will see more of it (and your desktop icons will look smaller, and take up less space), and if then you can use 1024 x 768 pixel resolution, you will have much more "space" on your screen to see everything and, feel you have "space to spare"!

As a compromise, it is best we assume (worst case scenario maybe) that many people are for now most likely to still be on 800 x 600 resolution and so construct web material to be displayed at this level. If you use 1024 x 768, then you can use your browser at a reduced size and still see it at effectively 800 x 600, without filling the whole screen. Remember too ... even on newer laptops .. 1024 x 768 is often a practical max.

It is possible of course to reset your resolution in the Windows 9x and up operating system - the video card is what ultimately determines how high you can adjust your screen resolution - and with 68 Mb and 128 Mb 3D cards more common now, higher resolutions are achievable more easily. Many earlier "Mac" users may still be working at 800 x 600 but probably much better too these days.

Depending too on your monitor's intrinsic resolution, you may have a screen ''DPI'' of 75, although some can achieve 96 ''DPI'' .... see below.


This is a way of expressing the resolution in an image, by referring to how many One inch square image at 75 dpi''dots per inch'' there are. Let's show this graphically .......... here we have a screen grab from this document .... of part of this page above - albeit with background color brightened for visibility. It was sized at exactly 1" x 1". So at 75 DPI resolution, that means there are 75 dots across and 75 dots down (total ....... 5625). If you were to place the dot from a letter ''i'' 75 times across or down ... it would be a one inch straight line! Of course, on the screen - for dots read pixels. Your own monitor resolution and screen settings will influence just how big this appears.

These figures are not so important when displaying pics on your computer (except for the effects on filesize - see below) but become very important when having pic's for printing. What can look large on a PC screen may only print out at something very small in dimension if a high DPI image.

Consider .... an image scanned at 600DPI ........ on your screen, that will be 600 pixels for every inch of picture.!! Of course, at screen res' of 75 DPI, that one inch of the pic will 'seem' like 8 inches across, when viewed at 100%.

To see graphic representations of the pixels in a photograph, look at the page on Image Basics, if you wish, and have not already done so.

Resolutions and file sizes -

The resolution of an image will affect the size of file. So also will factors like color settings and of course compression - if any. See some extra details on compression.

Suffice for now and simplicity's sake ..... let us look at two examples of image specifications and how they affect file size --

1] - At one extreme (lower) consider a 256 color (8 bit) BMP ....... of 291 x 294 pixels. Nominal size on disk (and in memory) is 85k. If we multiply the pixels ....... we get a total pixel count of 85554 .... in fact the size of the image in bytes, because each pixel information is carried by just 8 bits (one byte) ....... value between 0 to 255 (256 total states).

2] - A 16 million color BMP image (24 bit) ....... of 266 x 319 pixels. Nominal size on disk (and in memory) is 250k. If we multiply up the pixel count here, we get a total of 84854. This though is about one third the file size. Why? Because this is 24 bit (3 bytes) and so three times bigger than the last image (#1 above). Of course if this was compressed to a JPG at say, 10:1 compression, then it would store as only 25k nominal file size.

If we take this one stage further relative to the DPI matters discussed above, then if we scanned the same picture, of say 6" x 4" (a common size print) at 75 and 200 DPI resolution and compare, you may begin to see how file sizes potentially go way up as resolution increases.

Taking these dimensions, 6" x 4" = 24 square inches ......... each square inch scanned at 75 DPI will therefore contain 75x75 pixels (5625), so a total image pixel count of 135,000. For an 8 bit color set (256 color) this would be the actual max file size in bytes and size in memory when displayed. Go up to 24 bit color (16 mil colors) and we have to multiply this by three ... then a max file size of just over 400k and same in memory when displayed. This will display on the computer also as 6" x 4" but will have a fuller and smoother color rendition than 256 color.

Now - scan at 200 DPI, and we have a total pixel count of 960,000!! (200 x 200 x 24). Thus for an 8 bit (256 color) print at this size we already have a very large file ........ use that however as a 24 bit color image (16 million colors) and we again multiply by three .... giving a large 2.9Mb uncompressed file size, and size in memory.!! Of course, 10:1 compression to JPG will make that latter more bearable - at 290k. This image on the computer would, if shown in its entirety, be effectively 16" x 10.6" (because we will probably view the 200 DPI at screen 75 DPI). Usually, to see this as a full image on the screen it must be ''zoomed out'' somewhat and viewed at a lower percentage...... when not viewed at 100% real size, usually some perceived degredation can result, depending on the sizing algorithm - it is only temporary.

In all these cases we are looking to be able to print at a physical size of 6" x 4" ... but if we required an image for web use then, possibly a 500 pixel (or less) square image at 75 DPI might suffice ....... and at 24 bit this would give a max file size of 750k. JPG compression here though, of say 15:1 (as high as I go before losses start to really creep in) .. then that would reduce to a file size of 50k ........... very manageable and reasonably fast to download. On screen it would appear as a 5" square, dependent on screen resolution. Even at 1600 x 1200 96 DPI in fact - it will be very near to physical size - around 80%.

This is merely touching on some of the broader aspects of resolution ... much has been omitted and simplified but, as with most other sections here, the intention is to at least give you a ''feel' for the main principles. Sorry this section is rather ''text heavy''.

*A small addition regarding explanations of digital resolution, which is a subject that can be covered differently by various people........ here are some links that might prove some useful reading with more details and breadth.

Article - Resolution Explained: Megapixels 101

Article - About Resolution

Article - Demystifying Digital Camera Resolution

Article - Resolving Resolution

Article - What is Digital Image Resolution?

Article - Understanding Resolution and Resampling

Article - Resolution

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