At the risk of muddying rather than clarifying, can I give a little tutorial...
There are three things about image size, in terms of file size:- the number of pixels
- the number of bits per pixel
- the compression (if any)
(the size in terms of inches or cm. is of little relevance)
NB. Each pixel in an image will occupy a pixel on screen, unless scaling or zooming are in effect.
In various places, you can right-click or otherwise find properties that will show the dimensions in pixels.
Say your image is 1000x800 pixels: that's a total of 800,000pixels or 0.8Mp
If your image is in full colour, it probably uses 24 bits per pixel; and since there are 8 bits in a byte (ordinarily), that's 3bytes per pixel. (Black and white images use fewer bits per pixel, higher-spec images use more.)
So the image is 0.8 x 3 = 2.4Mb (megabytes).
CompressionIf the file is in a straightforward bitmap format, that will be the size of the file (give or take a little overhead).
But most images we deal with today that are derived from digital cameras are compressed. That requires the use of a format such as JPEG or GIF or PNG or TIFF. JPEG is the most common, and is well suited to full colour photos.
(I know I am ignoring the RAW formats used in higher-end cameras, in association with which TIFF is often used as a master format. Plenty more can be said on that, but not here.)
When saving a file in JPEG format (file extension usually .jpg), various levels of compression are possible. That allows you to save an image with a large number of pixels as a relatively small file. ...Which is good for emailing or posting on websites, or simply keeping your disk space consumption under control. But higher compression means lower quality. A compromise has to be struck. Fortunately it is possible to get very useful amount of compression while barely losing any quality. (But beware saving and re-saving an image many times, as quality will degrade - because typical JPEG uses a 'lossy' type of compression, unlike TIFF, eg..)
So, our example image, which was 2.4Mb in size, could be reduced to perhaps 0.2Mb when saved in JPEG format with moderate compression. The amount of compression achieved depends partly on the nature of the photo; some are intrinsically more compressible than others.
When wanting to make an image file smaller, consider also:- cropping (reducing the number of pixels by chopping some off the edges)
- the size of the screen the image is likely to be viewed on (eg. my (average) laptop screen is 1,280x800pixels)
- do you need full colour? (A page of black and white text or diagrams does not, eg.)
So:
- crop (eg. in Photoshop or more basic programs such as Paint or Microsoft PhotoEditor) to eliminate extraneous parts
- reduce the pixel dimensions to make the image appropriate for the display system of the person viewing the image (I could suggest limiting it to 400 pixels high/wide, as a rough guide for an image expected to be seen as just part of a web page). But if you expect the viewer to zoom-in, more pixels could be appropriate.
- reduce the bit-depth (change from colour to greyscale or to black and white). (You can usually get by without doing this, and it entails detail than I really want to go into here.)
Our example image file could now be around 50Kb in size.
How do you reduce the number of pixels?The different image editing programs have different methods for resampling. I can't really go into that here, but in Photoshop: Image --> Resize Image gets you to the right dialog. Always keep the pixel dimensions ratio fixed, otherwise you will stretch the image.
When I resample an image, I usually do
not use Photoshop. I find it much easier, certainly for web posting purposes, to use a utility accessible by right-clicking on an image file (ie. a 'shell extension' which gives access to the utility in the 'Context menu').
The one I use now is 'Image Resizer for Windows'. Go to
http://imageresizer.codeplex.com and the screenshots will give you a good idea how it works.
Some people will still be using the (perfectly good) image resizer that was available as part of the PowerToys for Windows XP; but has sadly gone from there now.
A good order to do things in:
1. decide roughly what file size you need (may depend on web site stipulations and/or your own preference)
2. check the size of the image file in Mb or Kb (maybe it is OK; it might already have been edited/downsized)
3. make a copy of the original file, work on that (the original serves as a backup, eg. in case you need to start again)
4. crop (reduce image dimensions (pixels) by removing extraneous parts of image)
5. resample (reduce dimensions (pixels) by combining adjacent pixels, using facilities within image editing program)
6. save as JPEG (compress file, moderately)
7. check the size of the file (if necessary, repeat some of the steps but crop or resample more)
(resampling, alternatively, can be done after saving, using right-click utility such as Image Resizer)I hope that helps!