There's always an aweful lot of confusion and bad advice given in relation to resolution so let me try and make this as easy to understand as possible.
For a start dpi only relates to the printers output (or scanning resolution) - nothing else. Many people make the mistake of using dpi instead of ppi (see above).
Images are made up of millions of pixels. So in relation to resolution the term should be ppi (pixels per inch).
The resolution coming out of your camera i.e 72ppi/240ppi etc., matters not a jot until you come to printing. The
only thing that matters is the
number of pixels.
My 20D produces a 3504x2336 pixel image. Whether it comes of my camera at 1ppi or 1000ppi the image is still a 3504 x 2336 image. Try converting in RAW at different ppi values and you'll see the total number of pixels stays the same but the print size falls - explained below.
Now when it comes to printing there are a couple of very easy ways to work out the unknown variable like print sizes or resolution.
1. Say you know the size of print you want (say a 10"x8") and you want to print it at 300ppi on your printer. You would need an image of 3000pixels x 2400 pixels in order to do this. If you have more pixels you can print at a higher resolution (or as many do - crop the image to the required pixel count). Quality from printing at higher than 300ppi is unlikely to be seen by the human eye so 300ppi seems to be the preferred quality for small images. To be honest you can actually print at around 240ppi without any noticable drop in quality in a small image like a 10x8 and for larger prints your resolution can drop even lower with no noticable drop in quality (at normal viewing distances).
2. Say you know the number of pixels you have (the camera default for instance) and you want to print a large 18" x 12" print. At what res will this print out at without resampling the image.
The easy formula is Resolution= pixels/print size
20D image is 3504 x 2336
so:
3504/18 = 194.666ppi and
2336/12 = 194.66ppi
Sp roughly 196ppi which contrary to popular belief, will provide a high quality image that will look fantastic from normal viewing distance.
Because viewing distance will be further than that of a 6x4, the ppi can be a lot lower for larger images. (look from a distance at a billboard then look close up and you'll see what I mean). By printing these larger images at 300ppi all you do is increase the image size (greatly) and at the normal viewing distance you will not see a difference in the print! I print my 19"x13" prints at just under 180ppi.
You can also add a third equation to work out how many pixels you need for an image. If you want a 12x8 print at 300ppi you need 12x300=3600 and 8x300=2400. So you need an image of 3600x2400 (just outside the normal range of an 8Mp camera.
You can increase the number of pixels in your image (use the resample button in photoshop's image resize menu). With resampling switched on, you can increase the pixel content of your image. I try not to resample if I can help it. Even printing a 6x4 @ 240ppi or slightly less will give perfectly acceptable prints from a decent printer.
As I say, I try not to resample too much because all you are doing is either adding information that is not there in the first place (adding pixels or upsampling) or deleting pixel information (downsampling).
There's so much misinformation on this subject around the web it's amazing how anyone gets to grips with this stuff.
Printers print in dots (
dpi) and my R2400 can print at about 5670dpi. Now no matter the pixel resolution of my image I can print it at this high setting. So my 72ppi file that is 6x4 will still print at 6x4 but my printer will print it at 5760dpi but it'll still look crap because the 72ppi is not a high enough image resolution to get a good print.
dpi is only for printing and scanning and should not be confused with images which are made up of pixels.
Does this make sense? Hope I can help those who are struggling to understand this. Your use of digital images will improve when you understand the basic concept.
Here's a pretty good link
http://www.steves-digicams.com/techc...uary_2005.html
Cheers
Jim
PM me if you have any queries as this is my first post here.
Regards
Jim