Hi All,
If you disregard the file types and concentrate on the pixels, it should become (sort of) clearer.
Both tif and jpegs save as a resolution size, but as noted earlier, jpegs compress the information, so if continuously altered, will lose the information to some extent.
If we look at a 2.1megapixel camera as an example, at its best resolution it will save files at about 1800x1200pixels per inch, (1800x1200=2160000) and whether this is as a tiff or jpeg can be seen as irrelevant as both these dimensions are the same, whether tiff or jpeg, therefore, when working out how big it will print, it is the actual dimensions that count.
As a rule of thumb, (not quite correct, but certainly close enough for these purposes) if you assume 1 pixel is equal to 1 dot of ink, then dividing the picture resolution dimensions by the print resolution and you will have a print size, so 1800(ppi) divided by 300 (dpi) will give 6(inches) and likewise the 1200 dimension equates to 4 inches, so a file of 1800x1200 pixels per inch will print at roughly 6x4 inches at 300 dots per inch.
All you need to do is look at the file size and divide each by 300 to get the print size at 300 dpi (save the picture files to your computer, right click the file and read the info. It will give you the resolution info required)
So if a magazine asks for a picture that will print at 6x4inches at 300 dpi, you can work out how high the camera resolution should be set at.
As noted earlier, files should be sent 'as is' if they go through manipulation software such as Photoshop or Microsofts own PhotoEditor the files start to lose information.
To be honest, if you shoot at your highest Jpeg resolution you will be better off as magazines often crop pictures to remove irrelevant info, so an initially large file can be too small when cropped. Big pictures can be made smaller, but smaller ones pixelate when expanded!
I hope this makes some sort of sense!
Andy