For fax, scan the document at 200 dpi Line art mode (with the Scaling control reset to 100%). This is true for either Normal or High resolution mode. Then simply "print" that image from the File - Print menu of any photo program. Select your fax driver as the printer, and it will ask for phone number.
Fax uses line art mode, it is NOT optional. If you don't use line art, the fax driver will convert it sight unseen to line art. You may prefer to see this before you send it. Read the previous section about Threshold in line art mode. Threshold is very important to scanning for fax.
We say "200 dpi" for fax, and 200 dpi is the right number to use, but technically, NORMAL fax resolution is 203x98 dpi, and HIGH fax resolution is 203x196 dpi. I suspect 203 dpi comes from 8 dots per mm today. Fax is a very old specification, primitive almost, surely our oldest, and goes back to when the facsimile paper was attached to a rotating cylinder and marked with a vibrating ink pen.
You should scan at 200 dpi line art mode for fax. Fax drivers vary, and some will resample other values (and some won't), but 200 dpi is as good as any, and better than most. WinFax Pro accepts only 200 dpi. For MS Fax, I don't detect any visible difference scanning at 196, 200, or 203 dpi, nor at 250 or 300 dpi either, but not all fax drivers work alike. 200 dpi is always correct. If the fax page size is not coming out correct, be sure to scan at 200 dpi. Even if it is, you might as well scan at 200 dpi. However, you certainly can intentionally scale the image before printing to the fax driver. Scanning a small page at 400 dpi and scaling to 200 dpi before printing will double its size, as per normal scaling rules.
The difference in Normal and High fax resolution is the way the fax machine steps its paper motor. At High resolution, motor steps of 196 lines per inch are used for each scan line, and every line is sent. For Normal resolution, double sized vertical steps of 98 steps per inch are used, meaning Normal mode simply sends every other scan line, and omits remaining lines. Normal resolution requires larger text to be readable. High resolution therefore takes twice the Normal telephone time to transmit, because all scan lines are sent, twice as many.
Any area of blank paper on the page is also transmitted as real data (unless it is a short page), as a full count of scan lines of all white pixels. But solid color areas of all white or all black pixels compress extremely well, solid color areas are a very small cost in fax speed. It is the "detail" (the changes between white and black, like text) that adds bytes and takes time.
For Normal fax mode, 203x98 dpi, we should still scan at 200 dpi, because it is still 200 dpi horizontally. It is line art mode, so the memory requirements are relatively small, no more than 1/2 MB per page, and TIF G4 can often compress common data to about 50 or 100 KB.
Fax machines are NOT guaranteed to have High Resolution reception mode (although most do), so test your fax procedure for readability at Normal fax resolution, even if you intend to send High resolution. If we specify High resolution, the fax modem will send the best mode the other guy can handle, but that may not be High resolution. Not all fax machines are plain paper machines either. The quality you see at your end may not be what the receiver sees. It is wise to always set your word processor to use at least a 12 point font when faxing if you really want it to be read without error. Large fonts are not the prettiest pages, but you CANNOT force High fax resolution, the receiver does what it is able to do. Always consider increasing your font size for fax, it is good practice.
Text is really better faxed as text, printed from a word processor directly to the fax driver, instead of printing to paper and then scanning the graphics image from paper. Because if you print and then scan paper, then due to random paper alignment on the scanner bed, the printed dots are not precisely aligned with the scanner cells sensing them. Two scanner cells might often straddle one printed dot for example. But if you instead print the text file directly to the fax driver, then the fax driver creates the image. The fax driver can create every dot in every text character in exact alignment with the fax grid, so that every dot is in the precise location at the receiving fax machine. This comes out much better and sharper than if scanning a printed page. The text characters are thin perhaps, because dots don't straddle two pixels, but are sharp and clear.
But scanning and faxing an existing paper document is sometimes necessary too, and your scanner will do this as well as any fax machine. Maybe better, you can edit the image a little before sending. It is a little more work however.
You can fax a single page fax from the image program you scan from. Scan in line art mode at 200 dpi, and then "Print" or "Send" from the image program to the fax driver (like WinFax Pro).
You can simply print single page images to the fax software from any image program. Multiple pages are a bit harder. One very tedious way to fax multiple pages of scanned images in a fax transmission is to use an "image capable" word processor like MS Word or WordPerfect to combine the images into one document. Scan each fax page at line art 200 dpi. Save each page to a separate file, typically in TIF file format. Then insert each page into the documents at the cursor. In MS Word, use menu INSERT - PICTURE - FILE, then menu FORMAT - PICTURE to resize or arrange it if necessary. Or alternately, you can use the clipboard to copy each scanned image from a image program to Paste into the word processor. The word processor allows you to add additional associated text to the document of course. Word processors each have their own quirks, but the Word print driver should automatically add page breaks as needed to prevent if possible any image from being continued across a page border, that is, the scanned pages should arrive as whole fax pages. Always check the Print Preview to be sure it is as expected. But there are far better ways than a word processor for multi-page images (as images).
Most modems come with free fax software, and more fully featured fax software can be purchased. Most such fax software will support multiple pages. Fax software will allow you to include multiple pages from disk files.
For scanning multi-page files to fax, you need to be able to create multiple page document files, for "printing" to your fax software. PaperPort or Acrobat are two such utilities that will scan and create multiple page files .
Remember that you do need to test your fax procedure by faxing to a friends fax machine. If you're faxing something important, like your resume, it's very important to have tested your fax procedure, because you cannot otherwise see the results. Don't use tiny text characters.
We have been discussing faxing scanned images of text pages. Line art cannot reproduce photographs well of course, and there are additional requirements and complications when sending photographs via fax. Fax is 1-bit 2-color line art mode, meaning all pixels are either black or white, and there is no Gray possible. Conversion of color images to 1-bit color is a difficult problem, that is, if you want a good image.
Many scanners have a Halftone mode that will do the job directly. Or you can do it in software later too. Scan color images in Color mode and B&W photos in Grayscale mode. Set the histogram Black & White Points in the normal way. If scanned in color, convert to Grayscale.
When converting to line art, other methods like error diffusion dithering work pretty well (shown), but halftones can be better. Scan in gray or color mode at 150 dpi. Then for example, PhotoImpact menu Format - Data Type - Black & White to convert it, use 200 dpi for fax. It is VERY important to judge it on the video screen only when it is viewed at "actual" size.