So, you just got that brand new scanner hooked up and you're anxious to get started... wait! There's a few things you need to take into consideration before you do. Will you be printing the image, putting it on a Web page, or sharing them with friends via e-mail? Is there a chance you might want to edit the image at a later time?
By answering a few questions before you even switch on the scanner, you could save yourself a lot of frustration. The following pages contain basic suggestions and guidelines for anyone who's just getting started with scanning.
When scanning for screen display you need only be concerned with pixel dimensions. A computer monitor displays approximately 96 pixels per inch. For simplicity's sake, I prefer to round this off to 100 ppi. So, if I want an image to appear approximately 1 1/2 by 2 inches on my web page, I would scan it at 150 by 200 pixels.
When scanning for screen display you need only be concerned with pixel dimensions. A computer monitor displays approximately 96 pixels per inch. For simplicity's sake, I prefer to round this off to 100 ppi. So, if I want an image to appear approximately 1 1/2 by 2 inches on my web page, I would scan it at 150 by 200 pixels.
Each scanning interface is slightly different, but they all should have a resolution setting you can adjust. This will be labeled as either ppi or dpi. You'll want to set the resolution to 96 or 100 and set the dimension units to pixels. Then you can adjust the size percentage values to get the image to the desired pixel dimensions. Whether you adjust the dpi or the percentage does not matter, as long as the values you take into consideration are the output pixel dimensions.
Below is a partial screen capture of my scanner's interface. As you can see, I've set the scanner resolution to 96 dpi, and I've changed the dimensions to pixels. In this example, my image on the screen is going to be approximately 5 inches wide and 3 inches high because these are the values displayed for the final output (the grayed-out values).
Now let's assume I want the image height and width to be displayed at half that size. I can adjust the percentage value to 50%. As you can see below, the input value remains the same, but the output values (the grayed-out values) are 250 by 150, approximately 2 1/2 by 1 1/2 inches, half the dimensions of the image above. 
Incidentally, you'll notice that the image file size is reduced to 1/4 of the original... from 439 KB to 110 KB in this example. This is because the total area of the image -- or the total number of pixels in the image -- has been reduced by one-fourth. As you can see, reducing the dimensions of an image can dramatically impact its file size and download time. For this reason, it's important that images you plan to transmit via email and the Web be cropped and sized appropriately.
To review:
Saving for the Web or Email
For images going on the Web, you'll want to go with either GIF or JPEG. And because these formats can be viewed by anyone with a Web browser, they are also the best choice for sending images via email. The general rule is to use JPEG for photographic images, and GIF for images that have text, sharp lines, or large areas of solid colors. It's important to strike a balance between the smallest possible image size and highest possible image quality, so you may want to do some additional reading on GIF and JPEG and the best ways to optimize each.
If you plan to do any editing of the image prior to putting it on the web, you'll want to save it to an uncompressed format while you're working on your image and only save it to GIF or JPEG as the final step before putting it on the Web. GIF images are limited to a palette of no more than 256, and JPEG images lose quality each time you edit and re-save the image.
When sending images by email, never send a non-standard format, unless you know the recipient has software that can read that format.
Saving for Later Editing
For images you want to edit, it's best to save as either TIFF, PNG, or BMP (bitmap). Saving as a TIFF or PNG will result in a smaller file size because they use a compression scheme that does not result in a loss of image quality.
If you're sending the image over the Internet to someone else for further editing, PNG is probably the best choice since you'll get the smaller image size, and they'll be likely to have an imaging program that supports that format. BMP files tend to be quite large in file size, and I don't recommend using it unless you're sure your recipient does not have an image viewer other than the Paint program that comes with Windows. TIFF files can often be compressed quite small for sending via e-mail attachment by compressing them into a zip file.
Saving for Archival
For photographs that you've completed the editing process and you want to archive, you'll probably want to save as JPEG to conserve disk space. If you think that there is any chance that you might someday want to perform additional editing to the images, save them at the highest quality setting for JPEG, and use the same quality setting each consecutive time you edit the image. If you have the storage space, an even better solution would be to archive the images in a lossless compressed format such as those mentioned in the section above.
It is also highly recommended that you save a second copy of your working files in the native file format of your image editing software. Many people also find it useful to archive a copy of the original, unedited scan or photo.
Saving for Print
Scanning for print is a topic unto itself and you can read more about that in the next section, Printing Digital Photos and Scans. However, the information presented here regarding file formats is just as applicable to images intended for print.
To review:
The information here pertains to both color and grayscale images that will be printed on an inkjet printer. It does not apply to printing line art images. For more discussion on printing line art images see Scanning Line Art by Desktop Publishing Guide Jacci Howard Bear.
PPI vs. DPI
The first important point to understand is the differences between PPI (pixels per inch) and DPI (dots per inch). Many software programs and scanner interfaces use these two terms interchangeably but that's not exactly accurate. As a general rule, and for the purposes of this article, the term PPI should be used when referring to image resolution, and the term DPI should be used when referring to printing resolution. How can you remember this? Monitors display pixels, and printers produce dots. So, as you follow this discussion, whether your software calls it DPI or PPI, when I talk about the resolution of your image, I will be using the terminology PPI.
Pixel Resolution
Digital photos and scans are all bitmap graphic types, no matter what format the image is saved to. That means they are made up of a grid of individual, tiny pixels with each pixel representing a single color in the image. If you zoom in on one of these images (see example) in your imaging software you'll be able to see the individual squares of color.
What this means in relation to printing is that these types of images are resolution-dependent. In other words, the quality of the print and the size of the printed image is limited by the number of pixels in the image. You can't increase one value without effectively decreasing the other.
Inkjet Printer DPI
Today's inkjet printers have three standard output settings:
normal: 300 x 300 or 320 x 320 dpi
high quality: 600 x 600 or 720 x 720 dpi, 1440 x 720
photo quality: 1200 x 1200, 1440 x 1440 dpi, 2880 x 1440 and up
You might also have a draft or economy setting, but you should never use this setting for printing images. It's primarily used for printing text and rough drafts.
A popular myth is that it's necessary to scan an image at the same resolution that you will be using to print. In the case of color and grayscale images, this only results in excessive file sizes. At one time, a good general rule for inkjet printing was that you needed half to one-third of the PPI of the printer's DPI setting that you intend to use. So if you're using your printer's "normal" setting (300 dpi), your image needed to have at least 150 ppi. When using the higher quality printer settings (720 dpi and up), you can bring the PPI down to about 1/3 of the output resolution. That would be 240 ppi for your printer's 720 dpi setting.
Since writing this article photo printers have gotten much better--the dots are smaller and more compact--but still, you rarely need your image resolution to be higher than 240-300 ppi for inkjet printing.
Calculating Image Size and PPI
So how do you calculate the image size you need? It's simpler than you might think. First, decide what size you want your printed image to be, then multiply the height and width in inches by the PPI you need for the print resolution you'll be using. Here's a reference chart for some common print sizes:
Printer Quality | |||
Draft/Economy | Standard | High/Photo | |
Print @ | 300 - 320 dpi | 600 - 720 dpi | 1200 - 2880+ dpi |
Scan @ | 150 ppi | 150-240 ppi | 240-360 ppi |
Printed Size | Actual Pixel Dimensions (Average) | ||
2" x 3" | 300 x 450 pixels | 400 x 600 pixels | 600 x 900 pixels |
4" x 6" | 600 x 900 pixels | 800 x 1200 pixels | 1200 x 1800 pixels |
5" x 7" | 750 x 1050 pixels | 1000 x 1400 pixels | 1500 x 2100 pixels |
8" x 10" | 1200 x 1500 pixels | 1600 x 2000 pixels | 2400 x 3000 pixels |
Generally, printing photographic images requires more pixels per inch. For photo printing, you will get better results using the higher end of the scale. For scanned illustrations, cartoons, or paintings, you can use a lower PPI and not see a noticeable change in the printed quality.
Why is my scan so HUGE?
Keep in mind that when you scan an image at anything higher than 72-100 ppi, it's going to display much larger on your monitor than the original picture or page. That's because your monitor can only display a fixed number of pixels per inch, usually 96. When you place the image into a page layout program, most software will be able to read the PPI information and will place your image into the page at the proper size. Some of the low-end printing software may not, however, and you'll have a very large image placed into your page. When that happens, you'll have to size the image in your layout program to the printed size you based your calculations on.
But I don't know what the output will be!
Whenever you increase or decrease the pixel dimensions of an image you lose some quality, but it's less destructive to reduce pixel dimensions rather than increase pixel dimensions. If you're scanning for archival, or you don't know what the intended output will be, it's better to scan at a higher resolution and reduce the image later. However, it is of no advantage to scan higher than your scanner's maximum optical resolution.
PPI and Digital Cameras
When you're printing images from a digital camera, you have to calculate things a little differently. With a digital camera, you have a fixed number of pixels which is the maximum your camera is able to capture. Today's lower-priced cameras have an average maximum resolution of 1200 x 1600 pixels (2 Megapixels). In this case, we have the pixel dimensions, so we need to work in reverse to figure out the best quality setting and printed size.
Using the formula above we can calculate that with 1200 x 1600 pixels we can either get a normal quality print at 8 by 10 inches, a high quality print at 5 by 7 inches, or a photo quality print at 4 by 5 inches maximum. That's not a very big image for a photo-quality print, so if you thought you'd be producing photo-quality 8 by 10 prints with your $200 digital camera and inkjet printer, you're probably going to be somewhat disappointed. As you can see, there is a choice to be made as to whether you want to sacrifice printed size or image quality. If you plan to print many 8 by 10 prints, you should have at least a 3 megapixel camera, although I have gotten acceptable results printing 8 by 10s from a 2 megapixel original on a new photo-printer.
To review:PPI refers to image resolution
MGM
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