A few scanning tips

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Scanning questions:


What scanning resolution should I use? 300 dpi?

It is not possible to be specific with knowing your goal and situation. There are many situations. It is like asking "how long a rope is needed?" It depends - what will you do with the rope? Wash windows on a skyscraper? Make a handle for a wooden bucket? Purpose matters. Scanning is the same, you define your purpose, and then create an image size that is appropriate for that purpose. Any answer depends on what are you are scanning, and what size image do you need?

Then the rules are very simple: Scan resolution (pixels per inch) determines image size (pixels). Digital image size is dimensioned in pixels. Dpi means pixels per inch, meaning that if you scan 6 inches of object at 100 dpi, you will create 6 inches x 100 dpi = 600 pixels image size. Or scanning 1 inch at 600 dpi will also create 600 pixels. Or 2 inches at 300 dpi, or 3 inches at 200 dpi, or 12 inches at 50 dpi, or 1/2 inch at 1200 dpi, all of these also create an image with a 600 pixel dimension. Just plug in any appropriate numbers, what you have, and what you want.

So planning depends on knowing what size you have to scan (inches), and knowing what size image you want from it (pixels). We simply select the appropriate scanning resolution which creates the appropriate image size to be appropriate for our goal. More resolution creates a larger image, less resolution creates a smaller image. A larger original (like a full size page) needs much less resolution than a small original (like 35 mm film). Your scanner software should compute and indicate the final image size (pixels) for your current settings (crop size and scan resolution), but of course you need to know what size image you want (pixels)?

Dpi means pixels per inch, which means that if we scan 6x4 inches at 150 dpi, then we create (6 inches x 150 dpi) x (4 inches x 150 dpi) = 900 x 600 pixels. But if we are scanning 3x2 inches, then 300 dpi creates the same 900 x 600 pixels.

Video screens are dimensioned in pixels too. To show the image on a 1024x768 pixel video screen, we need an image that is no larger than the 1024x768 pixel screen size (video screens vary in size, they are not all the same size). The viewing program's window will be even smaller than the full screen. Most images that are appropriately sized for printing will be too huge for the video screen. See section one and following pages about video resolution.

If the purpose is about printing, perhaps the goal is to print 8x10 inches at 300 dpi. Then to do that, we need an image size of (8 inches x 300 dpi) x (10 inches x 300 dpi) = 2400 x 3000 pixels. If scanning an 8x10 original, then 300 dpi will create 2400x3000 pixels. But if scanning a 4x5 inch original, then it takes 600 dpi to create 2400x3000 pixels. However if we only intend to print 6x4 inches at 300 dpi, then 1800x1200 pixels is the right goal. You must decide what you want, but the answer is entirely about pixels per inch (often called dpi).

Said a different way, for printing, the ratio (scanning resolution / printing resolution) is the enlargement factor. Scan at 300 dpi, print at 300 dpi, giving 300/300 = 100% size, or original size when printed. To print at double size, scan at double the printing resolution. For example,

Scan at 600 dpi, print at 300 dpi, for 600/300 = 2X size (double size or 200% size)
Scan at 300 dpi, print at 300 dpi, for 300/300 = 1X size (original size or 100% size)
Scan at 150 dpi, print at 300 dpi, for 150/300 = 1/2X size (half size or 50% size)

Or scan film at 2700 dpi, print at 300 dpi, for 2700/300 = 9X size. 9X is about 8x12 inches (about A4 size) from full frame 35 mm (about 0.9 x 1.4 inches). The ratio of (scanning resolution / printing resolution) is the enlargement factor. This is named scaling, and it is how we print different sizes.

Small film needs much enlargement, therefore small film needs higher scan resolution to enlarge it more. There is more about this in the next section below, about printing, and for more, see section two and following pages about printing resolution. There is also a scanning calculator that may help to show this concept.

So, the answer depends on the size of what you are scanning (inches) and the size of image that you want (pixels) for your purpose. Scanning dpi gives the "pixels per inch" of scanned object which determines the final image size (pixels).

My scanned images are too huge?

If your images are too huge for your purpose, then the solution is very simple - you are scanning at excessively high resolution. You just need to scan at a lower resolution value, at an appropriate value for the goal. You should not scan at 2400 dpi just because your scanner can do it, same idea as not driving to the grocery store at 120 MPH just because your car can do it. There are rules. <grin>   Instead just use the basic principles to properly scan to create the size of image that you actually need and want. Or, another choice is that you can resample a too-large existing image to be smaller, to have smaller dimensions in pixels.

So what image size do I need?

It depends on the goal, the purpose for the scanned image.

If the goal is showing the image on the computer video screen:

Try scanning at 75 to 100 dpi for images to be displayed on the video screen, for example images for email or web pages. This "depends" of course, on the size of the photo or page being scanned, and on what size image you WANT to create. Your specific situation may be better size at 50 dpi or 200 dpi, but 75 to 100 dpi is slightly special, only because it turns out that images scanned at 75 to 100 dpi will view at near original size on most computer video screens (very roughly speaking, a ballpark number - NOT exact size). It can only be very rough approximate original size, because screens vary in size.

You must realize that your monitor likely sees things at a different size than your friend's monitor sees it - we don't all see the same size. There is no concept of exact size on the screen, because screens simply differ in size themselves. For an image on our web page, we have no clue what size monitor will view it, so we can only plan for the smaller side of average, like a 800x600 pixel screen. Many people will have that smaller screen size, regardless of what you have. So when creating your own web pages, it is a mistake to assume everyone has the same size screen that you have. However, scanning at 75 or 100 dpi will likely be viewed in the rough ballpark of original document size (closer than say 300 dpi) on most common screen sizes.

On the video screen, higher resolution creates a larger image, and lower resolution creates smaller. And you may well want it larger too. So simply choose the scan resolution value that creates the size of image that you want to see, from the size of object you have to scan. This is very easy to plan, for example, if you scan a 6x4 inch photo at 100 dpi, then you will necessarily create an image size of:

   (6 inches x 100 dpi) x (4 inches x 100 dpi) = 600 x 400 pixels.

Dpi means "pixels per inch" - when scanning, it determines the number of pixels that are created from the inches of the object being scanned. You don't have to do any arithmetic, your scanner software does it and shows in advance the size of the image that it is about to create.

Digital images and video screens are both dimensioned in pixels (not inches). The meaning is that this 600x400 pixel image will fill 600x400 pixels of the computer video screen, which perhaps might be 1024x768 pixels in size itself.

See the section on video resolution (and following pages there) for how this works.

If the goal is printing the image on paper:

Printing and paper are very different concepts than video screens. The routine quick and easy one answer, generally acceptable, is to scan at 300 dpi for images that are to be printed at original size.

If you want to print the image to be 6x4 inches at 300 dpi on paper, then the requirements for the image are:
   (6 inches x 300 dpi) x (4 inches x 300 dpi) = 1800x1200 pixels.
This image is larger than most video screens. Printing typically requires a larger image (more pixels) than does the video screen.

A shortcut:    If you want to enlarge the printed copy,
the ratio of (scanning resolution / printing resolution) is the enlargement factor.   For example:

Scan at 600 dpi, print at 300 dpi, for 600/300 = 2X size (double size or 200% size)

Scan at 300 dpi, print at 300 dpi, for 300/300 = 1X size (original size or 100% size)

Scan at 150 dpi, print at 300 dpi, for 150/300 = 1/2X size (half size or 50% size)

Film is typically small, and needs much enlargement, and thus small film needs higher scanning resolution. So maybe scan film at 2700 dpi, print at 300 dpi, for 2700/300 = 9X size. From full frame 35 mm film (roughly about 0.9 x 1.4 inches), 9X is about 8x12 inches (about A4 size). To emphasize the concept, a 4000 dpi film scanner would allow 300 dpi printing at 4000/300 = 13.3 times the size of the film frame (about 18x12 inches from 35 mm film). Or if printed at 200 dpi, then 4000/200 = 20x original size (28x18 inches from 35 mm film). The purpose of high resolution is for greater image enlargement.

You can scale an existing image to print it at any desired size, by printing the exact same pixels, but by varying the printing resolution dpi value. Printing resolution is just a separate number in the image file, not actually part of the image content, but is simply a number used for the declaration of how these same pixels are to spaced on paper, at so many pixels per inch. From the same pixels, higher printing resolution scales to print a smaller image on paper (spacing the pixels closer), and lower printing resolution scales to print a larger image on paper (spacing the pixels wider). The ratio of (scanning resolution / printing resolution) is the enlargement factor.

The image dimension in pixels is all important. You should plan things to have sufficient pixels to be able to print the desired size at about 300 dpi on paper. You can print at any resolution value, but around 300 dpi is special, a very good ballpark value to print photo-quality. In that ballpark, but the exact value is not important... 257 dpi or 313 dpi are perfectly fine values too, if the situation works out that way. But 300 dpi is a fine target value.

This image resizing with different printing dpi values is called "scaling". The concept of scaling is about the spacing of pixels, but the key fact of scaling is that the pixels are not changed or affected at all (which is a very different concept from resampling). This enlargement concept by scaling applies for scanning and printing anything, photo prints, documents, film, etc.

See the section about Scaling and Printing Resolution (and following pages there) for how this works.

And the section about Printer Resolution (and following pages there).

How can I make my image files smaller?

Image "size" has two meanings - two very different definitions: Image size in pixels, dimensioned like 1200x800 pixels, or image file size in bytes, like 3 MB. See just above about pixel dimensions, and this part is about file size.

Image dimensions do affect file size in bytes, how many pixels there are. If for example, the image dimensions are 1024x768 pixels, then there are 1024x768 = 786,432 pixels. If it is a regular 24 bit RGB color image, then its size in bytes of memory is always exactly 3 bytes of RGB color data per pixel, or 1024 pixels x 768 pixels x 3 bytes per pixel = 2,359,296 bytes. This size is always valid in memory, when the image is uncompressed and open in memory, but data compression methods may make the file size be smaller than this.

Data compression affects file size. A JPG file uses the most efficient compression (for smallest file size), so that JPG is possibly the answer you seek, but it is not necessarily a good answer... JPG compression is lossy, meaning there are resulting high costs in image quality (see about JPG files). A TIF file with LZW compression is less efficient compression by far, but LZW is lossless compression for perfect quality. TIF is a much larger file, closer to the size the data actually is, but image quality is not an issue (see about TIF files).

Image scan mode affects file size:

3 bytes per pixel for 24 bit RGB color mode (16.7 million colors),
1 byte per pixel for 8 bit 256 shade grayscale mode,
and 1 bit (8 bits per byte) for line art mode
(more details about size of scan modes).

Scan mode affects file size, but mode is mostly about purpose.

Using the correct mode for the purpose works wonders. Scanning a black and white page of text in color mode is counter-productive, there is no color to be captured, but the size of color mode is still 3 bytes per pixel. For text documents, line art images are 1/8 the size of grayscale and 1/24 the size of color, and line art compresses really well (see line art). Line art is perfect for text, however of course line art mode will not reproduce photo images, for example magazine pages with photos (but it may not always matter). And you might sometimes want to use grayscale instead of line art on web pages, because the aliasing of grayscale helps minimize jaggies at the low screen resolutions.

There is no one answer for this one. Lower scan resolution creates a smaller image (smaller image dimension, in pixels), which is probably the best answer if possibly your images are currently excessive size for the purpose. But quality images require pixels and pixels make the file size large... However scanning at the appropriate resolution for the job's purpose (instead of always using the maximum possible resolution) is the very best possible answer.

For graphics (logos, etc), GIF files with limited colors (like 16 colors when applicable) is the smallest file and absolutely ideal, at least for video uses, like web pages. Graphics normally use very few colors (see GIF and indexed color).

GIF format allows only 256 colors and so is not desirable for photographs. JPG files are very small, wonderful for web or email purposes, but JPG compression causes quality losses that are not appropriate for your one archived master copy. Check the File Type Basics Section about file types and compression.

I get odd patterns in the images I scan?

You may just need to clean the scanner's glass.

If you're scanning printed material (books, magazines, newspapers, postcards, calendars, etc) instead of real genuine original photographs, then that's Moiré patterns, and it is expected.   Magazine images are not at all the same thing as a real original photograph.   See the Descreen Basics Section for more, but the first rule is to scan at least at 300 dpi, and then resample smaller if necessary.

Photos with a rough paper surface

If you are scanning photographs on paper that has a distinct surface texture, then you are seeing the texture, and reflections of the scanner lamp from the irregularities of that surface.   I really don't have a solution, it's a terrible problem.   If you can see the texture, the scanner can see it better.   You want to scan glossy photos, but of course, that is not a choice for this photo.   To reduce the reflection, you need to be able to control the angle of the light (like you can do with a camera), but the scanner does not allow that. Even a fine matte finish is somewhat detrimental to scans.  

If your SLR camera has a macro lens for closeups, the best solution is to photograph the original print, and then scan the copy. This allows you to control the direction and angle and diffusion of the lighting to reduce the reflections. However larger work may require more pixels than the camera can give.

The document is larger than the scanner bed?

There are a few A3 12x17 inch flatbed scanners, but they are quite expensive. Otherwise, you can scan two or more partial images, and join them. The manual way is to open the images in a photo program, and enlarge the canvas size (meaning background frame) of one image large enough to hold all images, and then copy them in. Layers are easier to align than Copy-Paste operations (layers float independently, more tries at alignment). Manually adjust the positions to align pixels.

Joining images this way is called stitching. Ulead PhotoImpact and Corel Photo-Paint and Micrografx Picture Publisher 8 have menus EDIT - STITCH, which are specific tools for this purpose. See their STITCH Help menu. Photoshop Elements has File - Photo Merge (which is more manual). Several panoramic photo programs also do this, search www.google.com for panoramic.

Turn off all automatic contrast settings when scanning the parts. Adjust the contrast of the first using manual settings, and leave it the same for the second. Then the contrast results from one scan to the next will stay the same and won't auto-adjust differently based on content. Any additional correction of this type must be done in the photo editor program. Scan the document sections with a slight overlap.

Keeping things straight to eliminate skew is very important, measuring and marking the original helps. The Stitch menu usually has edge feathering options, but a quick and easy way to hide a visible seam is to simply use the Clone Tool to blend and blur the seam. Plan ahead to keep the seam out of the main detailed area of the image. Choose your touch up area in advance.

Suppose you scan a 12x12 inch LP album cover as two 6x12 strips (assuming a 14 inch bed), to print the total as 5x5 inches at 240 ppi for a CD cover. A shorter bed means four parts, or maybe 11.7 inches is enough to include? Scaling from (5/12 size x 240 ppi) = 100 ppi would do it, but for descreen of printed material, I'd scan at higher resolution, and then resample to 5 inches at 240 ppi.

The ends of fluorescent lamps darken with age, which can darken the image at the edges of the bed. This is normally difficult to notice, but stitching a lighter central section to a darker edge section can be visible. To work around this, place the document edges next to the edge of the scanner glass, one side to the left and one to the right, to keep the stitched center seam closer to the center of the scanner bed. The "cold cathode" scanner lamps used today are not user replaceable.

Adding images to MS Word documents?

In Word, you insert image files with the menu INSERT - PICTURE - FROM FILE. Those images are inserted at the cursor point, and you can also use the clipboard PASTE. You can move them to reposition, although Word will often fight vigorously about this if the images are large. Persevere, you can win.

The main concern is about printing resolution and image size. The ordinary rules of scaling apply, the same as is fully discussed in the Printing Basics section.

If you will fax this document by "printing" to a software fax driver, then scan at 200 dpi in Line art mode (see Fax section).

If the image is Line art and you are going to print it, then you can scan at 600 dpi for your printer, altho 300 dpi will be plenty to look good. If you will reduce or increase the image size, then scale accordingly to end up at 600 dpi at the new size.

If Grayscale or Color, and if your image will be printed on a 600 dpi laser, then you want images scaled to about 150 dpi (although 120 dpi will often be as good). And about the same 150 dpi for an inkjet if printing this Word document on plain paper, and up to about 180 dpi if printing on good coated paper.

Scaling is the normal procedure used to print any image. If you want your image to print at 3 inches size, and if you want it to print at 150 dpi, then you scan to obtain an image of size 3 x 150 = 450 pixels. Size this 450 pixel image in Word to be 3 inches, and you will have 150 dpi on paper.

Most file formats are supported. GIF files do not store the image scaled resolution in the file, so Word will not know the image size, and will rescale the image to be either 96 dpi or 120 dpi, depending only if you have Small Fonts or Large Fonts selected in your Windows video setup. Also, Word 97 seems to have a bug for TIF files, and these too are unnecessarily rescaled to 96 or 120 dpi (earlier or later versions handle TIF OK). Just resize them back to the size you want. Do the dpi arithmetic yourself.

The important thing is to keep up with the details yourself. If you know that you want a 3 inch image printed, and you know that you want it to be 150 dpi on paper, then it is YOUR responsibility to provide the 450 pixel image for your document. It might be only 400 pixels or it might be 500 pixels, and that might be OK, but you need to get it into the ballpark. You can do the arithmetic to understand the repercussion of this. 400 pixels / 3 inches = 133 dpi. It might be OK, especially if on plain paper.

If your image is not the correct size for your goal, and if you cannot rescan it (an image captured from a video screen say), then DO NOT resample the image to make it be the right size. That serves no good purpose, and if your image contains text (like a screen dialog box), that text will instantly become unreadable when you resample it smaller.

Instead, for printing, always SCALE the image to the right size in Word, the normal procedure for printed images. Scaling keeps all of the image pixels intact as in the original, and the sharpness will be retained, especially the text which suffers most from losses. I can only show this example in the printed book version, but scaling to a half-size image DOUBLES the resolution of text, while resampling to a half size image reduces resolution to HALF. See the Printing Basics section.


Copyright © 1997-2010 by Wayne Fulton - All rights are reserved.


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