A few scanning tips

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Printing and Scanning Resolution DPI Calculator

The full calculator is just below this first one.

First, a simple Existing Image Size Calculator

  Numbers Only.   A NaN result means an entry was Not a Number
Image Width: pixels Image Height: pixels
Image Type:
If Printed at: dpi

For reference, image sizes to fit the paper at 300 dpi are:

Aspect Ratio - a Printing Basic

Aspect Ratio is the "shape" of the image - the simple ratio of the images long side to its short side, maybe long and thin or short and wide. To print it, we can always enlarge the Size, but the image Shape needs to match the paper Shape (which is a crop). If this subject is new, see Aspect Ratio.

The calculator below specifies just one paper dimension, either the short or long side. The other dimension will float (will vary, to be whatever the image shape actually is). Ideally, you will have already chosen and cropped the image shape to match the selected paper shape properly. See Image Resize about how to handle this necessary resize and/or crop.




The Printing and Scanning dpi Calculator
Fitting an Image to the Paper Size

First - the Desired Output Print Size to be Printed

Specify either the Short or Long Dimension to be printed. This is the size the image should print, and could be a smaller area on a page, or it can be the full borderless paper size.
Select the important fixed side you want to to be exactly filled. The other dimension will float, determined by the image shape (aspect ratio).

  Short   Long    inch   mm

If this is full Paper Size, you may specify the size of an unprintable Border, equal all around.

  Border Width inch   mm
  (Omit from Print area, reduces size)

Then 2 choices
for this print size:

Calculator Help



For Existing Images (and digital camera images)
Specify size (pixels) of the Existing Image
to be printed on that selected paper size

Two ways:

 

  Width x Height pixels

Compute Printing Resolution to give Printed Size (above) from this Image Size



 
Or for Scanners, Specify Input size to be Scanned
from Photo/Film/Document
to be printed on that selected paper size

Two ways:

    (area scanned)

  Width x Height   inch   mm

Often we don't or can't scan to the edges. You may specify a Cropped Margin width to reduce the scanned area, equal all around (1.0 mm or 0.04 inch is suggested for mounted slides)

Cropped Margin inch   mm
(Omit from Scan area, reduces size)

For this Input Scan Size,   and for the Print Size above, then:
Calculate Scanning Resolution needed to Print at dpi

Calculate Printing Resolution if Scanning at dpi

The straight-forward way is to simple compute "pixels per inch" for the inches scanned, and then recomputing those pixels over the inches printed (called scaling, as mentioned in the scanning Results). We have photo editor tools to make this be easy. See Image Resize and Scaling.

However a shortcut for the same scaling concept is this:

The ratio of (scanning resolution / printing resolution) is the enlargement factor.

For example (in general - speaking of any size original):

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

Or scan film at 2700 dpi, print at 300 dpi, for 2700/300 = 9X size. If from full frame 35 mm film (roughly 0.92 x 1.41 inches), then 9X is about 8x12 inches (near A4 size). Film is typically small, requiring more scan resolution for more pixels for more print enlargement. The reason to scan at high resolution is to create enough pixels to print at about 300 pixels per inch.

This is called "scaling", and this enlargement concept is true for scanning anything, photo prints, documents, film, etc.

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Copyright © 2003-2014   by Wayne Fulton - All rights are reserved.

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