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Image Resolution - Scanning and Printing Below is a short tutorial for beginners to digital scanning and printing explaining the basics of image resolution. Many people shy away from doing the maths, but a very little arithmetic goes a long way - honestly!! It's well worth taking the trouble to understand the effect that resolution has on file size. The tutorial contains the absolute minimum of arithmetic you need to get the best out of your scanner and printer. Scanning
from a print
Why scan from a print? The disadvantage is that some of the information in the original negative or slide has already been lost at the printing process. But sometimes the negative or slide is not available - such as when you want to carry out restoration work on an old photograph - in this case scanning from a print is the only option. A flatbed scanner typically has an advertised resolution of "1200 x 2400" or "2400 x4800" pixels per inch and so on. In each case use the lower of the two figures when selecting the scanning resolution in the TWAIN dialogue box. The lower figure is the "optical resolution" of your scanner and it's generally best to scan at the optical resolution. The scanner specification will probably quote up to something like 9600 ppi or more of interpolated resolution. Interpolation is a mathematical process where the extra pixels are estimated from the values of surrounding pixels. This has the effect of slightly degrading the image and does not give any real improvement. If you do have to interpolate, use Photoshop, which will usually make a much better job of it. Now for just a
little bit of arithmetic: Suppose you have a print which
measures 5 x 7 inches and a scanner whose optical resolution
is 300
ppi. Along the 7 inch side of the print the scanner will
create 7
x 300 pixels (2100 pixels) and along the 5 inch side
there will be
5 x 300 pixels (1500 pixels). The total number of pixels
in the image will be 2100 x 1500 = 3,150,000. |
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7 x 300 = 2100 pixels along
this side |
| Take this one step further. Each pixel is stored in the computer's memory in 3 bytes (for a colour image). This means that this image requires 3 x 3,150,000 bytes of memory. This works out at 9,450,000 bytes or 9.45 Megabytes (9.45 Mb). Larger originals scanned at the same optical resolution will give proportionally larger files. Scanning slides and negatives For 35mm film you may wish to buy a dedicated film scanner. This is a small desktop module that takes in strips of film or single frames. Older models typically have an optical resolution of 2700 ppi, more recent scanners have an optical resolution of 4000 ppi and upwards. If you are aimimg for the best quality and your computer can handle large files, scan your film using the optical resolution. Bear in mind that there are other things that affect the quality of outcome of your scan as well as resolution. For example, the quality of the original - if you are scanning a fast film, there comes a point when the grain is so sharp that you can't get any more detail out of it however high your scanner resolution is. Some flatbed e.g.(A4) scanners have a strip down the middle which allows high resolution scanning of strips of film. This may be a very good compromise as it will also scan documents and prints. Some of the best as at 2008 is the Epson range of flatbeds. Let us say that you are using a scanner with optical resolution of 2700 ppi. The image size can be calculated in the same way as before: A 35 mm negative is approximately 1.4 x 0.9 inch. On the long side, the image will have 1.4 x 2700 pixels and on the short side 0.9 x 2700 pixels. i.e. 3780 x 2430 pixels. This works out at 9,185,400 pixels. Multiply by 3 bytes gives a 27.5 Mb file size(approx). It is easy to see that this image contains far more pixels (and therefore more detail) than the image scanned from the print in the previous example....27.5 megabytes as opposed to 9.45 megabytes. More detail in the image means you can make a bigger print. Print resolution Prints
are best made at a print resolution of between 150
and 300 pixels
per inch (ppi). To go much higher than this produces
little discernable improvement in quality. At much lower resolutions
than 150
ppi, the edges of objects start to look jagged as the
pixels become
large enough to be seen. If you are making a print from the flatbed scan of a print, it is unwise to enlarge it too much. For a print scanned at 300 ppi, a 2 times enlargement (printout at 150 ppi) is about the limit without interpolation, so from your 6 by 4 inch print you will get a 12 by 8 inch print. This is suitable for old family photos where the negatives are no longer available. With interpolation you may be able to go higher before the quality drops off toomuch. From film scans: the long side of your negative or slide scanned at 2700 ppi will have approximately 3900 pixels. printed out at 300 ppi, the long side of the print will be 3900 divided by 300 inches = 13 inches. Similarly at 200 ppi the long side will be 3900 divided by 200 inches = 19.5 inches. But don't worry, you don't have to do the arithmetic yourself !!! |
| Image size dialogue box in Photoshop Learn to use the image size dialogue box in Photoshop (found in Image>Image size). This will show you the image dimensions in pixels and Mb. You won't have to do the arithmetic yourself; the software will do it for you, but now that you understand how it works, you will be better able to appreciate what the numbers in the boxes mean.
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In the example above, the file size is 18.1 Megabytes and consists of 3000 pixels along the long side and 2100 along the short side. Printing at 300 dpi will therefore produce a 10 x 7 inch print. Photoshop will work out the dimensions for you in the Image Size dialogue box. Put in the width of print you want and it will automatically tell you what the height and resolution is. Try not to go below 180 pixels per inch, or your photo will start to look jagged along the edges. Below is another example of a sizing for print from the same 18.1 Mb file. This time, the resolution has been changed to 200 pixels/inch, by typing that figureinto the box, and this has automatically changed the size of the print. Notice that the pixel dimensions (size of file) stays the same in each case. Keep the "constrain proportions" box ticked to make sure the length and width of your image remain in proportion. Leave the "resample image" box un-ticked or Photoshop will change the number of pixels in your file by interpolation or by throwing away data. That's pretty
much all there is to it. |
Sizing Image in Adobe Photoshop  |
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Increasing or decreasing file size by resampling Just one last thing - if you do want to actually reduce the number of pixels in your file (or increase them) you will need to click on the "resample image" box. Photoshop will then make the necessary changes based on the print size and resolution figure which you have entered. But beware of increasing image size by too large an amount, because it will tend to result in loss of sharpness. Experience will tell you how far you can go with this. Conversely, once you have reduced file size by resampling, you can't restore the pixels which have been thrown away, so you could end up with poor quality if you need a large print. It's a good idea, therefore, to make a copy of your file before throwing away the unwanted pixels, save the smaller file under a different filename and archive the original higher resolution file in case you need a large print later. All
text and images on these pages are protected by copyright
and may not
be copied without permission. © Christine Widdall 2000-2008 |
