A Guide to Scanning-in
Clive R. Haynes FRPS
This topic, together with the issue of 'File Size and Resolution', is frequently one of the most problematical areas for newcomers to digital imaging to get to grips with.
I'm very aware of this 'bafflement' from the number of questions asked by my students when starting a Photoshop course and during workshops / demonstrations.
This page, together with the related subjects at the base, is intended to make this subject as understandable as possible. It's actually very logical and when it all 'clicks into place', you'll wonder why you were so baffled in the first place!
A note about photo-quality resolution:
Firstly we must bear in mind that we should view prints at a 'normal viewing distance' (this excludes a minority of individuals who insist upon looking at the 'mechanical structure' of the image as opposed to its 'picture content' - some camera club judges fall into this category I regret).
is 'Correct Resolution?'
At 'normal viewing distance', the human eye is capable of resolving separate details about 1 minute of arc apart and this translates to around 180 lines per inch - which from our point of view can further be translated into 180 pixels per inch.
To be on the safe side, and to give a little leeway, we can raise the pixels per inch to 200 or 220ppi. Resolutions in excess of this will not result in a sharper image but could offer the advantage of a greater enlargement, should this be required. Bear in mind that the finer the resolution, the greater the ppi and the larger the file size will become.
The aim in view
To reduce some of the confusion when
it comes to making decisions about scanning-in, the following information
and the chart below will provide a guide.
I have taken as a reference point the
necessity to produce an output image ready for printing of at least 200ppi
as this is considered to be acceptable minimum at normal viewing distance.
(See below - 'What is Correct Resolution' for an explanation).
Remember, reducing image size (shrinking the picture) will increase the number of pixels per inch (ppi) conversely, enlarging the image size will result in fewer ppi (and the poorer the quality).
Note: Once the scanned image is imported into Photoshop, all re-sizing of the image should be done with 'Resampling' turned off unless one isn't worried about either corrupting the image or increasing the file-size. Generally speaking, 'Resampling' is not recommended.
|The 'worked examples' below are taken directly from my own flat-bed scanner|
Flat-Bed Scans - a Guide
ppi / file size @ different image sizes
Remember, for Photo-quality images - choose a ppi for the size you require of 200ppi or a little more
Using the chart to estimate the scanning-in ppi and corresponding file size
|1 Decide upon the final print size (A6, A5, A4, A3)|
|2 Look at the original image size you are to scan (left column) - choose the nearest to either A6, A5 or A4|
|3 Move to the right and find the||
|with 200 ppi that coincides with your final print size|
|4 Note the File size in Mb, we're using it later (step 7)|
|5 Note the Scanning-in ppi in the||column|
|6 'Open' the scanner in 'Pre Scan' / 'Preview' mode|
the scanner parameters to the file size required - noted at step 4.
Generally speaking this means adjusting either the ppi 'slider' or the 'Scale' slider or a combination of both.
It should not be necessary (in fact it may cause confusion) to make any adjustment to the 'Input' or 'Output' dimensions themselves.
If difficult (some scanners will not let you choose precise file sizes), choose the nearest next file size up - better to have too much than too little.
The object of the exercise is to generate the appropriate number of pixels for the image - that is shown as 'File Size' / Mb. So long as we have enough (for photographic reproduction), Photoshop will be able to interpret the amount and distribute it to suite the image size.
Once you have grasped the fundamental concept that the all-important relationship of 'File Size' / Mb then it all becomes a lot clearer and easier to understand.
|8 Scan, and you should have a file size appropriate for the final image size you require with sufficient pixels available to enlarge to that size - that is no less than 200 ppi at the final image size.|
|9 To check
that you have sufficient pixels (ppi) available in your image ready for
your final image size, do this:
After scanning-in, go to Image > Image Size and with resample OFF, change the width or height to whichever is the greater dimension for the final image and note the ppi in the resolution box - the number should be 200ppi or a little more. (If the reading is pixels/cm, click on the drop-down arrow and swap to pixels/inch)
and ppi are somewhat interchangeable terms.
Strictly speaking 'ppi' (that is, Pixels Per Inch) is best applied to the electronic generation of tiny, square, individual picture elements or 'Pixels' ('Picture-Cells' if you like). Whilst 'dpi (that is, Dots Per Inch) is best applied to the tiny dots of ink that the printer uses to create the image on paper.
I have used ppi (pixels per inch) for clarity. Dpi can be confused with printer specification and is not an issue here - except to say that for photo-quality, the printer should be set to at least 1440dpi.
some people say, "why then, if all I need for photo-quality images
is around 200ppi or a little more, do I need to buy a printer that gives
1,440 or 2,880dpi?"