A Guide to Scanning-in
for
Photo-Quality Images
from
Flat-Bed & 35mm Scanners

Clive R. Haynes FRPS
Baffled?
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!
 
Resolution? What resolution?
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).
 
What 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
This should be 'photographic quality' whenever possible.

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.
Note: if you're scanning-in black & white images (as 'Monochrome' / Greyscale' as opposed to 'RGB'), this will result in smaller file sizes, however ppi (resolution) recommendations remain the same. See the link at the base of this page to 'Bit Depth & File Size' - this will explain more.

 

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).
As you will see, a figure of less than 200ppi results when the image is enlarged beyond the image size for which the scan was intended.
Take, for example, an A5 original image destined to become an A5 print at 200ppi.
At A5 we have 200ppi and should we wish to print an A6 version, well, the resolution is even better at some 280ppi. If we stretch the image to A4 the resolution falls to 150ppi and for A3, oh dear it becomes a very poor 105ppi.

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

Original
Image
Size

Scanning-in
ppi

Approx
File Size generated
in mb

Required Print Size

A6 ppi

A5 ppi

A4 ppi
A3
ppi

A6

200

2.7mb

200

140

100

75

A6

300

6mb

300

220

155

112

A6

400

11mb

400

300

210

150

A6

600

24mb

600

440

315

220

A5 140 2.7mb 200 146 104 73

A5

200

5.5mb

280

200

150

105

A5

300

12mb

420

310

220

155

A5

400

27mb

560

420

300

210

A4 100 2.7 200 147 105 75
A4 142 5.5 280 210 150 105

A4

200

11mb

400 300

200

150

A4

300

24mb

600 440

315

220


The
cells above represent 'optimum' resolution'
The
cells above represent 'poor resolution'
The
cells above represent higher than 'optimum resolution'

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
box
 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
 
7 Adjust 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)
 
Note: Dpi 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.


Printers - A frequent query and an answer:

"Okay", 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?"
Here's the answer:
The reason is that printers need to squirt clusters of tiny droplets of ink at a finer resolution than the image definition is because from a mix of three to six inkjets, the printer has to create well defined individual pixels, each of which can be one of some 16 million colours - a tall order to say the least!
The printer, therefore needs to squirt a mixture of microscopic ink droplets as a 'mingle' to produce each individual pixel colour, then move on to the next one and do it all over again, and so on, and so on, until the end of the print. The technology of printers is quite amazing! And sometimes we complain about how long it takes to perform this incredible task!
Some printers can also print at 2,880ppi, though unless your image is particularly demanding or the size is very large, the difference in actual visible quality is very marginal.

 
Click here to continue for more information - including 35mm scanning

Related Topics
File size & Resolution
Bit Depth & File Size
Scanning to Scale
Know-How Contents
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