A GUIDE TO SCANNING-IN

FOR

PHOTO-QUALITY IMAGES

Clive R. Haynes FRPS

Our aim should be 'photographic quality' whenever possible.

To reduce some of the confusion when it comes to making decisions about scanning-in, the following information will provide a guide. Note: if scanning-in black & white images, this will result in smaller file sizes, however ppi recommendations remain the same.

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. As you will see, quality of less than 200ppi frequently will result when the image is enlarged too much.

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.

Once in 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.

FLAT-BED SCANS

ppi / file size @ different image sizes

ORIGINAL IMAGE

SCAN ppi

FILE SIZE Mb

A6 ppi

A5 ppi

A4 ppi

A3 ppi

A6

200

2.6mb

200

150

106

74

A6

300

6mb

300

225

160

112

A6

400

11mb

400

300

212

150

A6

600

24mb

600

444

317

222

A5

200

5.3mb

 

200

146

102

A5

300

12mb

 

300

220

153

A5

350

16.5mb

 

350

260

180

A5

400

21mb

 

400

220

153

A5

450

27mb

 

450

330

231

A4

200

11mb

   

200

147

A4

300

24mb

   

300

220

A4

350

33mb

   

350

257

 

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 Scan ppi

6 'Open' the scanner in 'Pre Scan' mode

7 Adjust the scanner parameters to the file size required - noted at step 4. 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.

8 Scan, and you should have a file size appropriate for the final image size you require with sufficient pixels available to stretch to that size - that is no less than 200 ppi at the final image size.

9 To prove that you have sufficient pixels (ppi) in Photoshop for your final image size. Go to Image > Image Size and with resample OFF, change the width or height to whichever is the dominant dimension for the final image and note the ppi in the resolution box (If necessary change the scale from pixels/cm to pixels/inch) - the number should be 200ppi or more. (That is 78.74 ppc - Pixels per Centimetre if you must use metric!)

SCANNING FROM 35mm COLOUR SLIDES & NEGS

In many ways this is much simpler as the original size is constant.

If in doubt, always Choose 2,700 ppi or greater for best quality.

A3 Prints from Slides & Negs A high quality scan of 2,700 ppi will give a file size of approx. 23mb to 25mb and is suitable for an A3 image, giving around 235ppi (If it was then taken down in size to an A4 print, the ppi would increase to around 335ppi - more than enough).

A4 prints from Slides & Negs Choose a scan of around 1800ppi to give a file size of approx. 11mb and 225ppi at A4. Enlarging the image to A3 will give an unacceptable 157ppi. If scanning at 1350ppi a file size of about 6.5mb will result, giving around 170ppi for the A4 image which is on the low side and could look poor. If enlarging to A3 this then becomes an even less acceptable, 118 ppi.

A5 prints from Slides & Negs Choose a scan of around 1200 ppi to give a file size of approx. 5Mb

A6 prints from Slides & Negs Choose a scan of around 900 ppi to give a file size of approx 2.8Mb

In General

When scanning-in the rule is to always be aware of your final image size. This will enable you to obtain the best quality picture. If in doubt scan in at the maximum ppi you think necessary.

Dpi and ppi are interchangeable terms. 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.

Use Photoshop as a 'Calculator'

One way of determining the required resolution is to go to File > New

In the dialogue box presented, enter the height and width of the print you require, set the resolution to the amount needed - say 200ppi or 220ppi (to be on the safe side, as a few additional pixels can do no harm), look at the Image Size (in Mb) and note this figure - this is the file size.

The file size can now be used when scanning in - from whatever source.

Adjust the file size at source (the scanner) to the figure you require and you will have sufficient pixels in the image to stretch to the final image/print dimensions.

Adjusting the File Size at the Scanner

Adjusting the file size 'at source' - that is in the scanner software, do this by either altering the ppi/dpi by the 'slider' control, or typing in a 'guestimatated' number an by a series of trials or by changing the percentage (%) of the scan - whatever works for you and your scanner.

It may be impossible to obtain the precise file size - this is no problem, simply choose a slightly higher number as a few more pixels per inch is certainly better than too few!

Think in terms of File Size (Mb)

Become accustomed to thinking in terms of file size rather than ppi - for no matter what the scanning source, the file size will govern the number a pixels available for the image, the guide below will help you to become familiar with common sizes all based upon a minimum of 200ppi.

For A3 print 20mb

For A4 print 10mb

For A5 print 6mb

For A6 print 3mb

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