Scanning-in from
35mm Colour Slides
&
Negatives

Clive R. Haynes FRPS
 

In many ways this is much simpler than scanning-in from a flatbed scanner, as, unless you're cropping the image, the original size is constant.

The simple recommendation is, If in doubt, always choose 2,700 ppi or greater for best quality.
Naturally, if significant cropping is taking place, adjustment to the 'scale' and/or ppi will be required to maintain sufficient resolution. The link to 'Scanning-in to Scale, at the base of this page gives more information about 'croping' and 'scaling'.
 

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. As mentioned above, if in doubt scan in at the maximum ppi you think necessary.

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.

 
Scanning-in
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 the image was reduced to an A4 print, the ppi would increase to around 335ppi - more than enough so not a problem).
Retaining a large file size keeps your options open.
 
A4 prints from Slides & Negs:
Choose a scan of around 1800ppi to give a file size of approx. 11mb and a resolution of around 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 and a resolution around 200ppi.
Remember that if the image is enlarged to greater than A5 it will have reduced resolution.
 
A6 prints from Slides & Negs:
Choose a scan of around 900 ppi to give a file size of approx. 2.8Mb and a resolution around 200ppi.
Remember that if the image is enlarged to greater than A6 it will have reduced resolution
 
In General - For all Scanning-in (Flat-bed or 35mm Scanner)
 

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 (220ppi is airing 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 'guestimated' number and by a series of trials or by changing the percentage (%) of the scanning scale - whatever works for you and your scanner. Some scanners and software have more flexibilty than others.

It may be impossible to obtain the precise file size you require - this is no problem, simply choose a slightly higher number (greater file size) 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 (mb) 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 the image at the size indicated.

For A3 print 20 - 25mb

For A4 print 10 -11mb

For A5 print 5 - 6mb

For A6 print 3 - 4mb
 
Why do the file sizes (mb) sometimes vary a little from image to image with the same original dimensions?
Without getting too technical, the 'content' of the image in terms of colour range and saturation, etc., has a bearing upon the information that needs to be 'captured'. Consequently, a highly detailed image with a wide range of vibrant colours will occupy more file space than one with near monochrome, soft and muted tones.
 
 
Related Topics
File size & Resolution
Bit Depth & File Size
Scanning to Scale
Know-How Contents
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