Getting
the Most Out of Your Scanner
Scanning
Resolution and Purpose
I often read
posts on usenet forums asking for the best resolution for scanning 4x6
pictures to print as 8x10s, or for making 240dpi prints from 35mm negatives.
While it can be fast and easy to scan at the correct resolution to send
directly to the printer, it is not necessarily going to give you the best
scan, or the best file to enhance.
The output you
plan for your image may change. After you’ve made that 8x10 you may decide
it would make a great 11x14. The equipment you are using to print it will
definitely change - and soon. Your next printer may be able to use 360
dpi for photos, and the one after that might use even more. So the scan
you do today should be as ready as possible for the uses and equipment
of tomorrow.
If you scan at
the correct output resolution you have made a single purpose image. If
you scan at the highest optical resolution of the scanner, you have made
an image you can re-purpose.
Optical
Resolution
If you have a
flatbed scanner it might take a little detective work to figure out the
optical resolution . In an effort to have the best possible specs in the
advertising, the manufacturers are "fudging" the facts. The smallest number
you find on the box is the only one that counts.
If the scanner
has 600x1200 dpi - it is a 600dpi scanner. If it is instead 1200x600 -
it’s still 600. They simply reversed the numbers to make it look better
than the other next to it on the shelf. If the scanner has 9600dpi Max…..
it is most likely 600dpi. The 9600dpi is an interpolated figure - the
scanner "makes up" samples for you. This only leads to gigantic files
that are in no way an improvement over the 600 dpi optical resolution.
There are scanners
now with higher than 600 dpi resolution. At this writing 800, 1200 and
1600 dpi models are on the shelves. There are no 3200 dpi scanners at
your local K-mart. Read the specifications carefully. It could be a 400dpi
scanner as easily as an 800.
Most 35mm film
scanners are more honestly rated. The optical resolution is the Highest
number available. Currently 2400, 2700, 2800, and 4000 dpi are available
although there are some lower resolution models still on the market. Remember
this high seeming number is going to drop quickly as you enlarge the image.
2400dpi produces a 2400x3600 pixel image that is only 1x1.5 inches in
area. By the time it is 4x6 inches the resolution will drop to 600dpi.
An 8x12 is 300dpi, or about the same resolution as you would have gotten
scanning a 4x6 print on a 600dpi flatbed.
Photoshop
Birds and Bees - Big Files Can Make Little Ones
If your scanner
produces a 24 bit image at 2700dpi your colour negative will produce a
file of about 27Megs. A 600dpi scan of a 4x6 print will be about the same
size. This scan will not only allow you to print to the largest size possible
from your original, but will allow you to make lower resolution files
for the web or email, without re-scanning. Big deal, you say. It only
takes a minute or so to make a new scan - it would take almost as long
to re-sample this one in Photoshop. This is quite true, and if you do
nothing to the file, you might as well rescan… if you still have the original
negative or print, and if it is still in good condition.
But to how many
files do you do "nothing"? Did you crop out that guy behind
your brother – the one nobody can identify? Have you cloned out the big
scratch running through Aunt Velda’s left eye? How about that sickly green
tinge you adjusted out in the Levels? Do you want to do all this over
again? If you are like me you are lazy and do not wish to repeat work.
Any changes you made to the file took time and effort. You want to keep
those changes.
The large scan
is also going to be the easiest to retouch. You can make smoother gradients,
and masks with a full resolution file. You can remove spots more easily
because they are larger and occupy more pixels. Instead of spotting it
out with one sample, you will be able to take samples from several points
around the scar to create a smooth, invisible patch.
So start with
the best possible scan and do your repairs once. It will save time.
The Next article
deals with High Bit Images – What they
are and why they are good for you.
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