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A Little Experiment

I scanned the cabinet card on the first page at 75 ppi, 150 ppi, 300 ppi, 600 ppi, 1200 ppi, and 2400 ppi.  I then enlarged a small section of the photo a ridiculous amount, to see when increasing the resolution of the scan quit giving me more information.  I used the Hewlett-Packard scanning software, and kept brightness, color, and crop size constant across the scans.

As you can see on this screen shot, the HP software will give a histogram of brightness levels of the scan, so that you can adjust the brightness levels manually.

The first impressive result was the scanning itself.  To scan the approximately 4.25" by 6.5" card took about 6 seconds at 75 ppi.  The same size scan done at 2400 ppi took 17 minutes, 45 seconds!  Remember that as you look at that stack of tintypes you need to scan.

The second impressive result was the file sizes.  Remember, there are two dimensions to a scan (height and width), so as you increase the resolution in ppi, the number of pixels goes up as the square of the increase, i.e. if you double the resolution, you get 4 times the number of pixels.  This makes for big files.

The following are the file sizes for the scans of the cabinet card at different resolutions, using uncompressed TIFF files:

Resolution File Size
75 ppi 0.45 Mb
150 ppi 1.8 Mb
300 ppi 7.2 Mb
600 ppi 28.6 Mb
1200 ppi 115 Mb
2400 ppi 458 Mb

The highest resolution scan file is over a thousand times larger than the smallest!  The 2400 ppi scan takes up almost half of a gigabyte on my hard disk (it is getting dumped now).  I have 256 Mb of RAM on my computer, so it is larger than the memory on my machine.  To open the file in picture window takes almost 10 minutes!  Conclusion #1: the higher the resolution, the bigger the file.

For the remainder of this session, we are only interested in one small part of the photo:

There looked to be a reasonable amount of detail in the faces and clothes of these two people, so I cropped to a small section, roughly 3/8" by 3/4".  We are going to do some enlargement torture testing by blowing up this small area by a factor of 9 in both dimensions.  To put this in perspective, the entire card enlarged to the same degree would result in a 38 in. by 58 in enlargement!

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