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Author Topic: What resolution to scan prints to archive?  (Read 3406 times)
AFairley
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« on: February 05, 2010, 11:37:14 AM »
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I am embarking on a project to scan family photos going back as far as 100 years using an Epson V700. The prints range in size from 2x3 to 8x10.  The main purpose is to archive the photos so they can (1) be preserved and (2) be distributed to multiple family members.  It is possible that at some point, some of the images would be printed.

My inclination is to scan everything at 360 dpi size to size, since that is the rez I would be sending anything printed to the printer, my thinking being that scanning at a higher dpi would not recover any usable detail from the picture and if printed would be downrezzed anyway. Or should I scan at whatever DPI is required to give me a 8X10 360 dpi image (i can't see printing any of these things larger than that). Or is 360 dpi would be overkill because there isn't that much information anyway, so I'm not capturing additonal information by increasing the scan DPI?

I've done some web research, but have not found an information source I really trust.

Thanks
« Last Edit: February 05, 2010, 12:13:32 PM by AFairley » Logged

Mark D Segal
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« Reply #1 on: February 05, 2010, 12:28:43 PM »
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First, let us not use the term "dpi". IT means dots per inch, not scanning. I shall use the term PPI because we are scanning pixels in this context.

The resolution of your prints is probably equivalent to about 200 PPI, and you should get very satisfactory print quality from files containing 240PPI at output print size. Therefore it may not make much sense to scan at greater than 240 OUTPUT PPI.

There is a distinction between INPUT PPI and OUTPUT PPI. OUTPUT PPI is the PPI per inch at the output size of the image. INPUT PPI is the PPI the scanner would create to get that value. If the Scale is the same (100%) between the Original and the Output (i.e. both are the same dimensions), then the INPUT PPI and the OUTPUT PPI are the same. If the Output dimensions will be larger than the input (Original) dimensions, then the INPUT PPI will have to be greater than the OUTPUT PPI if you want to have enough pixels to cover the larger Output dimensions at your required OUTPUT PPI.

You should also decide whether you wish to scan at 8 or 16 bit depth (which for the three channels is 24 or 48 bit depth). I recommend 16/48 bit depth. You'll get better quality files with smoother tonal gradations for printing.

At this bit depth, storage and file size considerations could make a difference to you as between scanning at 360 PPI or 240 PPI. At 240 PPI a scan which has an Output size of say 7 * 10.5 inches would make a 24.2 MB file, whereas at 360 PPI the same file would be 54.5 MB - this kind of difference adds-up over many files. Granted storage is cheap these days, but nonetheless something to consider.

Now, for your 2*3 inch Original, let us say you wish it to be printable at 240 PPI and at 7*10.5 inches (this maintains the original aspect ratio). Hence your output PPI would be 240. There is a scale factor of 350% between the dimensions of the original and the dimensions of the Output, hence to have your 240 PPI Output PPI, your INPUT PPI would be 900. If the software you are using allows you simply to state the OUTPUT PPI you want, then you need not concern yourself with this distinction, but good to know what's happening under the hood. If your software only allows you to specify INPUT PPI, then you need to take into account both the scale factor and the output PPI in order to know the correct INPUT PPI.
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Mark D Segal (formerly MarkDS)
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AFairley
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« Reply #2 on: February 05, 2010, 01:57:59 PM »
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Quote from: Mark D Segal
Now, for your 2*3 inch Original, let us say you wish it to be printable at 240 PPI and at 7*10.5 inches (this maintains the original aspect ratio). Hence your output PPI would be 240. There is a scale factor of 350% between the dimensions of the original and the dimensions of the Output, hence to have your 240 PPI Output PPI, your INPUT PPI would be 900. If the software you are using allows you simply to state the OUTPUT PPI you want, then you need not concern yourself with this distinction, but good to know what's happening under the hood. If your software only allows you to specify INPUT PPI, then you need to take into account both the scale factor and the output PPI in order to know the correct INPUT PPI.

Thanks for your response, Mark.

DPI v PPI - thanks, I should know better by now...  I do understand the distincition between input and output ppi, though that probably doesn't come across clearly in my original question.

Anyway, I think what my question is boiling down to is, given that the resolution of my hypothetical 2x3 original is 200 ppi, am I going to get any real benefit of scanning at 900 ppi in order to able to send the scanned file to my printer at 240 ppi for the the 7x10.5 print size, compared to just letting the printer driver uprez from a 200 ppi scan (i.e. the scanned file would be going to the printer at about 70 ppi at the 7x10.5 print size).  In other words, given the 200 ppi of the original, am I actually capturing more information by scanning at 900 ppi?  I am having a little difficulty wrapping my mind around this...  I hear an empirical test calling....
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Mark D Segal
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« Reply #3 on: February 05, 2010, 02:10:42 PM »
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If you have a 2*3 inch original and you want to print it 7.5 * 10 inches you need enough original pixels in the scan to get the 240 OUTPUT PPI you'll need for printing, so you MUST use the much higher INPUT PPI setting. Remember 2 inches from the original is going to become 7.5 inches in the Output. If you scan the original only at say 200 PPI, you will have only 400 pixels (2 inches by 200 INPUT PPI) to spread accross 7.5 inches of output. That would give you only 53.3 OUTPUT PPI (400/7.5), instead of the 240 PPI you need for printing. I hope that clarifies the distinction for you. Don't even bother testing for this - waste of time. The correct methodology is unmistakable.
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Mark D Segal (formerly MarkDS)
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dmerger
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« Reply #4 on: February 05, 2010, 02:36:32 PM »
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I know little about the Epson V700, but I suspect that your scanner uses a CCD light sensor.  Unlike a digital camera that uses a rectangular CCD, however, your scanner probably just uses a three row CCD (for red, green and blue) and then moves the CCD during scanning to record the entire photo. The number of pixels of the CCD is fixed.  The number of pixels resulting from the movement of the CCD during scanning might be variable to some extent by the scanner hardware.  

The bottom line, however, is any resolution you select that isnít a native hardware resolution for your scanner will be interpolated, either up-sampled or down-sampled, using whatever algorithm is incorporated into your scanning software.  My suggestion is to scan at the highest native hardware resolution of your scanner and save those scans as masters, using copies for any edits or resizing.  
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Dean Erger
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« Reply #5 on: February 05, 2010, 03:28:16 PM »
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Quote from: dmerger
I know little about the Epson V700, but I suspect that your scanner uses a CCD light sensor.  Unlike a digital camera that uses a rectangular CCD, however, your scanner probably just uses a three row CCD (for red, green and blue) and then moves the CCD during scanning to record the entire photo. The number of pixels of the CCD is fixed.  The number of pixels resulting from the movement of the CCD during scanning might be variable to some extent by the scanner hardware.  

The bottom line, however, is any resolution you select that isnít a native hardware resolution for your scanner will be interpolated, either up-sampled or down-sampled, using whatever algorithm is incorporated into your scanning software.  My suggestion is to scan at the highest native hardware resolution of your scanner and save those scans as masters, using copies for any edits or resizing.

Nice idea in theory, but the V700 has a native optical resolution of 4800 dpi (Epson's term). A 2x3 inch print would then be scanned to 138 megapixels, which at 8 bit color depth would be about a 400 megabyte file. Somehow I don't think this is a good idea!

I too have recently been scanning old snapshots and have been very happy with the results at 300 dpi. At this resolution I think you can be confident of capturing all the info in the print thus way.

The V750 is really a terrific scanner, I have been doing prints, slides, and negatives with excellent results.
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Peter
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Mark D Segal
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« Reply #6 on: February 05, 2010, 04:44:37 PM »
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Quote from: dmerger
I know little about the Epson V700, but I suspect that your scanner uses a CCD light sensor.  Unlike a digital camera that uses a rectangular CCD, however, your scanner probably just uses a three row CCD (for red, green and blue) and then moves the CCD during scanning to record the entire photo. The number of pixels of the CCD is fixed.  The number of pixels resulting from the movement of the CCD during scanning might be variable to some extent by the scanner hardware.  

The bottom line, however, is any resolution you select that isnít a native hardware resolution for your scanner will be interpolated, either up-sampled or down-sampled, using whatever algorithm is incorporated into your scanning software.  My suggestion is to scan at the highest native hardware resolution of your scanner and save those scans as masters, using copies for any edits or resizing.

Firstly, the scanner has quite a large number of internal resolution steps at which it scans - this is hardwired. The way this usually works is that you select an output resolution and an output image size, and it will scan at the nearest hard-wired input resolution commensurate with your stated output requirements.

Secondly, scanning at maximum resolution unless your output size requires it is counter-productive. This has been tested over and over again. It is also discussed in the SilverFast Studio Ai6 manual where they explicitly recommend against doing this. I have tested it myself and I agree with them. That said, it is good to be quite certain what the full usage of this material will be over time so that one doesn't scan-in too few pixels and then need to redo it if an unexpectedly large print needs to be made one day. Not that this is a problem to do, just time-consuming.
« Last Edit: February 05, 2010, 04:45:23 PM by Mark D Segal » Logged

Mark D Segal (formerly MarkDS)
Author: "Scanning Workflows with SilverFast 8....." http://www.luminous-landscape.com/reviews/film/scanning_workflows_with_silverfast_8.shtml
dmerger
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« Reply #7 on: February 05, 2010, 05:16:51 PM »
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Quote from: Mark D Segal
Firstly, the scanner has quite a large number of internal resolution steps at which it scans - this is hardwired.

Can you explain how the scanner does so?  
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Dean Erger
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« Reply #8 on: February 05, 2010, 07:12:03 PM »
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I'm not a firmware engineer so I can't explain the inner workings to you. What I learned from those who are in a position to know is that whatever INPUT scan resolution you select (or gets selected as a result of a combination of output settings), if it is not one of the scanner's default PPI scanning values along the ladder to maximum optical resolution, the scanner would default to the closest one below. Whether that applies to ALL scanners I don't know, but this discussion came up several years ago in the context of the Minolta Dimage Scan Elite 5400.
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Mark D Segal (formerly MarkDS)
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« Reply #9 on: February 05, 2010, 07:58:58 PM »
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Quote from: Mark D Segal
I'm not a firmware engineer so I can't explain the inner workings to you. What I learned from those who are in a position to know is that whatever INPUT scan resolution you select (or gets selected as a result of a combination of output settings), if it is not one of the scanner's default PPI scanning values along the ladder to maximum optical resolution, the scanner would default to the closest one below. Whether that applies to ALL scanners I don't know, but this discussion came up several years ago in the context of the Minolta Dimage Scan Elite 5400.

I think it works as follows. Suppose the scanner has a maximum optical resolution of 1200 dpi. This means the sensor array has 1200 individual sensors per linear inch. Suppose you choose a resolution of 600 dpi. Then the scanner combines the input from each pair of adjacent sensors to create one output pixel. If you select 400 dpi, the output from 3 sensors is combined, and so on.
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Peter
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dmerger
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« Reply #10 on: February 05, 2010, 10:04:17 PM »
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Quote from: PeterAit
Nice idea in theory, but the V700 has a native optical resolution of 4800 dpi (Epson's term). A 2x3 inch print would then be scanned to 138 megapixels, which at 8 bit color depth would be about a 400 megabyte file. Somehow I don't think this is a good idea!

I agree, way too much information.
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Dean Erger
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