Question About Scanning Terminology

Discussion in 'Digital Darkroom' started by lobalobo, Feb 17, 2014.

  1. Just looked at a film developing website, which, in an FAQ, said something like this:
    "Our enhanced scans are 3000 x 2000 pixels, scanned at 72 DPI. The 72 DPI may seem low, but don't worry, it's not, because it's the pixel dimensions that matter."​
    To say the least, I get the "it's the pixel dimensions that matter" part. What I don't understand is what it even means to say that the film is "scanned at 72 DPI." I know what it means to say that an image is displayed at 72DPI or printed at 72DPI; it means that a image scanned to yield, say, 720 x 720 pixels would display or print at 10 x 10 inches. But what in the world does it mean to say that the images are scanned at 72 DPI? Is this merely an indication that the JPEG file created will, by default, display on a computer monitor at 72 DPI? Maybe, but that seems unlikely and hardly worth mentioning. What am I missing? Thanks in advance.
     
  2. You're not missing anything. This dpi info is meaningless when it concerns dislplay or print. Typically filmscanners are
    somewhere in the few thousand lpi range, but that's on the scanning side of things.
     
  3. david_henderson

    david_henderson www.photography001.com

    Its the pixel dimensions that matter​
    That's true. What they might have gone on to say though is that if you want to make large-ish prints, you might need more pixels than that. Doesn't sound like a film scanner - otherwise the dimensions would surely be greater. Possibly something that might be produced as a relatively low-cost scan from a minilab.
     
  4. Thanks, Jos, for confirming that the 72 DPI is meaningless (and David, yes this is from a mini-lab and I understand that the resolution I stated is not suitable for large prints, but I just made up the resolution as an illustration as my question was about the meaning of "scanned at 72 DPI"). So although we agree that 72 DPI is not meaningful in the sense that it doesn't matter, the question still remains what does it mean, in the sense that it must refer to something and I still can't figure out what. Any ideas?
     
  5. digitaldog

    digitaldog Andrew Rodney

    What I don't understand is what it even means to say that the film is "scanned at 72 DPI.​
    It's rather meaningless. It implies that the resolution tag in the metadata specifies that one is using 72 as the value to divide up the existing pixels to end up with a size that doesn't exist. The image at this state has a size it takes up on a storage device and nothing more. IF someone output the data and used 72 of the pixels per inch, that reside in the scan, that's how large the output would be.
     
  6. Here's my way of putting it. Dpi is only metadata in the image file, and means absolutely nothing, unless it is associated with a "print size" or "document size". Apparently, some applications will show the dpi and not the print size, which has lead me astray. For example, in Apple's Preview 5.0.3., the Inspector will show the dpi and pixel dimensions, but not the print size.
    One time I asked a service to drum scan a few 35mm transparencies at 4,000 dpi. I meant for them to set the scanner to sample the original film at 4,000 pixels per inch. This happened before I used Photoshop. When I opened the image files with Apple's Preview, the Inspector showed 4,000 dpi as the resolution. So I was satisfied. I do remember wondering why the file sizes were only around 38MB each, but I didn't question whether the scans really consisted of a resolution of 4,000 pixels per inch of the original transparencies for over two years.
    Finally, I brought those image files into Photoshop and looked at Image Size. To my surprise, I saw pixel dimensions of only 2926x4326, and a document size of .0732 x 1.082 inches, about 75 percent of the size of a 35mm film frame. The scanning service, so accustomed to the standard of 300 dpi established by the graphic arts industry, was thrown by my request. I should have told them that I wanted 4,000 dpi at 100 percent. 300 dpi is meaningless unless you are intending to print at a certain size, you are a book or magazine designer, or something like that. Well, what if you have no immediate plans to print at all?
    The Noritsu scans of 35mm film negatives that I had done at A&I have 72 dpi at a print size of 87.569 x 58.069 inches in the metadata. The pixel dimensions are 6305x4181, which is the only thing that matters, because I will never be printing them at all, let alone at a size of 87.569 x 58.069 inches.
    Drum scans from West Coast Imaging very nicely have the true resolution in the metadata. For example, their lowest priced drum scans of 35mm film have 3800 dpi in the metadata, meaning they were sampled at 3800 pixels per inch at 100 percent. I like it that way. To me, it's better to think in terms of the amount of samples you are getting, just as audiophiles speak in terms of sampling rates of 44.1 kHz, 48 kHz, 96 kHz, etc., when digitizing music. But I think this way because I never print, unlike many people on this forum.
    At any rate, let your scanning service put whatever dpi and print size they want in the metadata. You can always change that metadata with an application like Photoshop, without actually changing the image data.
     
  7. The 72 dpi on the file of the scan is a meaningless tag. However, I would not tend to have high confidence in anyone who says that a 3000 x 2000 pixel scan of presumably 35mm film is a 72 dpi scan. Insofar as the film is 36 x 24 mm, ipso facto the scan has to be 2117 ppi.
    I say the tag is meaningless. I suppose with some software, if you print the file without further direction, and use a big enough printer, you will get a 42 x 28 inch print. 3000 pixels / 72 ppi = 42 inches (roughly). But if you send that same file to, say, Mpix, and tell them you want an 8 x 12 inch print, what you have is a file that equates to 8 x 12 inches at 250 ppi, which is the same pixel dimensions.
    Also, although you sometimes see people who know what they're talking about use dpi (dots per inch) to refer to a scan, that terminology is ambiguous because in the context of things like inkjet prints, dpi means something else. IMO ppi (pixels per inch) is the more appropriate descriptor.
     
  8. Thanks to all, particluarly to Dave and John (prior two responses). So it seems that the 72 DPI (which should be PPI, so as not to confuse with printing density) is just a useless instruction to software that will display or print the image, though even at that, any intelligent software will simply ignore that instruction. (So, e.g., opening a 3000 x 2000 pixel image in Picasa does not default to a size many multiples of a screen that displays at 72 PPI, and printing in Picasa will adjust the PPI so that all the available pixels are used for whatever paper size is chosen, flashing a warning if that number goes too low for a decent print.)
    This raises the question, then, why any vendor bothers to report this useless and confusing statistic along with the only numbers that matter for a scan: pixel dimensions. My elderly father-in-law, for instance, who is really pretty impressive technologically for his age, is continually confused when he opens a file that Photoshop Elements lists as "resolution" at 72 PPI (or DPI); he can't figure out why some images print great and others pixelate when the listed resolution is the same on each. I've tried to explain to him that the resolution he needs to care about is pixel dimensions, but he refuses to believe me because Photoshop labels the meaningless "72" as "resolution." Now I have a better chance of convincing him, though he will perhaps doubt that Adobe could be that dumb in what it chooses to present.
     
  9. digitaldog

    digitaldog Andrew Rodney

    I suspect something has to (or should be) saved in this metadata tag, meaningless or not. So by default, some products probably place 72 into the spot but the bottom line is, you can alter that value to anything you wish after the fact in Photoshop. As long as you do not interpolate (add or subtract pixels), you can enter any value you want. The same is true if you make a new document, that value is sticky from last used, you can update or alter any way you wish. Work with pixels (W&H), just ignore the resolution tag.
     
  10. -anyhow, "DPI" is "dots per inch", a measurement that only relevant to the PRINTER. A more correct term would be "Pixels per inch" (PPI) in this context, but it still only refers to output, whether to a printer or a screen.
     
  11. -anyhow, "DPI" is "dots per inch", a measurement that only relevant to the PRINTER. A more correct term would be "Pixels per inch" (PPI) in this context, but it still only refers to output, whether to a printer or a screen.​
    Right. A couple of posts in this thread make the same point.
     
  12. A couple of posts in this thread make the same point.​
    And took twice as many words to say it, without making the dpi/ppi distinction at all clear. Just trying to help out, you know.
    Sorry about that.~
     
  13. Sorry JDM. No criticism intended. Just saying that others agree. In any case, DPI is confusing and not quite accurate even for printers. Printers actually tend to use more than 300 ink dots per inch; the 300 DPI for printing, then, when used to describe the information density of a printed image, actually also refers to pixels per inch. But maybe that's what you meant. Again, sorry if I seemed to be critical.
     

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