What resolution for scanning slides?

Discussion in 'Digital Photography' started by edmarg1, Jun 24, 2005.

  1. edmarg1

    edmarg1 Guest

    Hello all,
    How does this compare to 4x5 transparencies? For example, with an output of
    300dpi and a scanner that can capture 2400dpi and a 4x5 inch transparency is
    it correct to say I can make a print of 30 inches ( 2400x 4= 9600dpi,
    9600dpi divided by 300 dpi = 32 inches)?
    I am also trying to learn the basics of digital printing and scanning.
    Since I shoot 4x5 I need to get a flatbed scanner( cannot afford the price
    of a drum scanner). Any thoughts on epson's 4990 or microtek's i900
    Ed Margiewicz
    edmarg1, Jun 24, 2005
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  2. What is the consensus on 'best' resolution setting for 35 mm slides

    In Control Panel>Properties>Scanners & Cameras>Epson Perfection 2480,
    I see my new scanner is specified as 2400/4800 dpi. The 'default'
    setting in the Resolution box always seems to be 300. Using the Fully
    Automated Mode I cannot see what resolution has been used. In
    Professional Mode, which I am now experimenting with, if I use 2400 I
    get what appear to be good quality results. But I notice that the file
    size in that case is typically about half of that using Full Auto. So
    I'm wondering if that means Auto uses 4800? Come to that, what does
    '2400/4800' actually mean anyway?
    Terry Pinnell, Jun 25, 2005
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  3. Hello Terry

    It's accepted that an input value of 4000 ppi is best simply because it
    covers all possible uses for the final image and wrings as much
    information as possible from the neg or slide. 4000 ppi is the way to
    go for larger print sizes and detailed editing requirements. 300 ppi
    (it will probably read dpi) is the output resolution for home printers
    and professional digital equipment that produce digital prints.

    Each print size requires a minimum number of pixels at 300 ppi
    resolution to ensure the best resolution. Beyond this, the resolution
    is just wasted - you'll never see it. So ideally, a 7x5 print should be
    around 2100x1500 pixels. If you ask too few pixels to fill too big a
    space for a specific print size, the image will start to break up with
    the individual pixels becoming more obvious.

    See here: http://www.theimageplane.net/interp.htm

    The bad news for you is that the image quality of flatbed film
    attachments fall some way behind dedicated film scanners, so for
    optimum results it's best to use a scanner specifically designed to
    scan 35mm film. However, if you're producing smaller prints, you may
    well find that your scanner will deliver good results if you scan for
    reasonable detail.

    Hope this helps.

    Sharp Shooter, Jun 25, 2005
  4. Sure does! Much appreciate that prompt and comprehensive reply. Hope
    you don't mind a few follow-ups please?

    The drop-down list in EPSON SCAN doesn't include 4000, so I've entered
    that manually. I've just done a couple of scans with that. Slides were
    approx 35 x 25mm. The program's JPG setting was at 'Compression Level
    16', whatever that means; in another dialog it's described as '85%
    high quality'! That resulted in files of about 1.4 MB. For an exact
    comparison, I scanned the same slide twice. Leaving aside the fact
    that this 30-year old slide was poor anyway, here's the comparison:

    1481 KB res = 4000 dpi

    659 KB res = 2400 dpi

    I suppose compression complicates the size ratio? If size was simply
    proportional to area and resolution, I'd expect the 4000 version to be
    (4000/2400)^2 = 2.78 times larger than the 2400 version, i.e. about
    1835 KB, instead of 1481 KB.

    I can't personally see any difference on my 1024 x 768 monitor, but I
    suppose I would if I made two 5" x 7" prints?

    BTW, what exactly does the 'd' in dpi stand for, and why is it shown
    as that instead of the unambiguous 'ppi'= pixels per inch?

    Will now study that ImagePlane site carefully!
    Terry Pinnell, Jun 25, 2005
  5. edmarg1

    MTBike1970 Guest

    Hi Terry:

    DPI means "dots per inch" and usually refers to your printer output. It is
    independent of the image PPI, or "pixels per inch" I'll usually size my
    photos at 300 ppi, and set my printer at 720 dpi.

    A couple of other comments:
    Your scanner has a maximum optical resolution of 2400 dpi. Above that, it
    is interpolating the extra pixels and does not give you more detail or
    resolution. Most people will recommend that you scan slides at the maximum
    optical resolution and avoid interpolation.
    If you're able to save your scans in a lossles format such as TIFF or
    Photoshop PSD, you'll avoid getting any jpeg compression artifacts in your
    images. Whether they're visible or not depends on the jpeg setting at the
    print size. If you have to save in jpeg, use the maximum (least
    compression) setting.

    MTBike1970, Jun 25, 2005
  6. Hello Terry

    I would think your scans would produce prints you'll be delighted with.
    It won't be in the same league as dedicated film scanners, but judging
    by these scans it will do a really good job! A flatbed scanner won't
    compete with a film scanner resolution wise, but scan at the best
    resolution you can without any digital trickery (the adding of pixels).

    But there are a couple of issues here:

    Firstly, if you're sure you will not be needing very large prints, scan
    at a resolution that will deliver the best print results, as we
    discussed above - 300 dpi (ppi) is the norm with film scanners. Or, to
    put it another way, it wouldn't make sense to scan at a 2400 dpi input
    value if you're only wanting 7x5 prints. The maths here works out at
    2100x1500 for optimum results for 7x5s. Divide your pixels by 300. I
    routinely scan at 4000 ppi input (@ 300 ppi output) because I know I
    can use the images for *every* purpose. If I need a big 18x12, no
    problem. If I need a small image for the website, no problem. If I need
    a 9x6, no problem. However, if I scan too small I'm snookered if I want
    a really big print.

    Your images are scanned in at 96 dpi. Just do the maths to make sure
    that the correct number of pixels is present in any image per print
    size before you get your prints done. If film is canned at 300 ppi,
    software can show its dimensions in inches. For example, a recent scan
    of mine is 5557x3640, which when shown in inches is 18.5233x12.1333, so
    I know what maximum print size I can get.

    Some consumer labs will produce print sizes in relation to submitted
    image sizes that go way below 300 ppi as a minimum requirement, and
    many are more than happy enough with the results. I can see it though.
    Get below 200 ppi and things start to really go downhill. But it
    doesn't make sense to compromise resolution in this way unless you're
    just after 'happy-snappies', and there's nothing wrong with that, of

    A second point: JPEG compression quality refers to the amount of data
    the software loses when it saves the image. This is why JPEGs are used
    for web pages. In terms of KBs, they can be quite small but still look
    ok. But if you compress too much you'll lose too much data and the
    detail and colour will go out the window. I recommend you set it to the
    very best quality possible, or even better, save your scans as TIFFs
    and convert to a JPEG when you have worked at the image, if that's what
    you intend to do.

    "The JPEG standard was written by the committee known as the Joint
    Photographic Experts Group, and it was designed for compressing full
    color or grayscale images (in particular, photographs and similar
    high-quality artwork). JPEG is a 'lossy' file format; when the JPEG
    algorithm compresses the image, it reduces the size by chucking bits of
    the image away. How does it know what to lose? JPEG compression plays
    on the fact that the human eye can only see so much. We have trouble
    seeing small color changes, so JPEG loses some of this subtle
    information" (Una Dooney).

    I would express the difference between dpi and ppi like this: "The
    resolution of a digital image is measured in pixels per inch, not dots
    per inch. Software may show the image resolution as DPI, but strictly
    speaking this is not correct. Unfortunately, the two terms - ppi and
    dpi - tend to be used interchangeably, even by some professionals.
    Dpi refers to a printer's output resolution, not the resolution of a
    digital image." There are no "dots" in a digital image, just pixels.

    Hope I haven't missed too much. Not too much time at the moment, but
    feel free to email me through the site if you feel I can be of any use.

    Good luck with those scans!
    Sharp Shooter, Jun 25, 2005
  7. Thanks both, very helpful.

    I'm going to have to get stuck in tomorrow to really get a grasp of
    this. I confess that at the moment I'm floundering, but that's partly
    because of unfamiliarity with this software. My crude test of any
    trial is to view it on my screen, so printing and print sizes don't
    figure much in my thinking. And my simplistic reasoning was simply
    that, as the source is so small (1" x 1.5"), I have to scale up the
    resolution proportionally. 2400 seems to work OK. But, as I say, I've
    plainly got a lot of basic studying to do!

    Husband duties now call, but back on the case tomorrow afternoon <g>.
    Terry Pinnell, Jun 25, 2005
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