Slide scanning - scan to print size or best resolution available?

Discussion in 'Digital Photography' started by Deep Thought, Aug 22, 2005.

  1. Deep Thought

    Deep Thought Guest

    This may be perceived as a stupid question but I use an on-line photo
    printing service whose equipment prints at a set 300dpi (I assume this is
    300ppi and "dpi" is being used as a generalisation) and my slide scanner is
    capable of 14bit/4000spi... if I am only looking to have, say, a number of
    5" x 7.5" prints made from a particular 35mm scan - would scanning get me
    the same printed result (in terms of detail, colour depth etc) if I only
    scan at 1500spi? Or should I scan at max resolution and assume the software
    at the printers will resample the resulting huge file down to the required
    size for printing?

    Reason I ask is that I often have to adjust white/black points, and other
    curves on the scan before saving the file and I have a suspicion that these
    adjustments are best done with as much DATA as possible to give smoother
    effects - hence I high bit-depth, high resolution scan... however I would
    much prefer to upload smaller file sizes and there is this nagging feeling
    that whatever kind of resampling is done at the other end it is likely to be
    detremental to the image quality that I took the trouble to produce...

    For archive purposes I assume the general recommendation is a high
    resolution, high bit depth TIFF file for those particularly important
    photographs.
    Deep Thought, Aug 22, 2005
    #1
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  2. Deep Thought

    piperut Guest

    Deep Thought wrote:
    > This may be perceived as a stupid question but I use an on-line photo
    > printing service whose equipment prints at a set 300dpi (I assume this is
    > 300ppi and "dpi" is being used as a generalisation) and my slide scanner is
    > capable of 14bit/4000spi... if I am only looking to have, say, a number of
    > 5" x 7.5" prints made from a particular 35mm scan - would scanning get me
    > the same printed result (in terms of detail, colour depth etc) if I only
    > scan at 1500spi? Or should I scan at max resolution and assume the software
    > at the printers will resample the resulting huge file down to the required
    > size for printing?
    >
    > Reason I ask is that I often have to adjust white/black points, and other
    > curves on the scan before saving the file and I have a suspicion that these
    > adjustments are best done with as much DATA as possible to give smoother
    > effects - hence I high bit-depth, high resolution scan... however I would
    > much prefer to upload smaller file sizes and there is this nagging feeling
    > that whatever kind of resampling is done at the other end it is likely to be
    > detremental to the image quality that I took the trouble to produce...
    >
    > For archive purposes I assume the general recommendation is a high
    > resolution, high bit depth TIFF file for those particularly important
    > photographs.


    There are a few things to know about scanning -

    The higher resolution the scan - the larger the file.
    The larger size the image - the larger the file.
    TIF files are larger then JPG files and drive the lab technician
    printing the photo crazy. (It is best to use JPG files, this is sort
    of a standard file type.)

    You don't say what size slides you are going to scan, or what scanner
    you are using. Also, what software you are using to scan the slides in
    with. All of these are going to affect the quality of your scan.

    First of all - keep the scanning surface clean. I have a Umax Astra
    2200 Scanner. The way it is set up, it get finger prints on the glass
    from time to time. I have to take some Windex and clean the glass from
    time to time, or a lens cleaning wipe if it is really bad. This really
    improves the quality of the scanned image. (This is a stupid thing,
    and it is annoying ... but it really changes the quality of the scanned
    image in both prints and slides.)

    I am yet to get a good quality negative scan. There is a blue mask on
    the negatives, and once you remove it from the scanned images, the
    quality goes in the toilet.


    Now as far as the size to scan - it depends on a number of factors.
    You stated the size of the desired output. Also consider that in the
    future you many want to make an 8x10 from one of the scans. This is
    possible. There are a couple of other factors. The power supply in
    your computer, the memory in your computer, the processor in your
    computer, the ammount of memory in your computer, what program you are
    using to scan photos, what operating system you are using, the amount
    of storage space on your computer, (in windows - how much virtual
    memory you have allocated), in linux how large of a swap partition you
    have. All of these thing have to be considered. I forgot about the
    power supply and was crashing my computer trying to do scans of 3x5
    photos at 600 dpi.
    The computer would reboot for no reason in the middle of a scan. I was
    overloading the power supply. The computer only came with a 250 watt
    power supply. It now has a larger power supply, and the scanning works
    fine.

    If you try to scan something too large for you computer - you will lock
    the computer up, or it will decide to reboot on its own, or in windows
    it will write a data dump file to the hard drive and fill up your hard
    drive with garbage files, or all three. None of these are very fun.
    In linux, the scans worked fine without too much trouble even with the
    lower wattage power supply at the same dpi.

    So, you have to experiment and find a good resolution for the printout
    you want to make. I would make a test print on an inkjet or laser
    printer before sending them to the lab, and see if the result is
    acceptable. The inkjet or laser print it not going to be the quality
    of a lab print, but it will tell you if your scan is going to be a high
    enough quality to get good results from the lab.

    On my scanner - for most things I end up scanning at 1200 dpi from a
    3x5 or 4x6 print. Slides I try to scan at 2400 or 3600 dpi, as slides
    are smaller, sometimes the slides work out at that dpi, and sometimes I
    have to scan them in at a lower dpi.

    You just have to play and see what works out.

    Hope this helps.
    Now if anybody can figure out the negatives?

    roland
    piperut, Aug 22, 2005
    #2
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  3. Deep Thought

    birdman Guest

    For practical purposes it can be considered that all printers print at 300
    dpi. In fact, most print at less and rarely a few print up to 360dpi.
    Printer dpi and scanner dpi have nothing useful to do with each other when
    you are scanning 35mm film originals so do not get hung up on that number
    when creating scans.
    The considerations for scanning photo prints are very different from
    considerations for scanning 35mm film originals.
    You can scan your 35mm film at any dpi you like. However it makes no sense
    to scan at anything less than 1800 dpi and 22-2400 dpi is probably the most
    practical for most users (not the ideal, the most practical) and generates a
    file of 22-24mbs in size. Ths image file size is large enough to survive
    reasonable cropping and has enough detail to make retouching easier (it is
    acutally easier to clone, composite etc when you have a larger file). A good
    quality scan of this size will easily print to 11x14 if you know what you
    are doing.
    You can can scan at higher resolutions and greater than 8 bit color depths
    if your choose. Hopefully you will learn why or why you would not want or
    need to to do so. If you like to make very large prints frequently you may
    want to scan at this size.
    Always save a copy of your masterpiece in a lossless format like tif or, if
    you use non-destructive layers in Photoshop, save in psd.
    If you do not print your own and understand such manipulations you can
    resize your prized image in Photoshop to whatever size you want (5x7, 8x10
    etc) at a specified dpi (300 if that makes your printing service happy) and
    send it to the printing service as a jpeg. However if you simply crop your
    image and save it at whatever dpi it happens to be at the printer will
    automatically resize it anyway.
    If you experience computer freezes working with image files of this size the
    problem has to do with memory, software or hardware or both, and rarely with
    power supplies unless you have an overloaded box like mine. Whether working
    with Windows or Mac you need at least 512mbs of RAM and the fastest
    processor you can afford. Depending on how you use your computer you may
    have to shut off background programs (like anti-virus) and services to
    minimize system freezes. Until the most recent version the MAC os actually
    was even worse than Windows at handling memory, now they are probably about
    equally unreliable.
    birdman, Aug 22, 2005
    #3
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