Saving for Web - Image Size

Discussion in 'Photoshop Tutorials' started by C Wright, May 27, 2006.

  1. C Wright

    C Wright Guest

    This is a simple nit of a question. Often, when preparing images to be
    uploaded to the Web, we are advised to save them at 72ppi because that is
    the resolution of many older monitors. Along with that advice we are often
    also told to restrict our images to a certain size, for example a long
    dimension of 600 pixels. Is there any point in doing both of these resizing
    operations? If I save an image as, for example, a 600x400 pixel image it
    will appear at that absolute size on a monitor regardless of whether it is
    saved at a resolution of 72ppi or 150ppi! If the image were to be printed
    that, of course, could make a difference but if the only concern is to
    properly display the image on other's monitors is there any point to
    resizing to both an absolute dimension AND a certain ppi size?
    Chuck
     
    C Wright, May 27, 2006
    #1
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  2. C Wright

    Harry Limey Guest


    Just use the save for web function in Photoshop, put in your dimensions
    600x400 or whatever - I don't believe you are even asked for ppi
     
    Harry Limey, May 27, 2006
    #2
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  3. C Wright

    Bill Hilton Guest

    C Wright asks ...
    No, as you say the ppi figure is meaningless for display purposes ...
     
    Bill Hilton, May 27, 2006
    #3
  4. C Wright

    edjh Guest

    As the others have said, no. The pixel dimensions are all you should
    concern yourself with.
     
    edjh, May 27, 2006
    #4
  5. C Wright

    tacit Guest

    No.

    The people who tell you to save at 72 ppi do not understand resolution.

    When an image is displayed in a Web browser, the Web browser strips off
    and disregards any ppi information. The only thing--the ONLY thing--that
    matters to a browser, any browser, is the total number of pixels. All
    Web browsers display an image at one pixel inthe image equals one pixel
    on the screen.

    A 300x200 pixel image at 72 pixels per inch is absolutely, completely
    identical, in all Web browsers and on all platforms, to the same 300x200
    pixel image at 96 pixels per inch, the same 300x200 pixel image at 300
    pixels per inch, and the same 300x200 pixel image at 3,000,000 pixels
    per inch. For Web use, resolution in terms of pixels per inch is
    completely irrelevant.
     
    tacit, May 27, 2006
    #5
  6. C Wright

    Owen Ransen Guest

    Owen Ransen, May 28, 2006
    #6
  7. C Wright

    Eric P. Guest

    It is my understanding that the human eye can't distinguish much
    above 72 or perhaps 75 dpi, so for images that will be viewed on
    a monitor, but not printed, there's no advantage to setting the
    resolution above this. For printing, though--and here my understanding
    isn't so clear--it appears preferable to set images much higher,
    so settings of 600 or more shouldn't be unreasonable. I don't
    know if what I said about the human eye extends to printed pages
    or not.

    On a somewhat related note, if scanning an image to bring into
    an image editing program, my instinct is to scan at a very high
    resolution, and then have the option to save/print edited versions
    at lower resolutions.

    Hoping for further clarification here...
    Happy computing,
    Eric
     
    Eric P., May 28, 2006
    #7
  8. It's actually ppi, not dpi at this stage. Most monitors now are around
    96 ppi, a rule of thumb when setting pixel dimensions for monitor viewing.
    Most folks are fine with 300 or so ppi for printing, and it's not clear
    that more ppi makes a discernable difference in final product, and it
    prints much slower. Here's where dpi comes in: Some printers can be set
    to print at 360, 720, 1440 etc. dpi, all from the same image. Other
    printers may call it HQ, or fine, or medium, or draft, etc.
    That instinct is right! Unless you have dozens of images to scan and
    they will go only onto a web page....
     
    John McWilliams, May 28, 2006
    #8
  9. C Wright

    tacit Guest

    This is factually incorrect. The human eye can easily distinguish detail
    above 72 pixels per inch.

    However, a monitor is a fixed pixel device. It is made up of a grid of
    pixels. A monitor who's resolution is set to 72 pixels per inch always
    displays all images under all circumstances at 72 pixels per inch,
    without exception. A monitor who's resolution is set to 96 pixels per
    inch always displays all images at 96 pixels per inch.
     
    tacit, May 28, 2006
    #9
  10. C Wright

    2 Guest

    Tacit: Isn't the Apple Cinema Display 120 ppi?
     
    2, May 28, 2006
    #10
  11. C Wright

    Eric P. Guest

    There is an upper limit somewhere. I just don't recall what it is.
    I didn't say anything to suggest that that's not the case. Again, though,
    there's an upper limit to a monitor's resolution, due to physical
    limitations. I don't know what that might be, either.

    Thanks,
    Eric
     
    Eric P., May 28, 2006
    #11
  12. C Wright

    tacit Guest

    The highest resolution Cinema Display has a pixel pitch of .25mm, which
    roughly translates into an effective resolution of about 96 pixels per
    inch in its highest resolution setting.

    That seems reasonable to me, as anything higher resolution might make
    fine detail on the screen hard to view.
     
    tacit, May 28, 2006
    #12
  13. C Wright

    Eric P. Guest

    I forgot how to assign this. My Monitors cp allows me to select
    resolutions at certain refresh rates, but I don't see where to
    assign a "per inch" setting.
    Oh, yes. In the case of .jpg files, I choose Save for Web in Photoshop.
    For .gif files, I don't like to alter things too much, because the more
    I do, the more degradation in quality I see.

    Thanks,
    Eric
     
    Eric P., May 28, 2006
    #13
  14. C Wright

    2 Guest

    Thanks for that. I thought I recalled a control-panel monitor setting of
    120.
    That must have something to do with the way the image is formed, because we
    don't have trouble seeing more detail on paper. FWIW, tests have shown that
    while the human eye has a nominal 'resolution' of 6 lp/mm it can often tell
    when an image (especially of type) is of a higher resolution; it can't count
    the lp/mm, only sense the difference.
     
    2, May 28, 2006
    #14
  15. C Wright

    2 Guest

     
    2, May 28, 2006
    #15
  16. C Wright

    2 Guest

    Well, it really shouldn't ever be put in such terms, but if you are using
    XP, then just right-click on the desktop, then take Properties, then the
    Settings tab. Click in there on Advanced and (daminit) there's "DPI"
    settings (96, 120, custom). Go crazy and take the Custom settings, like
    20%, and see how silly it can get.
     
    2, May 28, 2006
    #16
  17. C Wright

    Warren Sarle Guest

    That's not a useful "rule of thumb". The monitors I use range
    from about 32 to 120 ppi. But although you may well be interested
    in a web user's total screen resolution (say, 1024x768 or 800x600),
    there's no reason you should care how many pixels their monitors
    have per inch.
     
    Warren Sarle, May 28, 2006
    #17
  18. C Wright

    Warren Sarle Guest

    The resolving power of the human eye varies greatly from person
    to person, especially at close distances, but a typical value for
    a young person with good vision would be in the neighborhood of
    150 to 250 lines per inch. And a line requires at least two pixels.
     
    Warren Sarle, May 28, 2006
    #18
  19. SNIP
    It's called Vernier acuity:
    <http://www.photonicshistory.com/directory/dictionary/lookup.asp?url=lookup&entrynum=5559&letter=v>
    as relative to visual acuity:
    <http://www.photonicshistory.com/directory/dictionary/lookup.asp?url=lookup&entrynum=5605&letter=v>

    So according to that source, Vernier acuity is about 6x higher than
    visual acuity, which is why typeface needs to be printed at much
    higher resolution than continuous tone images.

    Bart
     
    Bart van der Wolf, May 29, 2006
    #19
  20. C Wright

    tacit Guest

    It has to do with the assumptions that programmers make when they design
    user interface controls, more likely.

    User interface objects on a computer screen are often designed to be a
    certain number of pixels in size; for example, the standard size of a
    button is typically 20 pixels high. If a screen has a resolution of 200
    pixels per inch, that makes the size of a button one-tenth of an inch
    high.

    Now, it's easy to see an object one-tenth of an inch high on paper. But
    we typically look at a sheet of paper from a much closer distance than
    we look at a computer monitor--and a sheet of paper isn't glowing, it's
    using reflected light. Looking at an object a tenth of an inch high on a
    glowing screen that's much farther away is a whole 'nother story. :)

    A computer screen that had a higher resolution but did not make the
    objects smaller would probably look significantly better. If the size of
    a button were the same on a 200 pixel per inch screen, say, and the
    button were composed of more pixels, it'd probably look quite good. But
    that'd involve dramatically re-thinking the way things are displayed on
    a screen.

    The Mac uses PDF (in OS X) or QuickDraw (in older versions) to draw
    images on the screen,a nd it is possible for a programmer to write a
    program that would display its user interface elements and text and so
    forth on the screen at the same physical size regardless of the
    resolution of the monitor if he so chose, because both PDF and QuickDraw
    are resolution independent. (He wouldn't be able to use pre-built OS
    controls; he'd have to draw them himself.) On Windows, which does not
    have a resolution-independent system for displaying things on the
    screen, it'd be much more difficult. (Windows Vista was supposed to
    include this kind of functionality, but Microsoft has announced that it
    has been dropped.)
     
    tacit, May 29, 2006
    #20
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