Image Size vs Image Resolution vs Pixel Count

Discussion in 'Digital Darkroom' started by vivid light photography, Sep 2, 2003.

  1. I have a question about image size vs image resolution vs pixel
    count. I've been shooting all my images so far at the highest image
    size or resolution of 2000 x 3008 pixels. When I check the image size
    in PS7 the default resolution is 300 dpi, the image size is about
    6x10 rougly and the pixel count is 2000 x 3008 pixels. If I crop with
    the option boxes cleared the image size and pixel count will adjust
    but not the resolution; it remains at 300 dpi. If I crop with an
    image size constraint in the crop options field (say 5 x 7) then the
    resolution adjusts as well as the pixel count and the image size; and
    this is expected since I specified the image size and the three are
    all dependent on each other. No resampling has been done at this
    point. I've read that to get a better quality print you should use an
    image resolution that divdes into the resolution of your printer
    evenly. I have a Canon i950 and the resolution is 4800 dpi so that
    leaves an image resolution of 480, 400, 320, 300, 240, 200, 192, etc.
    that can be used then. When I crop a 2000 x 3008 pixel 300 dpi image
    using a 8.5 x 11 image size contraint the resolution drops to some
    arbitrary dpi below 300. Is this going to be an issue when I print or
    should I resample to the next resolution that divides into 4800. I
    don't want to resample down and throw away resolution and I've read
    that you never really want to sample up either. I'm a little
    comfused. If I crop an image at 4.25 x 5.5 and the resolution is say
    300 and I fit to sheet during printing on an 8.5x11 sheet isn't it
    the same print quality and resolution as if I cropped the same image
    at 8.5x11 which would be and didn't have to fit to sheet? I like
    cropping with image size constraints turned on so I know I won't get
    any more cropping during printing. Should I even be conserned about
    the image resolution not divding into the printer resolution evenly?
    Any help would be appreciated. Thanks.
     
  2. There is a lot of information about this topic in the forum archives, so you can get something by searching about print resolution. You can also get some info at normankoren.com, luminous-landscape.com, and many other places.

    That said, I'd strongly urge you to try it a few different ways and see where you start to see degradation in the image. I have not found that it makes a huge difference to print in an even divisor of the printer resolution because of good printer drivers (though I get that the math says it should), but I'm not familiar with how the Canons handle it (can't say good or bad). Here's a way to try this out, though: try printing the same image (preferably a good, sharp section) at various resolutions on the same sheet, side by side. Try, say, 150, 180, 240, 300, 360, and see the difference. Try it with the same image size, but different resolutions and try it with the same image piece, but let the printed size change with different resolutions.

    Also, don't be terrified of upsampling. Good digital images can be upsampled to some extent before the quality loss becomes unbearable. You probably can't print your 3000 pixel image to 60" and view it up close without some problems, but you might be able to print it above 10" at 300dpi...
     
  3. A couple of things to note Darrel

    1) the i950 has a resolution quoted as 4800 x 1200 dpi. With printers its the second figure that determines the image resolution (ie the pixels per inch that get onto the paper). The first figure refers to the quality of the "overpainting" done by the printer to give good colour gradation. The second figure divided by the number of dots required to reproduce a pixel will give you the printed image reolution (that is not always the same as dividing by the number of inks). There is stacks of info on this topic on various groups on the net.

    2) The resolution of the image you supply a printer should be at the "native resolution" expected by the print driver. Some apps like Qimage give this figure and any other resolution will cause the print driver to interpolate the image up/down to get what it expects.

    3) Canon recommend that the optimum resolution for i950 (and i850) files to be printed is 360 pixels per inch.
     
  4. ... the i950 has a resolution quoted as 4800 x 1200 dpi ...[snip]... Canon recommend that the optimum resolution for i950 (and i850) files to be printed is 360 pixels per inch.
    Hmm... neither 4800 nor 1200 are evenly divisible by 360.
     
  5. Canon sets the native resolution of their DSLRs at 180 dpi, which despite
    being lower than the much touted 300dpi standard, works exceedingly well.
    The 300 figure comes from using a 150 line screen in the printing industry,
    and having a 2x oversampling rate for safety. It does NOT directly relate to
    direct digital imagery, and certainly not to inkjet prints which use stochastic
    patterns rather than conventional screens. More than one inkjet dot mix to
    reproduce one pixel, so the printer res. does not particularly matter.
    Certainly having the print driver do less math is convenient - for the print
    driver. But we buy computers so that THEY do the work...let them. Modern
    drivers are perfectly capable of doing the math, so all that really matters is
    image quality. In a nutshell, 180 is good for general use, 360 is the max
    EVER needed. Don't believe it? Try it and be pleasantly surprised. What really
    matters for printed image quality is sharpening and level/curves gradation.
    Resolution just isn't all that important.
     
  6. Thanks everyone so far for the responses. But here is the one question I really would like help on. If after opening an image in PS7 taken from my camera (default 2000 x 3008 pix, 300 dpi, 6 x 8 image size) and I do a little cropping therefore dropping the resolution a little in the process because I specified the image size to constrain the crop proportion (5x7, 8.5x11, etc.) should I just leave the new resolution alone or should I upsample the image back to 300 dpi before printing? Sounds like it really doesn't matter that much with todays print drivers. Thanks.
     
  7. Let's get one thing straight here, Photoshop could care less about resolution, that is
    until you send your file to an output device. Even then Photoshop only wants to know
    how tightly to pack the pixels so that they can fit onto the output device. What
    Photoshop really wants to know is how many pixels wide by how many pixels tall.
    Nearly all filters in Photoshop are resolution dependent, that is, the higher the
    resolution, the higher the filter settings will need to be.

    The same is true of digital cameras, all they really know is "how many pixels wide by
    how many pixels tall". Most digital camera are set to the arbitrary 72ppi as that is the
    "standard" resolution of most monitors. Image size and resolution are inversely
    proportional to each other for any given image size (in Megabytes). If you go to
    image>image size dialog box and uncheck Resample image ( think of that as the
    scale box) you will notice that as Document size increases, resolution decreases and
    Pixel dimentions remains constant.

    When you use the crop tool, the document size is scaled first and then resolution is
    established, not a good combo. I asked why at the Adobe developers conference and I
    was told "because it has always been that way". I personally go to Image>Image
    Size>uncheck resample image first. This establishes the maximum print (document)
    size without interpolating.

    The next issue is Printer Resolution. Photoshop's resolution and your printer's
    resolution are NOT the same thing. Photoshop is using square pixels and your printer
    is using (usually) round spots or halftone dots. Put a square peg in a round hole and
    something has got to give. A general rule of thumb is to devide the printer's (desktop
    printer that is) output resolution by 3 and that is the mazimum you should ever build
    you Photoshop document to. For practical purposes anything over 250-300 dpi is a
    waste of render time and ink. I have had people that claim they can tell a big
    difference at higher resolutions. To that I have a one word responce, Bull$*&t. Under
    a loope maybe, from 3 feet nearly impossible. Run a test for yourself and see. Ink dot
    gain and paper quality alone will kill higher print resolution.

    Regards,

    Will Hammond
    Adobe CTT Photoshop
     
  8. I have a recently purchased a Cannon XTi SLR. I found that opening jpg images directly in Photoshop CS2 (version 9.0.2) defaults to 72 ppi (and yes, I set the camera to Large/Fine - 3888X2592).

    I wanted to create 6" X 4" prints... when I changed the Document Size to 6" wide X 4" high, I did not get the image quality I expected. This was a consequence of (as Will Hammond mentioned) the total number of pixels used in the image: only 432 X 288 pixels.

    I found that if:

    - I open the original jpeg in "Digital Photo Professional, version 3.0.1.5" (it came on a CD with my XTi)
    -then go to "Tools|Transfer to Photoshop"

    the same image opens at 350 ppi! After doing my 6" X 4" resize, the Pixel Dimensions were 2100 X 1400 - almost 5 times better than the 72 ppi version.

    Don't ask me to explain why or how Photoshop opens the same image at different resolutions. All I know is that (when printing) I got better quality with the higher resolution.

    To Will's point, I probably should be somewhere between 350 ppi and 72ppi. My point though is that the 72 ppi default was not sufficient.

    If you're intent is to use your images on the web, this need not be a concern. In fact, you should consider the "Save for web" option in Photoshop to extract metadata that you may not want out there.

    FORege
     
  9. Spearhead

    Spearhead Moderator Staff Member

    Will is right, and there is a lot of wrong information here. It doesn't matter what shows as "dpi" in the image out of the camera, in the image in Photoshop, or the image on the web. It only matters what is set there when one goes to output to a device that measures in inches, i.e., a printer. Before that, it's irrelevant. You can easily see this by saving the same image at 1dpi and 100000dpi - they both have the same file size and the same images dimensions in pixels.

    Until you get to set up for printing, ignore the number that shows as "dpi" in any file.

    It's really important to get this - that "72dpi" coming out of your camera is meaningless. It affects absolutely nothing - repeat - nothing - and says nothing about the image file from the camera. It's an arbitrary number chosen because something has to be there when Photoshop imports the image. Digital image sizes are defined by number of pixels x number of pixels, as Will says. Physical print sizes are determined by number of inches (or the metric equivalent). DPI as it appears in an image, or PPI as it should be called, defines the relationshop betweeen the number of pixels and the number of inches when you go to print.
     

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