DPI in Photoshop

Discussion in 'Beyond the Basics' started by mashworth1, May 24, 2009.

  1. mashworth1

    mashworth1 TPF Noob!

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    Can anyone tell me exactly how to set a photos dpi without making any other changes. In other words I want my photos to be at 300 dpi at their largest possible size. I need step by step instructions to resize the dpi and let photoshop handle the relative change in height and width dimensions.

    Any help would be appreciated.
     
    Last edited: May 24, 2009
  2. Garbz

    Garbz No longer a newbie, moving up!

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    Well firstly realising that it's called PPI (Pixels Per Inch) will get you 90% of the way there ;)

    Click edit -> Image Size
    In the dialogue untick the checkbox at the bottom that says "Resample Image" and all you're left with is the option of selecting a Width, Height, or PPI.

    If you legitimately wanted to change the Dots Per Inch, then that's a setting in your printer driver. Photoshop has no choice over this, but I doubt that's what you meant.

    Also I'm not quite sure what you're doing. But the requirement for 300PPI is a misnomer since it assumes you're talking about a 6x4 photo. You can easily get away with printing at 100PPI 30" wide since no one will view that from about 20" away. Thus the perceived quality is the same. Rule of thumb is 300PPI means it can be viewed at a comfortable arms length with 20/20 vision and not see the pixels. So if someone is standing a meter away then they would perceive 250PPI as photo quality. From about 2m it's down to about 175PPI etc. Photo quality does not mean 300PPI in all instances.
     
  3. mashworth1

    mashworth1 TPF Noob!

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    I'm entering a photography contest. The submissions must be 300 dpi----not ppi.
     
  4. Andrew Sun

    Andrew Sun TPF Noob!

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    * PPI affects the print size of the image
    * DPI only refers to the printer

    However, in photoshop they are both determined by 'Resolution'

    @
    mashworth1, The photo DPI is mostly set and determined by the camera itself. Bump it to the best quality and the largest size and if your camera is not too old, it should be able to shoot 300dpi.

    In photoshop (assuming you have and use it), you can actually change the DPI / PPI settings but it's not recommended to touch it unless you completely understand image optimization and how the final product is going to turn out (printed). Once you shoot your photo in 300dpi, you don't even need to go through photoshop if you don't want to. But if you're talking about up-converting a photo to 300dpi, then you're going to see severe quality-loss because you'll be using limited amount of pixels to cover more space.

    Here's a screenshot of where the DPI / PPI is located in photoshop.
    Basically your photography competition asking for 300dpi means that where is says 'Resolution', it needs to be 300. Hope this helps.

    [​IMG]
     
  5. Dwig

    Dwig TPF Noob!

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    They are using DPI incorrectly. They should be saying PPI. DPI (Dots Per Inch) is NEVER a term than can properly be applied to a digital image under any circumstances. Many people use it incorrectly and many otherwise good software applications use it incorrectly.

    What counts in image quality is the size of the image in pixels. PPI, itselft is a meaningless term. Anyone who ever uses PPI as a measure of image quality without also specifying the image size refered to is simply showing their ignorance.

    A 300x300 pixel thumbnail can be set to 300ppi and it woun't be any higher resolution than if it was set to 100ppi. The PPI setting is mearly a note in the image header that tells a page layout, or similar, application how large to display the image on its virtual page. The 300ppi 300x300pixel image would be scaled on the page so that it is 1" x 1" and the 100ppi 300x300pixel image would be scaled to 3" x 3". Both have the same pixels and are of the same quality intrinsically and would have exactly the same detail.
     
  6. In2daBlue

    In2daBlue TPF Noob!

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    Well, they would be using it correctly if the photography contest requires prints and not digital images.
     
  7. Dwig

    Dwig TPF Noob!

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    Only if they are discussing the resolution of the printer (spacing of ink droplets) and not that of the digital image that was printed. Its still PPI when discussing the image itself even when its on paper.
     
  8. Moglex

    Moglex TPF Noob!

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    Not with current printing technologies.

    A particular printer may print at 1200 dots per linear inch which is 1440000 dots per areal inch but, because no current printing technology can produce droplets of any colour the printer has to use some of the available resolution for 'dithering' to obtain its available gamut.

    Unfortunately it's not even possible to come up with an accurate PPI figure for the printed output because the amount of dithereing required varies depending upon the colour in question.
     
  9. KmH

    KmH Helping photographers learn to fish Supporting Member

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    "For the average person any sufficiently advanced technology is indistinguishable from magic." Arthrur C. Clarke :lol: :lol: :lol: :lol:
     
  10. Garbz

    Garbz No longer a newbie, moving up!

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    In which case they want horrid quality prints. Tabloid magazine quality printers print 600 DPI. My very cheap inkjet here prints 3600DPI.

    God I love this quote. It works in so many ways. Especially since most forms of magic can be explained through the use of advanced technology. :lol:
     
  11. Moglex

    Moglex TPF Noob!

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    That depends on how you choose to interpret what is an absurdly ambiguous 'unit'.

    DPI means absolutely nothing when refering to anything existing within the computer. The earliest it can take on a meaning is when you actually initiate a print at at a certain size at which point there will be a certain number of 'dots', in the sense of 'pixels', available per inch of requested output.

    I'm pretty sure that is what the competition requires. It would provide a perferctly adequate quality.

    If you output (in linear terms) a 3000 pixel internal image to produce a 10" printed image that is 300 source pixels per inch and the program you are printing from will interpolate these to the specified resolution of the printer thus sending it 360000 linear pixels. Depending on how the printer engineers have produced the dithering algorithm those 360000 pixels will be processed to produce some other number of logical dots per inch based on such factors as the minimum physical dot size, variability of droplet size and number of inks.
     
  12. Garbz

    Garbz No longer a newbie, moving up!

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    Yep Molex, both of us are talking about PPI vs DPI.

    Or simply:

    PPI is the resolution of the image as it is sent to the printer.
    DPI is the number of dots the printer prints on the paper to make up the coloured pixels.
     

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