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PPI and DPI

Discussion in 'AP Magazine Feedback & Suggestions' started by PhotoEcosse, Jan 28, 2014.

  1. PhotoEcosse

    PhotoEcosse Well-Known Member

    I am not sure that Chris Gatum really undestands the matter of image resolution, image size and printing when he responds in this week's "Ask AP". In his defence, however, his misconceptions are very widely shared by writers and others.

    If I take a photograph with my Nikon D800, the image file measures 7360x4912 pixels. At that stage there is no ppi (or, of course, the totally unrelated dpi) involved. The resolution is 7360x4912. Full stop. If I use software to display that image on a computer monitor, I will probably want the software to downsize the image to give the best fit on my screens, both of which have a resolution of 1680x1050. The image on the screen will probably be around 1575x1050 pixels. It is only then that ppi has any meaning. If the image on my screen measures 18" x 12", then it is being displayed at around 87.5ppi. But - and this is the important but - that 87.5ppi is not in any sense a parameter of my image file. It is simply a characteristic of the way my software displays it on my monitor.

    If I want to print from that file, then my printer software and firmware will translate my image file to suit the parameters I have set for printing. It is quite likely that I will print at my printer's default of 300dpi. This bears no relation whatsoever to the 87.5ppi that I am seeing on my screen.

    How "big" a file, in terms of pixel dimensions, I need to produce a high quality print depends upon many factors - to name just a few: the number of ink cartridges in my printer, the dot pitch of the print head, the spacing between dots, the surface tension of the inks, the algorithm that my printer software uses to blend colours, the surface characteristics of the paper I use, etc., etc., etc.

    The main point that I want to make, however, is that the dimensions of the image file (not ppi or dpi) that will give me a great quality print are much less than any of the pundits seem to suggest. If, after processing and severe cropping, I am left with an image measuring 2000x1500 pixels, my Epson R3000 printer using Epson's own profile will produce a 16"x12" salon-quality print easily. That's a lot different from Chris's calculations. And the question of ppi does not enter the equation at all; the question of dpi is merely concerned with how my printer prints and is not a function of the image file.

    But don't feel too bad about it - it is relatively recently that Adobe sorted out the diffence and non-relationship between ppi and dpi.
     
    Last edited: Jan 28, 2014
  2. Roger Hicks

    Roger Hicks Well-Known Member

    Like many, I've always thought that equating PPI and DPI is (a) fairly sloppy but (b) a pretty good rule of thumb for photomechanical repro.

    To the variables already listed, add "subject matter". If you can always get a 12x16 inch "salon quality" pic out of 3 megapixels, you're doing well, but "salon quality" is a pretty wobbly definition of "quality". Don't feel too bad about it, though. People who postulate or seek absolutes in photography often fall flat on their faces.

    I think I'd back Chris. Otherwise I'd not see the difference between an M8 (10 megapixels) and an M9 (18 megapixels) with the same lenses. And I DO see a difference.

    Cheers,

    R.
     
    Last edited: Jan 28, 2014
  3. daft_biker

    daft_biker Action Man!

    I'm not sure you really understand the meaning of resolution as you have given us pixel counts......
     
  4. Fen

    Fen Well-Known Member

    Mr Jacobs wrote a brilliant post some time ago about dpi and ppi and even lpi, but I can't find it.

    Perhaps he'll see this and be able to post a link to it...
     
  5. spinno

    spinno Well-Known Member

    methinks you're being too modest....look at the stickies in the help section...
     
  6. Fen

    Fen Well-Known Member

  7. PhotoEcosse

    PhotoEcosse Well-Known Member

    It is you who fails to understand, daft_biker.

    How often have you seen test reports in AP refering to the resolution of a camera sensor as, for example, 12Mp or 24Mp or whatever? All the time. Similarly, just right click on your monitor and check "screen resolution" - once again it will give you something like 1680x1050.

    And that is where ppi becomes particularly meaningless - if an image is displayed on a 27" 1680x1050 screen you can determine the ppi by simply measuring the picture on the screen and applying simple arithmetic. Display the same image on a 21" 1680x1050 screen (taking up the same prorportion of the screen) and the resolution of the image is the same but the ppi is quite different. ppi is not a measure of resolution.

    Dpi, arguably, is a measure of resolution when applied to a printed photograph.

    We can all remember the days when we supplied magazines with MF colour transparencies. Those transparencies were then used to provide "separations" by scanning them through dot screens of a given dot pitch to create litho plates for each colour. The "resolution" of the printed photograph, in layman's terms, was determined by the dot pitch of the screens in much the same way as the resolution of an inkjet print is determined by the many factors (and more) listed in my original post.

    Yes - there will be a stage at which, for a given image, the file size (not ppi) becomes too small to produce a satisfactory print of given dimensions - but that is down mainly to lack of sufficient data to allow the printing software to cope adequately.
     
  8. Benchista

    Benchista Which Tyler


    I think he understands it better than you...


    Resolution has to have a spatial aspect. A number of pixels is NOT a resolution. International standards say so:

    "The term 'Resolution' shall not be used for the number of recorded pixels"

    A number of pixels in a given area is the resolution. Your D800's 7360x4912 pixels absolutely DO have a related PPI figure based on the size of your sensor - they don't exist in isolation. It has a resolution of approximately 520 PPI. Of course the file can be looked at at other resolutions, with a correspondingly different physical size on a monitor at 100%, for example, or different resolution for a given size, as in an A4 print. The issue in terms of printing is to get enough information into the picture for it to be of "photo quality", for want of a better term.


    The 300 PPI thing is just a rule of thumb, and relates to magazine (and book) printing, as he says. Chris is very clear:

    "Indeed, you could argue that the only measurement that really determines how large an image can be printed is the quality of the image. While the number of pixels plays a part, the fact is the higher the quality, the larger an image can be printed while still looking good. A pin-sharp shot taken using a 10-million-pixel camera will probably look a lot better at an A3 print size than a slightly soft image taken with a 36-million-pixel camera."

    For me, that's the most sensible thing I've seen written on the subject for a very long time, along with the rest of the answer.

    What on earth has he got to feel bad about, apart from the appalling Americanisation of using the phrase "tack-sharp" at one point - tacks really aren't as sharp as pins, there's no excuse for it - and the wanton cruelty to hyphens in the pixel counts? ;)
     
  9. Terrywoodenpic

    Terrywoodenpic Well-Known Member

    Put your hand in a bag of pins and shake it.. none will stick in to your fingers.
    Do the same with a bag of cut tintacks and your fingers will look like a hedge hog.
    Cut Tacks are incredibly sharp. It is only the modern stamped ones that are less so.
     
  10. daft_biker

    daft_biker Action Man!

    Sensors and screens have a physical size though. The point you were making was that your image did not yet have a physical size so it was only the use of the word resolution that I disagreed with.

    Aye, as a keen macro photographer I find ppmm more useful ;)
     
  11. AlexMonro

    AlexMonro Old Grand Part Deux

    While I'm happy that true resolution is a dimensioned quantity, usually expressed in terms of lines/length, I was wondering if we have an accepted term for "dimensionless image sharpness", that I've seen reported in some test reports in terms of lines per picture height (LPPH)?

    Although I think the reports were in relation to digital cameras, I can see that there could be a use for such a concept to compare film camera lenses for different formats, e.g. a 10x8 view camera lens and a 35mm compact. Or would you just use lines per mm of prints of the same dimensions, which would of course bring the resolution of the enlarging lens into the comparison, amongst other things.
     
  12. PhotoEcosse

    PhotoEcosse Well-Known Member

    In a discussion of this kind, it is very easy to lose sight of what the abbreviations actually stand for. In both "dpi" and "ppi", the "i" stands for "inch".

    In terms of a data file, that measurement has no currency.

    It only becomes meaningful when our file is converted to an image and is dispayed on a computer monitor or printed on to paper. Then we can measure the image in inches and count (or calculate) the ppi (on a monitor) or the dpi (on a print).

    As I suggested earlier, the "quality" of the displayed or printed photograph is dependent upon the file containing sufficient data for the software to use when converting the file for display or printing.
     
  13. Benchista

    Benchista Which Tyler

    Unless you're talking about its resolution...

    Which is quite correct, but paraphrasing what Chris actually said in his reply in the mag.
     
  14. Roger Hicks

    Roger Hicks Well-Known Member

    It does to most people in publishing, who are accustomed to dividing by 300 as a rule of thumb to guess how it might look in repro. Thus, you probably won't run a 900 x 1200 pixels image bigger than 3x4 inches. Or conversely, if you want a 5x7 inch image, you probably want 1500 x 2100 pixels. More pixels are no problem -- they're "spare" quality -- but most publishers will get nervous if they see fewer.

    Cheers,

    R.
     
  15. mark_jacobs

    mark_jacobs Retired

  16. Roger Hicks

    Roger Hicks Well-Known Member

  17. mark_jacobs

    mark_jacobs Retired

  18. PhotoEcosse

    PhotoEcosse Well-Known Member

    Maybe the proof of this particular pudding is in AP's own instructions to APOY entrants:

    No mention of ppi (or dpi of course)


     
  19. Fen

    Fen Well-Known Member

    Because there doesn't need to be.

    3000 pixels is 3000 pixels no matter what the ppi, dpi. A measurement of 3000 pixels is just that... a measurement. It has feck all to do with dpi or ppi.

    You could be viewing the image on your screen at whatever dpi/ppi you want. An image that is 3000 pixels will always be 3000 pixels.


    I'm beginning to think that you have selective reading, either that or just acting the troll!
     
  20. Benchista

    Benchista Which Tyler

    Yeah - AP know how big a file they need to print in the mag at 300 ppi, that's all that proves. Doesn't say it's a resolution either, because that would be plain wrong.
     

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