The Pixels Underneath Your Photos

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Ever wondered about the pixels that make up your images? Today Jodi Friedman from MCP Actions dissects an image to shed some light on them.

I often get asked why resolution is important and what it all means. Since a “picture says a thousand words” maybe this can help you understand it better – rather than getting too technical. Below is a crop of my daughter’s eye (and part of her hat). I cropped it, than resized it (throwing out pixels). My end result was 30px wide and 72ppi. So when zoomed in at 3200% – this is the result. Your photo is made up of tiny squares as you can see. The bigger the photo and the higher the resolution, the smaller the squares. For excellent quality images, you want more pixels per inch and you want larger sizes. The more pixel dense a photo, the larger you can print.

 eye-resized-to-30px.jpg

Next is the full photo resized for my blog (so it fits) 900px wide. And the resolution of 72ppi (which is considered appropriate for web).

pre-sandiego-2.jpg

If I wanted to print, here are two examples of numbers that work. Both mean EXACTLY THE SAME THING!!!! Go under image – image size – in photoshop. You will see the size of your photo. Photos come off your camera as digital files at 72 pixel per inch (at least all my cameras do). My photo started at: 72ppi and was 33.3×50 inches. 2400×3600 pixels.

 eye-1.jpg

If we want the photo at 300ppi since some labs ask for that and some people feel they need 300ppi resolution, here is the result: 300ppi and 8×12 inches. Note that the pixels are the same 2400×3600 as the prior one! This is the exact same quality as 72ppi and was 33.3×50 inches.

 eye-2.jpg

Hopefully this photograph helped you start understanding the pixels beneath your photos.

This post on The Pixels Underneath Your Photos has been submitted by Jodi Friedman of MCP Actions. MCP Actions offers customized photoshop training and online photoshop workshops, photoshop actions, and other photoshop resources for photographers. To see more Photography and Photoshop tips and tutorials visit her blog at http://www.mcpactions.com/blog.

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  • Martin!

    As I understand is, the dpi is set by our OS Screen resolution and physical size of the monitor. (1280X1024 on a 17 inch monitor as a higher DPI than 1280X1024 on a 19inch…) When I display a picture on my monitor, It is displayed as one image pixel to one display pixel, unless the software resize the picture on the fly. I still don’t understand how dpi is used on our monitor or the web.

  • spiny norman

    A lot of people are confused by this, including design professionals for some strange reason, but there is no such thing as DPI with digital images, there are only pixels. The DPI setting is just a metadata tag that tells software how big to print, and even then it’s easy to over-ride. Changing it does not change the image – 1024×768 (or whatever) is always the same resolution whether the flag is set to 72 or 600 dpi. The DPI box in photoshop is useful when resizing for a set print size (or you can set the size in the print preview dialog), but otherwise you can ignore it .

    As martin says, web images display on the screen dependent on their pixel dimensions and the screen’s resolution, which varies depending on the monitor and computer settings but in this millenium is almost certainly not 72dpi. DPI only really exists when scanning or printing, and then it’s the digital image’s linear size in pixels divided by the print or original’s size in inches.

  • for printing you would not always use the 300 dpi resolution of the image which is only a waste on file size

    by checking the image size then clicking auto (below cancel) and if you know how many line screen the printer would use which is usually 150 lines/inch (but for magazines it would be 175 lines/inch) then the equivalent resolution is 225 dpi and for 175 lines is 263 dpi

    this is from what i have learned through the years and to lower the file size and save disk space

    thanks

  • I’m glad the commenters above get it, while the article at least implies that DPI is an intrinsic part of the image, while before printing DPI is of no more value than any other part of the metadata (like focal length, copyright notice, author name, keywords etc.). DPI defines the relation between the pixels that make up the image, and the size at which it is displayed.
    (LCD) Screens, like said above, have a fixed number of pixels. A screen has a certain DPI (or PPI) value, at which all images are displayed. The 72 DPI value given to images in camera is meaningless, the image does NOT change at all when you change this value.

  • Really, the most sensible way to use DPI when you’re printing photos, is to just decide the printed dimensions that you want (8×10, 4×6, etc.) and let the printing service figure out the resulting resolution for you. Nobody ever says “I want a 300 dpi image, what size can I print it at?” — they say “I want a 5×7 image, how good of quality will I have?”

    I’m actually writing up an article about this right now, running the math on what sorts of resolutions you can get (and whether they matter at all).

  • Jiang Do

    I own a Canon 50D.
    Is a photo taken with S (equivalent 5 megapixels) looks the same or worsen than taken with L (equilavent 15 megapixels) on a postcard sized print?
    I appreciate any reply, thank you.

  • Paul Monaghan

    from what i understand, epson printers work best with 360dpi.

  • I think I ended up more confused! Does this mean that changing the dpi from 72 to 300 won’t affect the size of my image when printing? I always thought it would… help please!! :$

  • If you change the DPI from 72 to 300, you have to keep something constant:

    – Keep the same size in pixels: your print will become smaller in mm.
    – Keep the same size in mm: your print will become the same size, but your file will consist of much more pixels.

  • Jane

    It’s rather throw away to say some people ‘feel’ they need 300dpi. If your photographs are to be printed professionally, or in any major publication, they will insist on nothing less than 300dpi. Give them a 72dpi quality picture and you might as well give them nothing.

    Professional print houses want nothing less than high-resolution. If you have only 72dpi, then your image will only have a small physical size. Try blowing that up to 300 to increase it to a reasonable print size, and you wind up with gross pixellation, as it attempts to create colour information for the pixels it needs to generate.

    The simple rule is if your picture is for the web or at home printing, 72 is fine. If it is going professional in any sense, then no less than 300. Shoot in 300 always and ‘dumb it down’ for web copy, and keep your 300 for your print copy.

    This, of course, all depends on your output. But if there is a chance at all it’s going to print, don’t risk only having 72. It might not save space, but who is running out of space these days with terrabyte drives going cheap cheap.

  • @Jane:

    Shoot in 300 always and ‘dumb it down’ for web copy, and keep your 300 for your print copy.
    Read more: The Pixels Underneath Your Photos – https://digital-photography-school.com/the-pixels-underneath-your-photos#ixzz0C6S6rYP9

    There is no such thing as shooting in 300dpi (or 72, or whatever). DPI Only has meaning when talking about a physical print size in relation to the pixels in the image. A digital camera has a fixed number of pixels, which it may or may not save. Whether the image that comes out of the camera is saved ‘at’ 72 or 300 dpi has no effect on the pixels present in the image. For an example (in Dutch unfortunately, but the images are self-explaining) of the same image at 1, 72 and 300dpi, see: http://farawayplaces.nl/weblog/2008/12/dpi/ (images at the bottom of the post).

Some Older Comments

  • Joost van der Borg April 9, 2009 02:19 am

    @Jane:

    Shoot in 300 always and ‘dumb it down’ for web copy, and keep your 300 for your print copy.
    Read more: The Pixels Underneath Your Photos - https://digital-photography-school.com/the-pixels-underneath-your-photos#ixzz0C6S6rYP9

    There is no such thing as shooting in 300dpi (or 72, or whatever). DPI Only has meaning when talking about a physical print size in relation to the pixels in the image. A digital camera has a fixed number of pixels, which it may or may not save. Whether the image that comes out of the camera is saved 'at' 72 or 300 dpi has no effect on the pixels present in the image. For an example (in Dutch unfortunately, but the images are self-explaining) of the same image at 1, 72 and 300dpi, see: http://farawayplaces.nl/weblog/2008/12/dpi/ (images at the bottom of the post).

  • Jane April 7, 2009 07:33 pm

    It's rather throw away to say some people 'feel' they need 300dpi. If your photographs are to be printed professionally, or in any major publication, they will insist on nothing less than 300dpi. Give them a 72dpi quality picture and you might as well give them nothing.

    Professional print houses want nothing less than high-resolution. If you have only 72dpi, then your image will only have a small physical size. Try blowing that up to 300 to increase it to a reasonable print size, and you wind up with gross pixellation, as it attempts to create colour information for the pixels it needs to generate.

    The simple rule is if your picture is for the web or at home printing, 72 is fine. If it is going professional in any sense, then no less than 300. Shoot in 300 always and 'dumb it down' for web copy, and keep your 300 for your print copy.

    This, of course, all depends on your output. But if there is a chance at all it's going to print, don't risk only having 72. It might not save space, but who is running out of space these days with terrabyte drives going cheap cheap.

  • Sybren April 4, 2009 02:38 am

    If you change the DPI from 72 to 300, you have to keep something constant:

    - Keep the same size in pixels: your print will become smaller in mm.
    - Keep the same size in mm: your print will become the same size, but your file will consist of much more pixels.

  • Jele! March 31, 2009 12:38 pm

    I think I ended up more confused! Does this mean that changing the dpi from 72 to 300 won't affect the size of my image when printing? I always thought it would... help please!! :$

  • Paul Monaghan March 27, 2009 12:13 pm

    from what i understand, epson printers work best with 360dpi.

  • Jiang Do March 27, 2009 12:04 pm

    I own a Canon 50D.
    Is a photo taken with S (equivalent 5 megapixels) looks the same or worsen than taken with L (equilavent 15 megapixels) on a postcard sized print?
    I appreciate any reply, thank you.

  • dcclark March 27, 2009 01:22 am

    Really, the most sensible way to use DPI when you're printing photos, is to just decide the printed dimensions that you want (8x10, 4x6, etc.) and let the printing service figure out the resulting resolution for you. Nobody ever says "I want a 300 dpi image, what size can I print it at?" -- they say "I want a 5x7 image, how good of quality will I have?"

    I'm actually writing up an article about this right now, running the math on what sorts of resolutions you can get (and whether they matter at all).

  • Joost van der Borg March 26, 2009 06:47 pm

    I'm glad the commenters above get it, while the article at least implies that DPI is an intrinsic part of the image, while before printing DPI is of no more value than any other part of the metadata (like focal length, copyright notice, author name, keywords etc.). DPI defines the relation between the pixels that make up the image, and the size at which it is displayed.
    (LCD) Screens, like said above, have a fixed number of pixels. A screen has a certain DPI (or PPI) value, at which all images are displayed. The 72 DPI value given to images in camera is meaningless, the image does NOT change at all when you change this value.

  • Joey Rico March 26, 2009 10:34 am

    for printing you would not always use the 300 dpi resolution of the image which is only a waste on file size

    by checking the image size then clicking auto (below cancel) and if you know how many line screen the printer would use which is usually 150 lines/inch (but for magazines it would be 175 lines/inch) then the equivalent resolution is 225 dpi and for 175 lines is 263 dpi

    this is from what i have learned through the years and to lower the file size and save disk space

    thanks

  • spiny norman March 26, 2009 09:45 am

    A lot of people are confused by this, including design professionals for some strange reason, but there is no such thing as DPI with digital images, there are only pixels. The DPI setting is just a metadata tag that tells software how big to print, and even then it's easy to over-ride. Changing it does not change the image - 1024x768 (or whatever) is always the same resolution whether the flag is set to 72 or 600 dpi. The DPI box in photoshop is useful when resizing for a set print size (or you can set the size in the print preview dialog), but otherwise you can ignore it .

    As martin says, web images display on the screen dependent on their pixel dimensions and the screen's resolution, which varies depending on the monitor and computer settings but in this millenium is almost certainly not 72dpi. DPI only really exists when scanning or printing, and then it's the digital image's linear size in pixels divided by the print or original's size in inches.

  • Martin! March 26, 2009 08:28 am

    As I understand is, the dpi is set by our OS Screen resolution and physical size of the monitor. (1280X1024 on a 17 inch monitor as a higher DPI than 1280X1024 on a 19inch...) When I display a picture on my monitor, It is displayed as one image pixel to one display pixel, unless the software resize the picture on the fly. I still don't understand how dpi is used on our monitor or the web.

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