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Understanding Resolution in Photography

Understanding Resolution in Photography

This tutorial is the second in a multi-part series covering Aspect Ratios, Resolution, and Cropping vs. Resizing.

Resolution is one of those non-intuitive things that all digital photographers have to figure out eventually.  Why?  Because resolution has a direct effect on the quality of your printed photos.

Resolution is the number of pixels a digital image contains. This number is measured in megapixels.  If you buy a 17 megapixel camera, this means that the highest quality image that camera can produce will have 17 million pixels.  Think about a 4×6 with 17 million pixels – those pixels are going to be so tiny that you can’t see them, and your photo is going to look natural and realistic.

Say, however that the same 4×6 only has 100 pixels.  Divide that image up into 100 boxes, and fill in each box with a color.  Your image will look like a bunch of squares and will bear little resemblance to your subject.  This is what we call a pixelated image.

pixelated Understanding Resolution in Photography

When we refer to printed images, we discuss resolution in terms of dots per inch (or DPI). DPI refers to the number of color dots your printer puts into every inch of your image.

When we are talking about images on the internet, computer screens, tvs, etc, we discuss resolution in terms of pixels per inch (or PPI).

There are a couple of important questions for us as digital photographers.

First, how big can I make my image before it becomes pixel-y looking? In other words, does my digital file have enough pixels that can stretch across a large image without becoming visible to the eye?  The maximum print size of your image is limited by the number of pixels your camera put into it.  (Now, there are ways to add new pixels in Photoshop so that you can enlarge your image even further, but that’s a discussion for someone else to lead!)

Let’s return to the example of the 4×6.  Say that this image is 2400 pixels wide.  2400 divided by 6 inches = 400 pixels per inch.  That is more than enough to produce a quality print.

However, say we wanted to enlarge that 4×6 to a 40 x 60.  Now we have to divide 2400 pixels by 60 inches, giving us 40 pixels per inch.  That is not going to be a pretty print.

As far as the optimal DPI for a good print, it depends on the printer.  Photo labs or your home printer should have recommendations for you.  When I print, I aim for a minimum resolution of 240 DPI.

The 2nd question for digital photographers is, “What is the optimal size for displaying my pictures on the internet or emailing them?” The maximum PPI that that monitors, TVs, and other screens can display is 72 PPI.   If your image has a PPI larger than 72, those extra pixels are essentially wasted space.  This is an issue because they are going to slow down your upload and download times on the internet, and take up valuable hard drive space.

Technically, an image that is imported from a camera doesn’t have a DPI/or PPI setting.   But our importing software often assigns one for us, and sometimes cameras program a resolution number into the EXIF data of an image.  To get a good print, you might need to change this resolution of your SOOC picture, or you might not.

To view the current resolution/PPI of your image, type control+alt+i (command+opt+i on Mac) in either Photoshop or Photoshop Elements.  That is option+command+i on Macs.

resolution Understanding Resolution in Photography

Note that the resolution is only 72 PPI, but the width is 24 inches.  If I were to print it right now, as a small print, say 4×6 we would be fine.  If I tried printing it as a 24×36 with only 72 pixels per inch, it would be very pixelated.  To increase the resolution:

  1. Make sure Constrain Proportions is on
  2. Turn off Resample Image
  3. Change the resolution to your ideal print setting

Now you can see that the width of the image has changed to 7.2 inches wide.  Note that the pixel dimensions have not changed – we have not added or subtracted pixels because Resample Image was turned off.

res 2 Understanding Resolution in Photography

How big could I print this image?  It depends on the printer’s minimum resolution and whether I trust Photoshop to “Resample” the image by creating new pixels and trying to guess what they should look like.  (I usually don’t!)  I could probably push this image to a resolution of about 200 or so to get a larger print that still looks good. Consult your photo lab when printing larger prints to be sure you have enough information.

We’ve almost completed our journey to simplifying aspect ratio, resolution, cropping and resizing.  Any questions?

Want more information like this?  Take one of Jodi’s online Photoshop classes or Erin’s online Elements classes offered by MCP Actions.  Erin can also be found at Texas Chicks Blogs and Pics, where she documents her photography journey and caters to the Photoshop Elements crowd.

pixy3 Understanding Resolution in Photography
 Understanding Resolution in Photography

Erin Peloquin

Erin Peloquin is Lightroom & Photoshop Elements instructor. She also works for Jodi here at MCP to create great editing tools for Lightroom & Elements. Her portfolio is at Time in a Camera.

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15 Comments

  1. 1
    Marta says:

    Now that facebook has a download option for images, how do you size it so that people just can’t print their images straight off of facebook. I put my logo at the bottom, but it can easily be cropped. Is the only other option of protecting my images through watermarking? I just think it takes away from the images.

    • 1.1

      Marta, go to the image size box as described above and, with Resample checked, reduce the resolution to 72 ppi and the width in pixels to something less than 1000. People will still be able to download the images, but they will be low quality.

  2. 2
    christina says:

    wondering if you can answer a question about “slurping” blogs –I read awhile back that the 72ppi uploads to the web will be pretty grainy if you have your blog printed into a photo book, which I plan to (eventually) do. Do you know if this is true?

  3. 3
    Lillian Hoyt says:

    I so appreciate this post as well as the last post about aspect ratio. These are thingg I have never read anywhere else, but is so important to understand. Thank you for taking the time to share this info.

    As for this comment from you:
    “(Now, there are ways to add new pixels in Photoshop so that you can enlarge your image even further, but that’s a discussion for someone else to lead!)”

    I have been wondering how this works for awhile and would love a post on this (or a post on where I can find out more about this!).

    Thanks again so much. I love MCP Actions!

  4. 4
    Joshua says:

    Viewing Distance is another factor to consider.
    For instance, I had this image(2592×3888 pixels) printed on 24×36 canvas which translates to just 108 DPI. It’s only when viewing closer than an arm’s length that you can begin to notice that it’s digital.

  5. 5

    Coincidentally, I’m writing a post on the same subject. ;-) I find that people get hung up on the 300 dpi rule of thumb for print without really understanding it.

  6. 6
    DJH says:

    Good info. You could also use the save for web option in photoshop to resize for a web image…

  7. 7
    DJH says:

    I also use a tool called GenuneFractals. I trust this more than I do the PS tool

  8. 8
    Caylena says:

    Thanks for writing these posts.

    The aspect ratio post really helped me grasp it.

    I have understood ppi/dpi relatively and knew 72ppi and 300dpi as general guidelines – but was always confused with megapixels and how that fits into the mess.

    My DSLR produces images with 10.1 mp and when I open the RAW files in Photoshop it has 240 dpi/ppi. From your post, I gather that my images have 10,100 pixels and have a 2:3 aspect ratio. This much is clear to me. At this point, I have trouble understanding what to do with the numbers and at what size print the image quality will start to degrade/pixelate.

    Also, what will adding resolution – going from 240 to 300 dpi – for a print do to the image quality?

    • 8.1
      Erin says:

      Hi Caylena,the minimum dpi really depends on the specifications of the printer. Going from 240 to 300 might not do anything – again, it just depends on the printer.

      How many pixels wide is your image? Divide it by 240. That’s the size it could print with a resolution of 240. Print it larger, and quality will begin to degrade. How much? Depends on the photo, the printer, and the viewing distance. Does that help?

  9. 9
    Tina says:

    So after reading this i get it better.

    So i hope i have this right so 10,400/300= what i can print at?
    I know my photoshop takes FOREVER to save and edit these photos.

  10. 10
    Irena says:

    What is the point of changing resolution for facebook or flickr – it does it for you anyway? I’ve uploaded the same image in different sizes and resolution to see what happens and it does the same thing for both. Well, size might be still different (but smaller than original), but resolution is 96 dpi on facebook and 72 dpi in flickr.

    • 10.1
      Erin says:

      Hi Irena, if you change the resolution yourself, you have more control. You can also sharpen after resizing, to make sure that the sharpening is appropriate for the file size. The hope is that FB will compress your photos less (and therefore maintain more quality) if you’ve already resized the photo.

  11. 11
    Amanda says:

    Thanks so much for this info. Very helpful. I am also trying to figure out how to heavily crop an image and still keep enough pixels in the image for printing for a photobook project, prints, and canvas. Thank you

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