Back to Basics: Image Resolution

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Image resolution – how much detail an image holds – is measured in pixels per inch, or ppi. It is also referred to by the width and the height of the image as measured in pixels (i.e. 640×427, 1024×683, etc.). The more pixels the image possesses, the better the image quality.

Rules of Thumb

There are a couple of “rules of thumb” to consider when setting your image resolution.

For printing, you’ll want to use more pixels per inch, like 200-300. Increasing the ppi will allow you to print a larger physical copy while retaining the quality of the image.

For viewing on a monitor or on the web, which can only display a finite number of pixels per inch, setting the image at 72 ppi is just fine. Increasing the PPI for a picture you’re only going to look at on-line only makes the file size larger, it doesn’t improve the appearance of the image.

Do the Math

I know, I know, nobody said you’d have to do math! But in order to figure out how many ppi you need in order to print a high quality image of a specific size, you should at least know the calculations behind it. It’s easy, I promise.

Say you’d like to print an 8×10 image using a ppi of 300 (the standard recommended image quality):

8 inches tall X 300 pixels per inch = 2400 pixels
10 inches tall X 300 pixels per inch = 3000 pixels

So your image resolution would need to be at least 2400×3000 in order to print an 8×10 photo at 300 dpi. That would require a camera that can shoot at least 7.2 megapixels (2400 X 3000 = 7,200,000, or 7.2 megapixels).

You Don’t Have To Do Math

Thank goodness for photo editing software! Programs like Photoshop, Lightroom, Aperture, and Camera Raw have the ability to resize photographs and do the calculations for you. There are a couple of different techniques you can use, depending on if you’re shooting in RAW or JPG.

For RAW photos, you can set the pixels per inch in Camera Raw or Lightroom. For the purposes of this tutorial (and because I haven’t received my copy of Adobe Photoshop Lightroom 3 from Amazon yet), I’ll use screen shots from Camera Raw. It works exactly the same way in Lightroom.

Open the photo in Camera Raw. Perform whatever adjustments you deem necessary, then take a look at the blue hyperlink displayed at the bottom of the screen:

Click to enlarge

Click it, and a Workflow Options screen appears:

Click to enlarge

Under “resolution” you can see that my default is 240 pixels per inch (your default may vary). This is where you can change your ppi to whatever resolution you wish. Click on “OK”, then open the photo in your photo editing software and do your thing.

If you wish to adjust the image resolution for a JPG photo, the process is a bit different. For the purposes of this tutorial I am using Adobe Photoshop CS5.

Say you have a photo that was saved at only 72 ppi. In order to print a quality image you want to increase the ppi to 300. However, keep in mind that changing the ppi will reduce the physical dimensions of the photo, as it is “compressing” to fit more pixels into each inch.

Go to the “Image” menu and choose “Image Size”. The following dialog box appears:

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I kept the rulers in the screen shot for a reason, you’ll see why in a minute. Notice that according to the ruler, the physical size of the photo is about 45.5 inches by about 70.5 inches – a huge print that at 72 ppi would look very poor indeed.

Back to the dialog box. Notice that is split into two sections, “Pixel Dimensions” and “Document Size”. Leaving all other settings the same, un-check the “Resample Image” box, then change the “Resolution” figure to 300. Here is the result:

Click to enlarge

Notice that the pixel dimensions and the document size both changed, in accordance with the ppi set at 300. Click on OK to save the settings. Now when we look at the ruler, we see that the physical size of the image has been reduced dramatically:

Click to enlarge

Now the maximum physical printed size of this photo, at 300 ppi, can be no larger than about 11.5×17.5.

I hope this helps de-mystify any struggles you may have been experiencing with image resolutions. If you have any questions, please feel free to leave them in the comments.

Photo credit: “Macro of sharpened colored pencils” by Horia Varlan on Flickr Creative Commons.

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  • http://www.flickr.com/photos/mully410/ Mully410

    A crucial element to determining what DPI you need for a print is knowing what printer you will use.  The professional print shop I use has printers that are 240 dpi.  No need to set the DPI any higher.

  • Anonymous

    Excellent point, Mully.  Thanks!

  • Ed

    Thanks,
    Trying to get best setting for printing everyday 4×6 prints with little to no cropping, your overview of pixels and DPI should help me out much.