Along with all the conflicting approaches to image sharpening, the choice of an appropriate print resolution has more well intentioned but ill informed opinions that just about anything I can think of.  Print resolution is important because we want our prints to display all the fine detail our camera captured.  If your chosen resolution is too low, your print will look like a highway billboard viewed from three feet; if it’s too high you’ll waste ink, and might even end up with a lower quality print.

First off, don’t confuse print resolution with the dot patterns laid down by your printer:  the promotional literature for your printer might proclaim that it can lay down 2880 dots per inch, but this is not the same as print resolution.  Your printer lays down ink dots of varying sizes in complex patterns to achieve the perception of fine detail, using many ink dots to represent a single image pixel.  When we talk about setting print resolution, we’re talking about the numbers and settings in the Image>Image Size dialog in Photoshop.  Choosing the settings in this dialog will be one of the last decisions we make prior to sending an image to our printer (just before we do our final round of sharpening for our specific image size, resolution and output device).

If you were to open the Image Size dialog, you might see something like this:

This dialog is telling us that if we take the 5281 pixels making up the long side of our image, and spread them 240 to the inch across the page, we have just enough pixels to produce an image about 22 inches on the long side.  Now, if you use the same number of pixels to produce a smaller print, you would obviously end up with more pixels to the inch… go larger and you would end up with fewer to the inch.  And it is here, when we try to pick the appropriate number of pixels for a particular size of image, that all the confusion arises.  Some say that you should always print at 300 ppi, others say to print at 360ppi or some multiple of this (but only when the moon is full and Jupiter aligns with Mars!)

So how many pixels to the inch do you need to create an image that appears sharp and detailed?   Think about this for a minute, and you’ll realize that there isn’t much point in creating a print with detail finer than the human eye can see.  It’s also intuitively obvious that if you hold a print closer you can perceive finer detail than if you hold it farther away.  The "flip-side" of this is that a print that you would naturally view from a greater distance can have a lower resolution than one that you would naturally view close-up, and appear just as sharp.  (This is of course why that highway billboard can get away with golf ball size dots.)

When we view large prints we have a natural tendency to stand back and view them from a greater distance.  For most people a comfortable print viewing distance is something between one and two times the length of the diagonal of the print.  If you combine a comfortable viewing distance with the limits of the average person’s visual acuity, and translate all this into print resolution, you end up with ideal resolution numbers for prints of different sizes.

Let’s go back and look at Photoshop’s Image Size dialog:

Notice that the “Resample Image:” box is checked.  When this is so, I can change any of the Pixel Dimensions (Width and Height), the Document Size (Width and Height), or the Resolution.  If I change any one of them, the the number of pixels in our image will change.  When the Resample Image box is checked and we change the Size or Resolution of our image, Photoshop must either make up or remove pixels in our image to match the changes we make in this dialog.

Now, uncheck the “Resample Image” box.  The “Pixel Dimensions” boxes immediately becomes greyed out, and cannot be changed.  If we now change the size of the Document (say, for example I decide to print this image at 10” X 15”).  I am now taking the same number of pixels and spreading them over a smaller image, with the result of course that the number of pixels per inches increases.

“So what?”, you might say.  Well, the difference here is that when the “Resample Image” box is unchecked, Photoshop only sends the original “native” pixels to your printer, without making any up, or taking any away.  And here’s the kicker: it’s always better to send the native pixels to your printer, rather than having Photoshop make any up or take any away.  “Made up” pixels are just Photoshop’s best guess at the pixel information need to fill in the gaps, and will never equal the sharpness and detail of the original pixels in your image.  Your printer does a much better job at deciding how to arrange the pixel information on your print for maximum detail than does Photoshop (this is the opposite of the decision we make with color management, where there is no way we want the printer to mess with the colors in our images!)

So, go back to the table above relating different print sizes to optimal print resolution.  Turns out, with cameras of about 10 Megapixels and above, leaving the “Resample Image” box unchecked and using just the “native pixels” of your image will generally result in a print with resolution that falls within the guidelines of table.  For example, a 12.8 Megapixel Canon 5D (4368 X 2922 pixels) can produce an 8 X 12 print with 364 pixels per inch, and a 16 X 24 @ 180 pixels per inch, using just “native pixels” … both within the guidelines for producing images with all the detail that the average person can discern at a reasonable viewing distance.

Here’s the “Rule of Thumb”:

1. Start by un-checking the the “Resample Image” box when you set your final output size.
2. Set your print size as desired and have a look at the Resolution box, if you are between 180 and 480 pixels per inch at your final print size, you’re good to go!
3. One caution:  don’t be tempted to “push the envelope” beyond 480 ppi: your print will not improve, and most print drivers will have difficulty with this much data, possibly resulting in a less than perfect print.

Can’t get to 180 ppi with the native pixels in your image file?  That’s a topic for another day… and Lesson 3 in my course “The Art of Printing and Selling Your Art”.

This entry was posted on Saturday, November 14th, 2009 at 4:55 pm and is filed under Myths & Misconceptions. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.