Understanding Resolution Part 2-Resampling

Posted on Sunday, January 27, 2008:: 1084 Views

Now lets look at a big NO NO when resizing your images to send to the lab. The big bad word in this case is "Resampling". Resampling occurs when you enlarge an image by increasing the resolution (Resing-Up or Up-Resing as it is commonly referred to). When you tell Photoshop or any other digital imaging program to increase the number of pixels per inch you are telling it to literally cram more pixels into every inch. Those pixels aren't just magically created, they have to come from somewhere. That "somewhere" is basically up to Photoshop. Photoshop looks at the surrounding pixels, takes the average of the colors and creates it's own pixels based on that information.

Lets look at this in the Image Size dialogue box in Photoshop. We see that our original image is a 6 x 4 at 72 PPI with a pixel dimension of 432 x 288. Magnified to 500 percent you can clearly see the pixels in the image to the left of the dialogue box in example a.

When we increase the resolution to 300 with the Resample Image check box checked (example b.), we can see how the document size (print size) doesn't change, but the pixel dimensions do. We've just told Photoshop to cram 228 more pixels into every inch. Now instead of 432 x 288 we have 1800 x 1200! This means that Photoshop has created 2,035,584 pixels since the original pixel dimensions were 432 multiplied by 288 (which equals 124,416, well under a megapixel). As you can see, the result of Photoshop trying to fill in and smooth out the new pixels is a blurry image.

In example c. we have unchecked the Resample Image checkbox and you can see that we no longer have the option to change the pixel dimensions. This is essentially going to let you know how big you can actually print your image at various resolutions.

 

 

Now look at example d.

 

By increasing the resolution to 300, the document size becomes very small at 1.44 inches by .96 inches. That wouldn't even work for a passport photo. Scratching your head in frustration wondering "So why did the image size go down?" Since resampling is turned off, our pixel dimension is fixed. This means that A; all of your original image detail will remain intact should you increase or decrease the resolution, but B; the higher you go with the resolution the smaller the pixels get. Remember, when you increase the resolution of an image the pixels get smaller to fill up the space in an inch. So since our pixel dimension is fixed at 432 x 288, and we increased the resolution to 300 PPI, we know that one inch is going to be 300 pixels in length. That only leaves 132 pixels which brings the print size down to 1.44 inches.

For you photographers out there who design your own flush mount albums, unfavorable resampling results can also occur from time to time when resizing images on a canvas in Photoshop. It's the same principle. Say you've sized an image to be smaller with the Free Transform Tool, but then change your mind later on and want it to be larger. After free transforming the image again to be larger, it probably looked kind of blurry and nothing like it did before you resized it. You've already thrown out some detail when you shrank the image the first time. By making it larger you told Photoshop to, again, fill in the missing detail with it's own interpretation of what pixels should fill in the empty space.

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