It seems that we rarely get the details we really need from photolab and printing specialists whenever we ask if our image files are big enough for large-format or high-quality prints. For example, ask what size image would look good on 8r prints and most technicians will say 8 megapixels (MPs)–as if there was a correspondence between 4r prints and 4mp images, 5r and 5mp files, and so on. The actual minimum for an image to look as fine as it could on 8r print is 7.2mp and the math for figuring out this number is simple enough that we don’t have to settle for mere estimates.
When reproducing images, resolutions are expressed in dot pitch-72 pixels per inch or PPI for default screens at a low 640×480 setting, 96ppi for sharper 1024×678 displays, 180ppi for print publication minimums, and 300ppi for photographic reproduction. On the other hand, resolutions in digital cameras are expressed in mega-pixel or MP counts–like the 5mp that got digital SLRs to first be considered seriously, and the 12mp that is now common on point-and-shoot compacts.
Let’s look at the numbers behind these different takes on the resolution count. An 8 by 10 inch portrait photograph taken on your camera and printed for you by a photo-lab would have to be at least 2,400 pixels (8in. x 300ppi) wide and 3,000 (10in. x 300ppi) pixels high. And, a 2,400 by 3,000 pixel image measures 7.2mp in digicam resolution. It really is that simple-multiply 2,400 by 3,000 to get 7,200,000 or 7.2 million pixels.
Turning the equation around, a 12mp image with the typical aspect ratio of 4:3 (for every 4 units of length for one side, there are 3 units on the one perpendicular to it), would have side lengths of 4,000 and 3,000 pixels …
4a x 3a = 12,000,000
12a2 = 12,000,000
a2 = 12,000,000 / 12
a = square root of 1,000,000
a = 1,000
where 4a = 4,000 pixels
and 3a = 3,000 pixels
If you plan to reproduce a large 6 by 8 inch picture in a glossy magazine layout with sharp 300ppi images, how do you check if your JPEG file has enough resolution? Yes, it is as simple as it seems. Just multiply 6 inches by 300 and you have your width, multiply 8 by 300 again to get your height. So, you’ll need an image that is at least 1,800 pixels wide and 2,400 pixels high. In this case, even a 7.2mp image will be sufficient. All you have to do is down-size or crop it down from its 2,400 by 3,000 original. What you wouldn’t want to do is upsize a small, say 3.6mp, image and stretch it into the layout. Then, you’ll lose the one dot one pixel correspondence and your image will look all “pixelated” as they say–pixelated because the picture elements will be much larger than the ink-dots meant to reproduce each one of them.
Stop to think about it and you’ll realize that the resolution information stored with image files is just a vestigial convenience. Right-click the icon of your image file, click on “properties” in the pop-up menu, select the “details” tab, and finally scroll down to the “image” panel and you’ll find horizontal and vertical resolutions expressed in ppi, while image dimension is measured in pixels. What the resolution setting does is give your system the necessary variable to compute the dimension in inches if your file is outputted into various media. But, inches or any other real-world units for length hold little meaning while the image remains in digital form.
But in case you do need to tweak ppi resolutions (to have complete control of your photo-prints, or simply to help out your overworked team-mates) just do the following in Photoshop:
- Click on “Image” in the main menu and select “Image Size” in the drop-down list.
- In the “Image Size” window, look at the “Pixel Dimensions” panel in the top half of the window and note the pixel width and height of your image. For example, with a 7.2mp image, you’d note that it is 2,400 pixels wide and 3,000 pixels high.
- Then, look at the “Document Size” panel at the lower half of the window. Here, find the image’s width and height in inches, and the “Resolution” setting that yielded these measurements. Change the resolution from its camera default of 72ppi to a print-ready 300ppi.
- Don’t be surprised when the your change in resolution also changes numbers in the “Pixel Dimensions” panel up top, but not in the inch dimension for “Document Size” at the bottom of the window. For example, again with a 7.2mp image, the dimensions will have inflated to 12,500 x 10,000 pixels.
- Now, go back to the “Pixel Dimensions” panel and change back the pixel measurements to what they were before you changed resolution from 72 to 300ppi. Again with the 7.2mp image example, just change the dimensions back to 2,400 by 3,000 pixels.
- At the end of the exercise, you’ll have an image file that is essentially unchanged, without any kind of up-scaling or down-sizing, but with “Document Size” settings that are tailored for print reproduction, and that show you its actual size in inches if it is printed out.
At the end of the day though, these resolution settings are arbitrary, and your options for getting your images into fine and faithful hard-copies will always be determined by the pixel count.