Photoshop excels at preparing raster graphics like photos and scanned images for print. It offers to help images look their best, and can also help us make sure our images will print properly. Photoshop can also be used to adjust an image's resolution and print size. In this section, we'll examine an image that was taken with a digital camera for use in a research poster, and prepare it for printing. This will include checking the following:
What color model the image is using
The image's resolution
The image's print size
We'll want to make sure that the image is using the CMYK color model — given that the image was taken with a digital camera, it's likely using the RGB color model. We'll also want to ensure the resolution of the image is higher than 150 dpi, and that the print size of the image is no smaller than 4 by 6 inches. The image can be larger than the intended size, as we can size it down in a print layout program without sacrificing image quality, but we don't want the image to be too large.
The first things we'll check are the image's color mode and resolution, and adjust them as necessary.
Checking an image's color mode
The image we'll be working with is jules.png - let's open the file, then explore how to check the image's color mode and resolution.
NOTE: The following steps assume that you have downloaded the exercise files and extracted the folder Print_Graphics to your computer's desktop.
To open jules.png, in the Menu bar,
Click File, Click Open...
To move to the desktop,
To open the Print_Graphics folder,
To open the file, in the Print_Graphics folder,
Now that we have the image open, let's check what color mode the image is using. As discussed earlier, the two main color models (which in Photoshop are refered to as color modes) are RGB and CMYK. When we're working with graphics that will eventually be printed, we'll want to make sure they're using the CMYK color mode so the colors in the image print properly. Since this image was taken with a digital camera, chances are it's using the RGB color mode, but we should double-check before changing the image's color mode.
Let's check the color mode of this image, and change it if necessary.
To start the process of checking the color mode used, in the menu bar, Click Image, Point Mode
The different color modes will be listed in the menu, with the active color mode indicated with a checkmark, as seen below:
To start the process of converting the image's color mode to CMYK, in the menu bar, Click Edit, Click Convert To Profile...
To choose the proper profile, if necessary, in the Destination Space drop-down of the Convert to Profile dialog box, Click (ddarrow), Click Working CMYK - U.S. Web Coated (SWOP) v2
To finish converting the image to CMYK, Click (bttn_txt_ok)
The image has now been converted to CMYK — while there won't be any visible differences on your computer screen, the image is now using the appropriate color mode for printing.
NOTE: There are many different color profiles available in Photoshop — the one used today is a generic CMYK profile that's best for general use. If you're sending an image or document off to be professionally printed, you'll want to check with the printer to see what specific profile your files should be using.
Checking an image's resolution and print size
Now that the image's color has been converted to CMYK, let's check the image's resolution and print size. We can use the Image Size dialog box to check both of these values, as well as adjust the image's resolution and print size within reason. As you might remember, raster graphics are made up of individual pixels that, when viewed on a screen, make up an image. Each pixel in an image will correspond to a dot of color when printed. When adjusting an image's resolution to make it higher, we're telling the computer and printer to pack more pixels, or dots of color, per linear inch. In the process of raising the resolution, our image's print size will also change — the higher the resolution, the smaller the print size of the image. If we're adjusting the resolution of an image that's already rather small, raising the resolution will likely result in an image that is too small to print properly. The following table shows some different image dimensions and the highest their resolution can go while keeping print dimensions of roughly 3 by 4.5 inches, to help illustrate how changing the resolution can change the print size.
Print dimensions at 72 dpi
Highest resolution at print dimensions of 3 x 4.5 in
400 x 600 px
5 x 8 in
720 x 1080 px
10 x 15 in
1440 x 2160 px
20 x 30 in
When we indicate to Photoshop that we'd like to print more pixels per square inch of surface, Photoshop will reduce the size of the image in order to accomodate the change in resolution, as we're packing more pixels in an inch than previously. Images that have larger pixel dimensions will give us more freedom to adjust the image's resolution, as there's more data available in the image to work with. When adjusting an image's resolution, we'll want to make sure we turn off resampling, to ensure we keep all of the image's original information. With resampling turned on, Photoshop will add or remove pixels to our image (based on how we're resizing our image), and this can often result in lower quality images.
Let's use the Image Size dialog box to check the image's resolution and size, and adjust the resolution as needed.
To open the Image Size dialog box, in the menu bar, Click Image, Click Image Size
The Image Size dialog box will open:
(dbox_img_size) (highlight size and resolution)
The image's print size is incredibly large, but the resolution is only 72 pixels per inch. Since the image's resolution is too low to print well, the resolution will need to be changed in order for the image to print at a high quality.
To adjust the image's resolution without resampling the image, in the Image Size dialog box, Click to uncheck the checkbox next to Resample
To change the image's resolution, in the Resolution field, Press & Drag value, type: 300
To accept the changes we've made, Click (bttn_txt_ok)