Image Resolution
Print resolution, and monitor resolution are not the same. This leads to a great deal of confusion for new users of image editing programs.
If you have a one inch square picture that you want to print out on your inkjet printer at 700 ppi, which stands for pixels per inch, you will scan it into your computer with the scanner’s Output value set at 700 ppi, as well as the desired printout height, and width of one inch by one inch. Your printer and scanner understand each other.
Your monitor has a fixed number of pixels, depending on the resolution you have it set for. If you don't know your monitor’s display setting, right-click on a blank area of your desktop, and choose Properties from the menu. Click on the Settings tab. The slider labeled “Screen Area” indicates your monitor’s resolution in pixels.
If you scan an image into your computer with an intended output resolution of 700 ppi, the only way the monitor can display this information is to use the current screen resolution. Therefore, the 700 ppi, one inch square image will be displayed on the monitor as approximately ten inches square (using the common “average” monitor resolution of 72 pixels per inch, or ppi).
Output will be as intended, i.e. 700 ppi, and one inch square. It is confusing, nevertheless, to work on an image ten inches square while remembering that it is “really” going to be one inch square.
Please remember that there is no point at all in ever using anything other than pixel resolution for images intended solely for display on computer monitors (i.e. Web site images).
To find the resolution of the active image in Photoshop, choose Image > Image Size. The resolution will be found in the Document Size part of the dialog box that appears.
Below are several illustrations that should help you visualize what is going on.
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Second from the left is a 100 % magnification monitor display of the same image which was scanned in at 150 ppi, and is set to print at a resolution of 150 ppi.
Third from the left is a 100 % magnification monitor display of the same image which was now scanned at 300 ppi. It is interesting to note that Photoshop adjusts their rulers display to match the images print size. Therefore, an inch on the 300 ppi image is much larger than the inch shown with the 72 ppi picture.
At far right is a 100 % magnification monitor display of the image that will print at 600 ppi.
All of these images will print to be the same size. Only the resolution will be different.
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For those of you who are obsessive about this kind of thing, I’ll include an illustration of the above detail at comparable magnification. You will note that the 72 ppi image, above right, is at 500 % magnification, while the 600 ppi one is only at 66.7 % (see the image capture’s title bar). What does the 600 ppi image look like at high magnification?
The screen capture below shows the little knot at center in the 600 ppi detail above left, magnified to 600 %.
If you are printing exclusively to a desktop inkjet printer, it is generally best to use a resolution of between 200 and 300 ppi. Do some tests with your own images to see if you find any visible difference at higher resolutions.
Please note that the resolution setting that you will find in your printer’s dialog is not the same as that found within Photoshop. Printers use dpi, which stands for dots per inch. A printer can use multiple tiny dots to express the content of one pixel (depending on how dark it is—dark pixels will use more dots, light ones will use less). Therefore, your printer resolution can be set to, for example, 1440 dpi when printing your image that has its resolution set at 300 ppi.
Also, please note that scanner dialog’s often call their resolution settings dpi, which is incorrect. They should be called ppi. Note, also that the print setup dialog within Photoshop appears to have a maximum setting of 600 dpi. This can be disregarded. The printer will use the setting that you choose from within its own dialog box.
If you are doing prepress work (which I know very little about) you generally need a ppi resolution of from 1.5-2 x lpi (lines per inch). However, this is dependent on how close, and for how long viewers will be expected to look at your image. For example, billboards usually require only 18-36 ppi. The best way to find the resolution you should use is to call your printer and ask for their specifications.
If your artwork includes line art, text, or vector objects, you will need a higher resolution. For line art (which will not be screened during output) use 1200 ppi. For images with text or vector objects, use 600 ppi or higher.
If you need to resize an image that is already in Photoshop, here are some pointers:
- As a rule, you should try very hard not to change the pixel dimensions of your image without changing the resolution. Doing so requires Photoshop to either add or subtract pixels using a “best guess” procedure known as resampling.
- To resize without resampling, choose Image > Image Size. In the dialog box that appears, deselect the Resample Image checkbox at the bottom of the box. As soon as you do this, the Pixel Dimensions portion of the dialog box will be grayed out. This is good. Make sure the Constrain Proportions box is checked.
- If you have a target resolution that you want for your image, change the Resolution text box to that setting. 300 ppi is a reasonable setting for an inkjet printer. Anything less than 200 ppi will probably give a poor quality result. If you are planning on printing this image on the same sheet of paper as other images, you will want to make them all the same resolution.
- You will notice that changing the resolution changes your Document Size dimensions. Increasing the resolution decreases the size. Decreasing the resolution increases the size as already describe on this page.
- If you are unable to get the desired dimensions at the resolution you want, you may then resort to resampling. Moderate resampling down (to smaller pixel dimensions) is not too bad. Resampling up should be avoided at all costs.
- To resize with resampling, select (check) the Resample Image checkbox. Choose Bicubic from the menu to the right of the checkbox. Make a note of the pixel dimensions showing at the top of the dialog box. This is important. You do not want to end up with settings that show an increase in pixel dimensions. Make sure the Constrain Proportions box is checked.
- Now, set your target resolution in the Resolution box, and then set your Document Size dimensions to what you are after. You will notice that changing one dimension also changes the other. This is because Constrain Proportions prevents disproportional resizing of the image which would result in distortion.
- If your image is intended for use on the Web, and not for printout, disregard all of the above. Simply open the Resize Image dialog box, make sure Resample Image is checked and set to Bicubic, make sure Constrain Proportions is checked, set your resolution to 72 ppi and use the Pixel Dimensions setting at the top of the box to resize to the desired dimensions.
- If, in the course of using the Resize Image dialog box, you get confused and want to start over, press the Alt key. The Cancel button will change to be a Reset button. Click on that and try again.
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