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Image Size Dialog Box and Resolution
The Image Size Dialog box is a great tool for calculating image size requirements and resolution. It's an easy to use calculator found within Photoshop.

This exercise in Photoshop that will help to introduce you to the IMAGE SIZE dialog in Photoshop and aid you in understanding resolution. The IMAGE SIZE dialog can also be used as a resolution calculator. The IMAGE SIZE Dialog can introduce you to how the resolution of a file can affect the actual size when output. In the article "Image Resolution Versus Output Resolution" it was shown how different color files are larger due to the number of color channels in the article .

1. Launch Photoshop.

2. Create a new file by selecting NEW under the edit menu or Command/Control N.

3. In the NEW dialog box name the file resolutiontutorial.jpg.

4. Notice the WIDTH and HEIGHT fields and the fact that you can specify how you want to work; pixels, inches, percentage, etc. Using the pop-down menu, select PIXELS and enter 400 in the WIDTH and 500 in the HEIGHT field as you see in Figure 1. In the RESOLUTION Field make it 72 ppi. This is meaningless, as we will see.
Figure 1
(enlarge view)

5. Set the mode pop-down menu to GRAYSCALE. It’s also possible to create a file in CMYK, RGB or GRAYSCALE but experiment with this later. Each colorspace produces a different size file for storage only. This does not change the resolution!

6. The CONTENTS radio buttons allow you to set the color of the BACKGROUND in your new document. Set to WHITE.

7. Click on the OK button or use the Return/Enter Key to create the new file.

The new file appears and should be 196K in size as indicated on the small size label in the lower left corner of the document window if DOCUMENT SIZE is selected as seen in Figure 2. Click on this label and get more information as seen in Figure 3. Notice that this little pop-up shows you information such as the number of pixels in WIDTH and HEIGHT, the number of CHANNELS and the RESOLUTION. The resolution indicated is 72 pixels/inch as specified in step 2. Notice the actual size in inches? This dialog states that the SIZE is 5.556 by 6,944 pixels How did Photoshop come up with these figures? Divide 72 (RESOLUTION) into 400 (WIDTH) and you end up with 5.556 inches. Divide 72 (RESOLUTION) into 500 (HEIGHT) and you end up with 6.944 inches. Photoshop is doing the math for you based on what was set (72 ppi for the resolution of this file. In other words, If you output this 400x500-pixel file to an output device that’s laying down 72 dots per inch, the size would be 5.556x6.994 inches. What would the size be if instead of the resolution being 72 pixels/inch (another measurements option), the size were 100 pixels/inch?
Figure 2
(enlarge view)
 
 

Figure 3
(enlarge view)

Now we will alter the resolution figure with this file and see how the size changes.

8. Under the IMAGE menu, select IMAGE SIZE (Image->Image size). The dialog will look like the Figure 4. The dialog box is divided into three important sections.

The top section labeled PIXEL DIMENSION shows the actual pixels in the width and high. Using the pop down menus, this can also be set to PERCENTAGE.
Figure 4
(enlarge view)

The second section is labeled PRINT SIZE. We can work with a number of different units depending on the pop-down menus next to the data entry fields. In this example, Other units of measure could be PERCENT, CM, POINTS or PICA. Notice that in our example the print size shows up as 5.556 by 6.994 inches because the field labeled RESOLUTION is set to 72 ppi.

The last section is very important in how it controls the entire Image size dialog. Here are two check boxes that allow the addition or subtraction of pixels when the RESAMPLE image check box is on. Notice the pop-down menu that controls the interpolation algorithm. Leave BICUBIC on for best quality of photo originals as shown in the example. In 99 out of 100 times the PROPORTIONS check box will be set to ON, or else the file can be resampled and distorted. For more information on why these were selected consult your Adobe Photoshop manual.

9. The RESAMPLE IMAGE check box is extremely important. Be sure that it is not checked at this point thus preventing Photoshop from adding or subtracting aka resampling of the pixels in the file. We only want to deal with the original image data. When RESAMPLE is unchecked, the top portion of the dialog where the number of pixels appears becomes locked. These values can no longer be changed. This will allow us to alter figures in the width and height pop-down menu and the resolution fields to see how pixels, inches and resolution work interact. When the RESAMPLE Check Box is OFF, the dialog box becomes a resolution calculator.

10. Click on both the WIDTH and HEIGHT pop-down menus and set to INCHES. Notice that the size is still set to 5.556" x 6.994".

11. Change the figure in the resolution field from 72 to 100. Instantly the WIDTH and HEIGHT change to 4 and 5 respectively.

12. Change the pop-down menu from INCHES to PERCENTAGE. Notice that the figures haven not changed. The file is still set to 100% (original size). The top portion of the dialog still shows the file contains 400x500 pixels. The resulting output size was modified by changing the resolution setting. This instructs Photoshop how large to output the file based on the number of pixels and the size of the resulting dot.

13. Set the RESAMPLE IMAGE check box on, unlocking fields. Now figures can be changed in any of the above fields allowing pixels to be added or subtracted.

14. In the PIXEL DIMENSION WIDTH field in the top portion of the dialog, enter 1400. Several changes are taking place. First, the HEIGHT field has been updated from 500 to 1750. The PROPORTIONS Check Box remained on this because Photoshop kept those proportions and updated the figures while providing the correct number of pixels for that dimension. Also, notice the new file size has been updated in the top portion of the dialog. The file size increased to 2.34 MB because more pixels were added to the file. This is also known as resampling or interpolating the file up in size. Photoshop is good enough to also indicates the previous file size as well which was 196k. If the WIDTH field in the print size has been set to PERCENTAGE, the new percentage size would have been changed to indicate 350%. If the HEIGHT field had been set to INCHES, it would have indicated a size of 17.5 inches. Experiment by changing the various pop-down menus in this portion of the dialog to different options. When done, reset the pop-downs back to inches.

15. Examine the PRINT SIZE fields in the center portion of the dialog. The size has changed because more pixels were added to the file. It will now output at a larger size of 19.444x 24.306 inches at 72 dpi.

16. Make sure the RESAMPLE Check Box is set to OFF. To reset the dialog back to the original setting, hold down the OPTION/ALT key. The CANCEL button changes to read RESET. Press RESET. The dialog box stays open but the figures change to reflect the original document data. Type in different figures into the RESOLUTION field and notice that only the print size changes. If you type in 300 pixels/Inch, the size reduces to 1.3x1.6 inches. By typing different output resolution figures into this field, it’s easy to see how the pixels in the file will change for different output sizes.

Keep experimenting with the IMAGE SIZE dialog box with different figures until you feel comfortable with these concepts. If you think in terms of pixels along a width and height, and then think about the actual output resolution of your device, it’s easy to determine the size at which a file will output.

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