1. Increasing the Resolution for Prints

For large prints, you may need to increase the number of pixels in an image to be about 300 pixels per inch, although 240 pixels per inch is generally good enough.  For example, a 5MP image from a Nikon 5700 will have pixel resolution of 2560x1920.  If an 8x6 inch print is made, you would have 320 pixels per inch.  If a 10x8 inch print is made, you would have 240 pixels per inch, and the result would look fine.  However, if you were to make 11x14 inch or 16x20 inch print, you would only have about 175 and 120 pixels per inch, respectively.  In these last two cases, the resulting print would probably have some undesirable artifacts.  You can increase the pixel resolution in Photoshop, but be aware that the software can not create accurate details in a larger image from a smaller image.  The best you can hope for is that you will eliminate many of the undesirable artifacts when you have too few pixels per inch.  The menu operation of Image > Image Size ...   will do the job for you, but do not expect to gain accurate detail from a smaller image file.  You can generally get reasonably good results if you are increasing the image size by less than 75% in each dimension, and it is likely that you will not get good results if you increase the size by more than 200%.  This means that you can often increase the pixel resolution of a 5MP image so that it will yield a good 11x14 inch print, but a 16x20 inch print will likely have artifacts.

My preference is to perform all edit enhancements before resizing the image for a large print because I do not want to introduce any inaccuracies until the very end.  If I am resizing up by less than 50% (which is generally my situation),  or resizing down less than 50%, then it does not make much difference if you sharpen the image before or after the resizing.  However, for size changes more than 50%, the difference can be significant, thus it is generally preferred to resize the image before sharpening.

Fig. 1 shows the Image Size dialog box that appears after the menu Image > Image Size ...  For any resizing, you want to have a check mark next to Constrain Proportion near the bottom so the image will not become distorted.  With that checked, you can only change one of the two variables for Width and Height, because the ratio of Width to Height is fixed.   For example, if I changed the Width to be 6000 pixels, the the Height would automatically be changed to 4000 pixels if the original image has a 3 to 2 proportion.  Most digital SLR's have a 3 to 2 ratio, while most fixed lens digital cameras have a 4 to 3 width to height ratio.  For increasing the pixel resolution, also check the Resample Image box and select either Bicubic or Bicubic Smoother.  I prefer using Bicubic because Bicubic Smoother can blur some details in other to yield a less jaggy smoother image.  There are several ways to increase the image resolution, and some of the following are equivalent.  The other option of Bicubic Sharper can be used to make a smaller image for the web, but it should not be generally used for increasing the pixel resolution because it will accentuate pixel boundaries and the jaggies.  If your image is in 16 bit Mode, leave it in 16 bit mode while you change the image size for better accuracy.  After the image size process is complete, convert to 8 bit Mode and then save because your printer and your eyes will not gain anything with 16 bits.
 

Fig. 1  Image Size dialog

Suppose that you want a 18x12 inch print and the original image has the 3 to 2 ratio with a width of 3504 and a height of 2336 (from an 8MP Canon 20D).  If you want 300 pixels per inch, a little math tells you that the new height should be 3600 pixels.  If you type in 3600 for the Height in the top region, the width will change to 5400 automatically.  A different way to get the same result is to modify the Document Size Height to 12 inches and to also change the Resolution to 300 pixels/inch.

An alternative approach is to increase the Width or Height by a percentage.  To get from a Height of 2336 to approximately 3600, you would need to enter a little more than 150%.  It is not critical to get an exact 300 (or whatever) pixels per inch because there is not a one-to-one match between the computer and the printer.

Another approach that some think is superior is to repeatedly increase the size by 110% over and over again, until the size is large enough.  If you did this 5 times, the net result would be a 161% increase in size, thus you would have more than 300 pixels per inch for this example.  I have tried this approach and it does work well most of the time, often with a slight improvement.  However, this approach generally yields similar results and the differences are not usually observable.

2. Reducing the Resolution for Web or Digital Presentations

To reduce the size of an image for a web page or email, be aware that most computer monitors have a resolution of 1024x768.  An image about 600x400 is a reasonably large image.  You will also need to change the Mode to 8 bits, convert to sRGB color mode, and then Save As a jpg image with a fair amount of compression or lower image quality of about 5 for the web or email.  For digital presentations where compression is not needed, set the image quality to the max of 12 and have the image fill up the maximum space inside 1024x768.  Details on this follow.  One key is that the maximum Height is 768 and the maximum Width is 1024.  When you change one of the sizes for Width or Height, the other will be automatically adjusted because you only have one degree of freedom if the aspect ration is maintained.
 

Suppose that you have a high resolution image that you want to convert to an image limited to 1024x768 resolution for a digital presentation.  Before doing the following, save your original image, duplicate the image by making a copy, and then close the original image.

• If you have layers, then Layer > Flatten Image.
• If you are using 16 Bit Mode, then Image > Mode > 8 Bits/Channel.
• Convert to sRGB profile if you are using Adobe RGB mode by the following:
• Image > Mode > Convert to Profile ...
• When the dialog box in Fig. 2 comes up (shown below), click on the arrow in the "Profile" field underneath "Destination", then select the option "sRGB ICE61966-2.1"
• Click OK.

• Resize the image using  Image > Image Size ... and the dialog box in Fig. 1 appears (shown above).  Note that the maximum pixel Width allowed is 1024 and the maximum pixel Height allowed is 768 for this constrained example.
• If the image is vertical (portrait), then set the Pixel Height to 768.  The Width should be adjusted automatically.
• If the image is horizontal (landscape), then there are two possible cases depending on the width to height ratio.
• First enter 1024 into the Pixel Width box.
• If the automatically adjusted Height is not larger than 768, then you are done with this part.
• If the automatically adjusted Height is larger than 768, then change Height to 768, and you are done with this part.
• Option: When resizing to a smaller resolution, the Resample Image (at the bottom of Fig. 1) is generally the recommended to be Bicubic Sharper, while the default of Bicubic is generally the recommended choice for resizing to a larger resolution.  I usually use Bicibic Sharper for resizing to a smaller resolution.
• Click OK

Save the file as a JPG image:

• File > Save AS, and then locate the folder where you want to save the image.
• In the Format field (about halfway down), select JPEG.
• Change the name of the file to whatever you want, but do not change the extension at the end that has ".jpg"
• Click Save.
• The JPEG dialog box in Fig. 3 appears.  For the higest quality, move the slider to the right (large file) or change the
• image quality number to a max of 12.  (For web pages and email, move the slider to the middle range of 6 to 8 to obtain a smaller file size, but it will be of lower quality.)
• Click OK.

Fig. 2  Convert to sRGB dialog

Fig. 3  JPEG Option dialog