Troubleshooting Photos –
About Resizing Photos I – How
This page boils resizing a photo down to its essentials. First, it addresses a common misunderstanding, the difference between resizing a photo's canvas and resizing the photo itself. Next, a recipe: how to resize a few different types of pictures. As with most recipes, there's no "why", only "how". For the "why" portion, see Resizing Photos II – Why.
Canvas size versus image size
The difference between Canvas resizing and Image resizing is a common source of confusion. Perhaps it will help to think of the Canvas as a sheet of photo paper on which your photo is printed that is normally exactly the same size as your photo. For some modifications to the photo, such as adding a decorative border or adding additional images to make a panorama, you must work with a larger "sheet", and that's the Canvas.
When you expand the canvas (Image >> Canvas size... in Photoshop and Paint Shop Pro, Image >> Resize >> Canvas size... in Photoshop Elements), the photo will expand (the pixel dimensions will grow, as will the file size), but the picture itself will stay the same size. Depending on the settings you chose in the Canvas size dialog box, the photo will acquire a regular border or it may expand on one, two or three sides with a color you chose. Typically you can specify a finished size for the newly canvassed photo or the amount by which you want the photo to grow.
If you want the original picture to be larger, you select Image >> Image size... in Photoshop, Image >> Resize... in Paint Shop Pro, Image >> Resize >> Image size... in Photoshop Elements.
The original photo occupies the whole of the 400 x 338 pixel space within the frame. Resizing the canvas by using Canvas size or a similar command, as in canvas +, adds a grey margin around the picture. This expands the dimensions to 450 x 381 pixels and the uncompressed file size by twenty percent, but does not change the size of the picture. Resizing the image by using an Image size or similar command, as in image +, enlarges the picture itself.The Recipe
Resizing photos depends on a process called resampling. Resampling means that your image-editing program takes note of the color of every pixel in the image, and based on the routine that you chose to use in coloring new pixels, uses that information to determine the color and tone of those new pixels.
- For enlargements, the new pixels will be inserted between the pixels that were resampled.
- For reductions, a new pixel will replace a number of original pixels.
- If the resampling option is not checked in your photo program's Resize dialog, the photo will not change in size, although some variables may appear to change.
The same, but different – these two image resizing dialogs from Paint Shop Pro Photo (Paint Shop) and Photoshop Elements (Photoshop) contain essentially the same elements, but with slightly different names for functions and organization. (For instance, Paint Shop Pro hides complexity behind the 'Advanced Settings' checkbox, which has been checked to show everything here, while Photoshop Elements keeps everything in the open .Expect to see comparable differences and similarities in other programs.
Both programs give you two ways of resizing photos. If you're resizing for the Web or for an e-mail, you'll want to use Pixel Dimensions (or percentage, available by pulling down the Pixels box). If you're making a print, the Print Size/Document Size method may be simpler.
Note that in both dialogs, there is a link between the two dimensions (a padlock or chain), so as one changes the other follows in proportion. This is on by default in both applications but you can change this with the 'Lock aspect ratio/Constrain Proportions' checkboxes). Also on by default is resampling the image using the Bicubic routine. Resampling (explained on the About Resizing Photos II – Why page) creates new pixels (averages of pixels that are removed in shrinking an image or expanded numbers of pixels required to enlarge an image).
Both dialogs give a Resolution figure; this is normally shown as 72 pixels per inch (28.3 pixels per cm), which is the standard resolution of a monitor screen. If you use the Print Size/Document Size method to resize a photo for printing, you need to set the dimensions and resolution so that the target size of the print has a resolution between 225 pixels per inch (ppi) and 300 ppi (88 to 118 pixels/cm).
Both dialogs also give the option of controlling smoothness or sharpness during resizing, either with a slider or by selecting a variation of Bicubic resampling.Locate the Image Size or Resize dialog in your application and use the following.
- For photos, use 'Bicubic'.
- For graphics with hard edges, use 'Nearest Neighbor", which may be called 'Pixel Resize' or something similar.
- Other options can safely be ignored.
That's it. As with Photoshop, your photo-editing application may provide options like "Bicubic Smoother (for enlargements)" and "Bicubic Sharper (for reductions)". Instead of using these options, which alter the photo in subtle ways, you may be better off just using "Bicubic", then looking at the result. At that point you can decide if more processing, such as sharpening, is necessary, and you can apply the amount you want, rather than letting the application decide. On the other hand, if you like the effects these options provide, use them.
- There is also an option called Bilinear; it's faster than Bicubic, which mattered ten years ago when computers were slower, but the results aren't nearly as good. Ignore it.
- Ignore "Smart" resampling options that pick a resampling routine; making your own choices teaches you more and saves you from the occasional dumb choices that "smart" functions sometimes make.
- Do your color-correcting and other editing functions on a photo before you enlarge it; the process will be quicker. The only exception is sharpening, which should be done on the photo once it's at the size you want.
- Resize an image only once. If you change your mind about the size, go back to the original by undoing your previous resize or by opening a new copy of the original. Re-sizing a photo that's already been resized lowers the quality of the image and risks creating artifacts.
- Work on a copy and experiment. No change is permanent until you click Save, so feel free to click around and try things. If you accidentally click Save, it's a copy, right? – you just make a new copy to play with.
- Finally, don't assume that if 300 ppi is good, 400 ppi or more will be better. Many programs will automatically resample a file to a smaller size before printing if it's over 300 ppi and the results can be unpredictable (for starters, you don't get a chance to set an appropriate level of sharpening). You are also giving up control if you let your program resize a photo to fit the paper size (Photoshop and Photoshop Elements have a checkbox in the print dialog called 'Scale to fit media'; Paint Shop Pro has 'Fit to page'). Set your photo to the right size and make sure it looks the way you want before you start to print.
Related reading
- About Resizing Photos II – Why
- About the Printable Size of Photos
- Print is soft or fuzzy
- Enlargement Dominated By Unrealistic Visible Squares Of Color
