Changing the sizes of image files is one of the most common software-based tasks in digital photography. Regardless of the type of digital camera you own, there will be times when the files it produces have to be resized for either printing or emailing. The process is simple, and can be done with virtually any editing application. It allows you to match the image parameters to the way the image will be viewed, so you can share your photos more effectively. It can also save you both time and money when you want to have prints made at a lab.

 

Changing the sizes of image files is one of the most common software-based tasks in digital photography. Regardless of the type of digital camera you own, there will be times when the files it produces have to be resized for either printing or emailing. The process is simple, and can be done with virtually any editing application. It allows you to match the image parameters to the way the image will be viewed, so you can share your photos more effectively. It can also save you both time and money when you want to have prints made at a lab.

Note: always work on a copy of the image, saving the original as a master you can return to if the resampling is not satisfactory.

Down-sizing image files

‘Downsampling’ is used when you have taken shots with the top resolution and quality settings (which we recommend for images that will be printed) and want to email copies.Most cameras with 2-3 megapixel sensors save files with an output resolution of 72 pixels/inch (ppi), which is ideal for screen viewing. However, the actual images may be too large to fit onto a standard computer screen (most of which are 640 x 480 pixels or 1042 x 768 pixels). If you have a 5-8 megapixel camera, the default resolution will probably be higher.

Illustrations: camera file.jpg and DSLR standard file.jpg

Caption: The dialogue boxes above show the default image size parameters for two different digital cameras; on the left is a standard 2-megapixel camera, while on the right is a 6-megapixel digital SLR. Note the differences in the Resolution settings as well as the different pixel dimensions and document sizes.

To resize the 72ppi file for screen viewing or emailing, simply type in 640 to replace the 1600 pixels reading in the Width box in the Pixel Dimensions section. Note the change in the file and document sizes.

Illustration: camera file resized for emailing.jpg

The digital SLR file requires four steps:

  1. First uncheck the Resample Image box (Illustration: uncheck Resample Image.jpg)
  2. Then type 72 into the Resolution box (Illustration: type 72.jpg)
  3. Next, recheck the Resample Image box to ensure the size of the file will be adjusted.
  4. Finally, adjust the Pixel Dimensions to the desired output size (Illustration: adjust to output size.jpg)

To resample an image so it prints out at a particular size, follow the steps outlined above but set the resolution at 300ppi (or 400ppi for some photolab printers; check with your favourite lab for the best resolution setting) and the Document Size dimensions to match the desired print dimensions (in this case 10 x 15cm). A sample dialogue box is shown below.

Illustration: downsample for printing.jpg

Up-sizing Image Files

The same procedures are used for increasing the size of image files. Because the upsizing process involves interpolation, the success of the operation depends on the nature of the original image and the extent of the ‘upsampling’. Note: interpolation adds extra pixels, based on existing pixels; it can’t insert information that wasn’t captured in the original shot. Images will start to look ‘softer’ as they are enlarged because the amount of information per pixel is reduced.

Many different upsampling systems are available, most of them calculating the hue, saturation and brightness values for the new pixels as a weighted average of values from surrounding pixels. Some algorithms are better than others, and success can depend on the nature of the original photograph, so it’s worth experimenting with several resizing systems to see which one works best for a particular shot.

Illustration: Upsampling options.jpg

Caption: Upsampling options.

Photoshop Elements provides three resizing methods, the effects of which are shown in the illustrations on this page. The nearest neighbour method calculates new pixels according to the value of the nearest pixel in the original. It can produce a blocky result when upsampling and a grainy effect when downsampling. Bilinear resampling operates on a 2 x 2 cell of pixels surrounding each new pixel. The resulting images are smoother but not usually as sharp. The bicubic method looks at the closest 16 pixels (a 4 x 4 pixel cell), computing new pixel values using cubic splines. This method usually yields good results, which can be improved by additional sharpening (not more than 20%).

Illustrations: Nearest neighbour.jpg, Bilinear.jpg, Bicubic.jpg

Label each image accordingly.

Other interpolation methods are also available. One popular option is Lanczos resampling, which operates on a 4 x 4, 6 x 6 or 8 x 8 cell of pixels surrounding each new pixel location. The 4 x 4 method is so similar to bicubic interpolation that most photographers prefer the other options, which produce better results – but take longer to compute. Lanczos resampling is provided in the freeware IrfanView Image Editor.

An alternative is Genuine Fractals, a Photoshop plug-in developed byAltamira (which is now part of the Celartem Group). Genuine Fractals can work well, provided it’s applied to large enough files. Files are first saved in the application’s native format, after which they can be enlarged as if they were vector images. Any further editing should be done after resizing to avoid potential artefacts. Fred Miranda’s Stair Interpolation (SI) also works as a Photoshop plug-in. It uses Photoshop’s bicubic method, one small step at a time, but in a rapid automated succession. The Dutch Shortcut Software company’s S-Spline 2 applies bicubic interpolation on the smoother parts of the pictures, but uses proprietary interpolation techniques in sharper areas. Like IrfanView, S-Spline is a stand-alone program.

Resampling Tips

Because many calculations are involved, resampling can take a long time. Best results are obtained if you lock the aspect ratio of the image before you start and enlarge in small steps. A good starting point is 110%. Most applications allow you to go back and resample at a different setting if your original attempt was unsatisfactory (either not enough enlargement or too much quality lost). Success depends largely on the nature of the original image. For example, where a hard edge is defined by adjacent yellow and red pixels, upsizing will place an orange pixel between them and contrast is reduced. Soft edges, such as skin tones can be up-sampled more successfully.

Your own eyes must be the ultimate judge and jury. Watch out for artefacts, which can appear as dark or bright lines along regions of high contrast, or the blocky effects that characterise JPEG compression. The latter can be reduced by a Photoshop plug-in called JPEG Repair, which is part of a suite known as Image Doctor. It’s available from Alien Skin (www.alienskin.com). Failing that, consider shooting at a higher resolution.