Resizing Digital Images

By John Munno

Make up of Digital Image

• Digital images are made up of small squares called pixels. The word pixel comes from combining the words "PICture ELement." 

On the left the full image, on the right the area in the red square magnified to show individual pixels

Make up of Digital Image

• Though a digital photograph looks smooth and continuous just like a regular photograph, it's actually composed of millions of tiny squares.

• Each pixel in the image has a numerical value of between 0 and 255 and is made up of three color channels, Red Green and Blue. So for example a pixel could be 37-red, 76-green and 125-blue and it would then look like this.

• If it was 162-red, 27-green and 12-blue, it would look like this.

• There are over 16 million possible combinations using this scheme and each one represents a different color.

RGB Color Scheme

Resolution• Resolution of an image is a very confusing

term in that it can mean different things according to the source

• # of pixels in an image in number of megapixels

• Height and width of an image in pixels• Fineness or detail of an image• Button in photoshop that determines pixels

per inch

Image Size and Resolution

• Image size and resolution are terms often used to describe different but related concepts. These concepts are important for those who wish to manage and use their digital images.

Image Size• For our discussion I will use the term

image size (or pixel dimensions) of an image as a measure of the number of pixels along an image’s width and height. For example, your digital camera may take a photo that is 3000 pixels wide and 2000 pixels high. These two measurements have a direct correlation to the image’s file size, and both are an indication of the amount of image data in a photo.

Resolution

• For our discussion I will use the word resolution to mean the fineness of detail you can see in an image. It is measured in pixels per inch (ppi), sometimes (dpi) dots per inch. The more pixels per inch, the greater the resolution. Generally, the higher the resolution of your image, the better the printed image quality.

Image size and resolution

• Although a digital image contains a specific amount of image data, it doesn’t have a specific physical output size or resolution. As you change the resolution of a file, its physical dimensions change, and as you change the width or height of an image, its resolution changes.

Image Sizing dialogue box

Example # 1

• A simple example would be an image with dimensions of 1200 pixels by 1800 pixels with an assigned print resolution of 300 PPI would print to 4 by 6 inches. Math formula as follows:  Height and width dimensions of the image in pixels divided by assigned PPI. 1200 pixels divided by 300 = 4 and 1800 divided by 300 = 6.

Example # 2

• If you take an image that is 800 pixels wide and 600 pixels high, and you print it with a PPI setting of 100 pixels per inch, the print will be 8 inches wide by 6 inches high. If you print at 200 PPI you get a print 4" wide by 3" high. Now the print at 200 PPI will be higher in quality but smaller.

Choosing correct resolution for images.

• Where will the image be used?

• Print

• Email

• Website

Resolution for printing images

• The higher the amount of pixels, the less each pixel is stretched when the image is enlarged. The resolution of an image is the most important factor when thinking about enlarging an image. The higher the resolution of the image, the less pixelation that will occur and the better the image quality will be when printed.

Printing images

• The standard for prints done in a photo lab is about 300 PPI. It also depends on the application; magazine and newspaper prints can get away with much less than 300 PPI.  240-360 PPI is generally used for printing photos. The more you try to enlarge a given image, the lower its PPI will become (assuming the same number of pixels).

Emailing Photos

• The main concern when emailing photos is to make them small enough that they can be emailed and received on the other end to view on a computer monitor.

• A recommendation for emailing size is a photo with a horizontal dimension of about 800 pixels and a JPEG compression of between 35 to 50%.

Save for web dialogue box

Image Size for computer monitors

• When you display a digital image on a monitor, the only thing that determines the size of the image is the pixel count. DPI and PPI, sometimes called resolution, mean absolutely nothing. If your image is 800 pixels wide by 600 pixels wide, it will display as a full screen image if you are using an 800x600 display. It doesn't matter if your DPI or PPI is set to 1 or 1000.

Image size for web and computer monitors

• The way you control how large an image appears on someone's monitor screen when viewing your images on the web is by changing the pixel count. 

• If your original image is 1600x1200 pixels it will probably be too large to see all at once on 95% of the video monitors out there. It will also be slow to load since it will be a large file. If you want someone using an 800x600 display to be able to see your image clearly, you need to change the size to, say, 600x400 pixels (remember the browser window is smaller than the full monitor display).

Image size for web images

• Digital images that are intended for web use or email attachments usually run between 150X250 pixels for small "thumbnail" images to 600X800 pixels for full or nearly full page images. Digital images of 600X800 pixels and smaller will fit without scrolling on almost all recently manufactured computer set ups.

Resolution for computer monitors

• Windows allows a monitor to display a maximum screen resolution of 72 PPI, so there's no point in saving an image at more than 72 PPI; doing so will only cause the image to load slower without giving the visitor any increase in quality.

Image Sizing dialogue box

Upsizing vs downsizing

• You can resize a digital image to smaller resolution without any noticeable quality loss. Resizing an existing digital image to higher resolutions always incurs some image quality loss since the program doing the resizing has to make educated guesses (interpolation) when assigning the colors of the pixels it is adding to build up the resolution of the image.

Enlarging Digital Photos

• Digital photos do not take well to being overly enlarged and this is generally to be avoided. If you want to print an image it is usually best to first try printing using the original size and quality of your digital photo. Your printer software will make the adjustments.

Image size for Photo Review Wednesday

• With regards to our Photo Review Wednesday, some of you may have notice during the digital showing that some of the photos did not fill the screen where others did. This is caused by the image size of your file, i.e., an image size of 640 X 427 pixel will not fill the screen! The recommended image size is 1024 X 768 pixel with a resolution of 100.

Cropping Digital Photos

• A common size change for a digital photo is cropping. When you crop an image you change the physical size of the image but not its resolution. This is what you want to use when printing images.

Conclusion

• I hoped I could take some of the mystery out of image resizing.

• It can be a mind boggling topic for sure.

• I hope you can feel more confident about manipulating your digital images for the various uses.

• Happy shooting and editing