Resizing Images

One of the most basis skills or processes we must master as photographers is resizing images.

Our cameras generally shoot images with lots of pixels and definition. A camera with an APSC, or “crop frame” sized sensor like the Canon 70D has a resolution of 5472 x 3648 pixels. Images from a full frame camera will have even higher resolution. Such hi-res images enable us to create beautiful, rich and detailed photographic prints as big as posters.

In this digital era, our images are more often displayed on computer or tv screens which have relatively low resolutions. Whereas a photographic print or image printed in a book or magazine requires a resolution of 400 dots per inch, computer or tv screens mostly display at 72 dpi. The nominal resolution for a HD TV and many PC screens is 1920×1080, which is an aspect ratio of 7×4. Aspect ratios vary from device to device, so the actual pixel ratio will vary accordingly.

In our club competition, we are asked to upload images for judging. The rules stipulate that images are sized to 1920×1080 pixels. Limiting images to these dimensions keeps file sizes low enough to be uploaded easily, stored and sent to judges without creating massive files to move around the web. This system limits the file size of an image to 3Mb. Correctly sized, all of our images should fall within this limit.

1920×1080 pixels is an image ratio of 7×4. Not all of our images are these dimensions, and intention of the the rule is not to force everyone to use make their images 7×4. Part of the creation of an image is the choice of an appropriate aspect ratio. We may have an image that works because it is 1×1 (square) or panoramic at 5×1 or wider. So the way we interpret the rule is to make to longest side no longer than 1920 pixels and the shortest no more than 1080 pixels.

Taking a 5×1 panorama as an example, the longest side must be 1920 pixels. Proportionally, the short side will come in at 384 pixels, with a file size 0.75Mb. If we were to size the short side at 1080 pixels the long side would be 5400pixels and the file would 5.8Mb – too big. Looking at a square (1×1) image, both sides are the same length, so 1080 pixels, the maximum for a short side is the measurement we use. 1080×1080 gives us a file size of 1.1Mb.

In short, the rule is to make the longest side no longer than 1920 pixels and the shortest side, no longer than 1080pixels.

Images can be easily resized in Adobe Lightroom or Photoshop and almost all PC and mobile applications. There are also web pages where images can be uploaded to be resized and then you can download them back to your device.

Below is a link to a youtube tutorial which shows how to use Lightroom and Photoshop to resize images. It deals with images for PSNZ’s Camera Talk publication, which are are different size to our club competition, but you can apply the principles in the same way.