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Aspect Ratios and Cropping Considerations for Digital Images

By R. Anderson - 2/2009

The ratio of the width to height (or long to short side) of an image or print is the aspect ratio. Cameras and scanners capture images in varied aspect ratios, and the ratio also varies for common print sizes. This means that decisions must be made about cropping portions of many images to size them for specific uses. This article describes common aspect ratios and image cropping considerations.

Aspect Ratios and Image Capture

Before discussing image display and printing, it is useful to understand common image capture aspect ratios.

The image sensors in most common compact digital cameras record pixel information in a ratio of 4:3 (4 pixels of width for each 3 pixels of height) for an aspect ratio of 1.33. The larger sensors typically found in digital SLR cameras capture pixel information in a ratio of 3:2 resulting in an aspect ratio of 1.5.

Images captured by scanning will simply reflect the same aspect ratio as the item being scanned. This can vary considerably, since over the years, photos and documents have been produced in many different sizes and aspect ratios. These photos of varied sizes reflect past iterations of competition among equipment and film makers, as well as different approaches taken by photo developers.

Aspect ratios for non-photo images will vary widely as scans are created of other images such as vintage postcards, documents , and other sorts of memorabilia.

Aspect Ratios and Display and Printing

Common image print sizes reflect a variety of aspect ratios, as can be seen below:

Print Size	Aspect Ratio
4 x 6	1.50
5 x 7	1.40
8 x 10	1.25
11 x 14	1.27
16 x 20	1.25
20 x 30	1.50

It is interesting to note that the compact digital camera aspect ratio of 1.33 does not perfectly match any of the above common print sizes. However, the 1.33 is a pretty flexible compromise. On the other hand, the digital SLR aspect ratio of 1.5 is a good fit for some, but less so for the majority.

Computer display monitors are typically engineered to an aspect ratio of 1.33 (600 x 800 pixels a common setting).

This understanding of aspect ratios can be important as you begin to contemplate how a captured image will be displayed in print and on monitors.

Cropping and Resizing Images

If we are planning to print an image in a different aspect ratio, we can often simply allow our software or printing service to use default and arbitrary cropping methodology to make our image fit a specified size. Alternatively, we can often specify no cropping, in which case white borders will be added to two sides as necessary to keep all image information in the boundaries of the print size.

A third option is to crop each image to the proper aspect ratio with digital image editing software.

It is current industry default practice for most photo printers, including online printers, to enlarge an image as necessary to fill the print area of a specified size of print. If the aspect ratio of the image is different from the aspect ratio of the specified print size, the portion of the image enlarged outside of the targeted aspect ratio will be cropped/trimmed equally on both sides. This default is often referred to as a "zoom & trim" approach. Alternatively, you are often provided the opportunity to turn off this default crop. When the default is disabled, your full mage is placed onto the designated print size, and borders are added as necessary.

To demonstrate the importance of aspect ratio, The left hand image below is a scan of an original 1926 photo print, which measured 2.75" x 4.5" for an aspect ratio of 1.64. If our intent is to print it as an 8" x 10" enlargement, we would have to find a way to adjust the aspect ratio to 1.25, a fairly meaningful adjustment.

The middle image shows a crop off the top of the original image to maintain the full width of the image and to include the date at the bottom of the photo. Unfortunately, the only way to save the date is to lose the head. The right hand image shows a second manual crop to attain the 1.25 aspect ratio needed for an 8" x 10" enlargement while retaining the key elements of the image.

1929 photo 1929 Photo Cropped

Tip / Hint: If the aspect ratio of the original image is greater than the aspect ratio of the planned print size, the longer dimension will be cropped. If the aspect ratio of the original mage is smaller than the aspect ratio of the planned print size, the shorter dimension will be cropped.

You can quickly calculate the amount of image to be lost in a default crop by comparing the aspect ratios. In our example above, an aspect ratio of 1.25 is roughly 76% of 1.64, resulting in the need to crop roughly 24% off the height of the first photo in order to print the entire width.

Current versions of image editing software, including Photoshop Elements, include the ability to select an aspect ratio and then crop within an image to this ratio. This provides strong control over the cropping process, and allows you to manually crop to an optimal aspect ratio before uploading to an online printer or for another purpose.

Learning to manually crop your original digital or scanned images is also very useful in eliminating unwanted elements that have found their way into the image, and in re-composing an image to highlight certain elements.

Assuming the subject of an image is not tight to the edges of an image, most default cropping will provide satisfactory results. Most online printers also provide you some control over the process by allowing you to opt for filler border strips instead of a default crop. For example, If more than 20% of an image would be cropped by default, Shutterfly.commakes the default assumption that you would prefer to have the entire image printed with filler borders instead of a crop.

Tip / Hint: If you are working with fairly common sizes of images and prints, you can quickly inspect your image to make sure that no important elements are close to edges. You can then feel confident that a default crop will likely be successful.

If you are working with unusual aspect ratio images, or you have important elements near the edges of the images, it is useful to assume more control by cropping your images using image editing software such as Adobe Photoshop Elements 7, or to learn how to use cropping controls at online printers.

When you crop an image to adjust the composition or aspect ratio, keep in mind that you are cropping pixels. Once the mage is cropped and saved, the image pixels outside the crop area are no longer available for future uses.

Tip / Hint: If you often crop your images, you may want to get in the habit of routinely capturing more pixels. Then, even if you crop aggressively, you will still have enough pixels for quality print enlargements. It is also a good habit to always use a copy of an original camera image or scan for your varied projects. This will prevent a situation where you may have cropped an image for one project, and then find that the remaining pixels are too few for another purpose.