A healthy crop

The straightening tools in image editors automatically crop the original image to maintain the largest possible image size.

The web has been awash with cropping tools almost from the time the first consumer digital cameras appeared, so photographers can be excused for thinking images should be cropped at will. Cropping is also one of the few editing actions permissible in modern photojournalism, along with tonal balancing, colour correction and sharpening.

However, when you think about what’s really involved in the cropping process, you will soon see cropping as a practice that requires deep consideration. The reason? When you crop an image, you discard pixels.

It doesn’t make sense to pay thousands of dollars for a camera with a high pixel count if you routinely throw away a percentage of the image data. A possible exception would be if you only display your photos on a computer or TV screen.

The highest resolution for consumer TV screens is 1920×1080 pixels, which equates to a bit over two megapixels, while the highest resolution available for computer monitors is 2560×1600 pixels, or a little more than four megapixels. You don’t need a 20-megapixel camera to produce images that look good on these screens; even images from smartphones should appear fine.

The main factor to remember before you crop a digital image is that the pixels you discard cannot be restored. Always keep a copy of the uncropped original as your back-up. (An upcrop plug-in is available for the GIMP image editor but even the developers state it is ‘hit and miss’.)

Reasons to Crop

There are some valid reasons to crop images, including to improve framing, accentuate a part of the subject or change the aspect ratio of the image. Some cropping can be required to straighten slanting horizons and more sophisticated editors include a straightening tool for cropping with minimal loss of pixels.

Cropping may also be required in situations where you don’t have a long enough lens to cover the main subject. Wildlife photographers are often forced to crop when their subjects are potentially dangerous or small and easily spooked simply because they can’t get close enough to fill the frame.

An example of a situation where cropping is necessary. Even with a 300mm telephoto lens, it was impossible to get any closer to this lioness and her cubs. Cropping provided the best way to obtain a worthwhile result.

The display format will often dictate the size and format of the crop. Images that will be displayed on widescreen TV sets will generally look best if they fill the screen, which requires them to be cropped to a 16:9 aspect ratio.

For prints, the paper size (or frame) being used can influence the shape of the image that suits it. Aesthetic considerations play a role here in determining whether you want a borderless print, or the widths of the borders and the position of the image within the frame.

For borderless prints, an image with a 3:2 aspect ratio will fit perfectly on a 6×4-inch (15x10cm) sheet of paper but will have to be cropped to fit on an A4 sheet when printed without borders. The borders of the same image will be uneven if the aspect ratio is retained when printing on A4 or A3 paper.

When to Crop

Images can be cropped at two stages of the creation process: when the shot is taken and when it is edited. Cropping when the shot is taken can be either through the choice of the image’s aspect ratio or the use of some kind of digital zoom function. Many cameras provide four basic aspect ratio crops: 4×3, 3×2, 16×9 and 1×1. Their effects are illustrated on this page.

The standard in-camera cropping ratios.

Digicams ““ and some CSCs ““ include digital zoom settings, which also crop the image, although in this case equal amounts are removed from the edges of the frame without changing the aspect ratio. Most cameras preserve the pixel dimensions of the original with interpolation, which adds in pixels based upon existing pixels in the cropped image. (This may create visible artefacts.)

Different manufacturers use different interpolation processes and some may offer two or more options in the camera’s menu. ‘Pixel binning’, which combines several adjacent pixels into a single pixel, can produce digitally-zoomed images with quality close to optically-zoomed shots, but resolution decreases with magnification. It’s useful for producing small zoomed images with reasonably high quality.

Post capture cropping in an image editor provides much greater scope for setting the aspect ratio to match the subject. It can also yield results equal or superior to digital zoom with interpolation ““ particularly if you start with a raw file.

With JPEGs, correctly interpolated digital zoom images can be superior to those produced by enlargement in post-processing, because the camera may apply its interpolation before detail is lost to compression. However, because raw files are losslessly compressed (if they are compressed at all), details can be preserved when resizing in post-production.

Cropping for Composition

One of the most common reasons for cropping is to straighten skewed horizons. Most image editors and raw file converters provide alignment tools to make this easy; you simply draw a line along the horizon. The image is cropped automatically to ensure as little as possible of the original is lost.

In some situations it is possible to shoot with an intention to crop the image in mind. An example would be for scenes containing large areas of blank sky and uninteresting foregrounds in which you can see a worthwhile subject in the area around the horizon.

A shot taken with intentional cropping in mind. The featureless sky and uninteresting foreground were cropped away in post-processing, leaving an image (below) with a non-standard aspect ratio.

Cropping can also be used to remove distracting elements that are peripheral to the main subject. Some subjects, particularly symmetrical ones, lend themselves to being placed dead centre in a picture, while others benefit from asymmetrical placement. If the original doesn’t meet expectations, it can be worthwhile duplicating it and trying different crops.

Another use for cropping is to change the orientation of the shot ““ which will also remove a large part of the image. You could also rotate the image before applying the crop and add movement by changing the position of the horizon.

A comparison of the original and cropped images shows how cropping can be used to change the orientation of the shot and direct focus to the main subject.

One final warning: when you crop you eliminate pixels, so the cropped version will have fewer pixels than the original. Whether you can make an enlargement that is as crisp and detailed as you could make from the uncropped image will depend on how much you cut off. For images that will only be displayed online, that probably won’t be a problem as the screens used for viewing are much lower in resolution than most camera images.

 

This article is an excerpt from  Photo Review magazine Issue 61  

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