Resizing and cropping are two of the most basic image editing functions. Both require careful consideration as they can affect image quality. Resizing changes the dimensions of the image, which usually affects the file size (and, thereby, image quality). Cropping always involves cutting away part of the original image and results in some of the pixels being discarded.
There are plenty of reasons to resize images, the most common being reducing the size of large files to make them easier to email or share online. Many cameras can do this in-camera but it’s easy to accomplish with basic image editors and there are plenty to be found via a web search. Before you use them on your own photos it’s important to understand what is involved because resizing requires a great deal of care.
This illustration shows how excessive resizing can affect image quality.
Resizing can go in two ways: to”¨ make images smaller or larger. Making images smaller simply discards pixels. Making them larger adds pixels, which can result in a loss of quality. Excessive enlargement will produce blocky-looking and/or fuzzy pictures.
Each pixel in a digital image has a particular brightness level, colour and specific location within the frame. When you reduce the size of an image, these pixels are reduced in size and some will be discarded, using a process known”¨as ‘compression’. Enlarging the image makes each pixel larger and eventually you will reach a stage where individual pixels appear as little boxes.
The effects of upsizing and downsizing depend on how much data you start with, which depends on the file format used to store the digital image. The file format will also determine how much storage space a photo occupies and how long it takes to send as an email attachment.
The default file format for all cameras is JPEG, which itself compresses images to make them easier to store and share. Cameras that support raw file capture can provide compressed or uncompressed options but both will be much larger than JPEGs. So, if you want to use your images online, it’s best to start with JPEGs.
The first step in resizing a JPEG image. By default, the dialog box comes up with the width, height and resolution of the images linked (this is circled in red in the upper box in the screen grab below).
When you resize a JPEG image, opening the dialog box will display its dimensions in pixels, show its resolution and provide several adjustment options. More sophisticated applications provide a drop-down menu that lets you fit the image to a particular output size, as shown below.
Pre-set standard sizes, shown in the drop-down menu above, let you resize images for different applications.
You can also change the pixel or length dimensions by adjusting the figures in the dialog box. If you change one figure, the others are automatically re-set to keep the image the same overall size (in the example shown, 8.83 megapixels).
Checking the Resample box below the link allows you to change the resolution of the image, as shown in the lower box.
Look what happens as the image is enlarged; sharp edges vanish and the entire image becomes mushy-looking. Look how much the image file has been enlarged; simply changing the resolution from 72 to 300 has increased the file size from 8.83 megapixels to 153.3 megapixels. This is far too much enlargement.
The cropping tool is indicated by a reversed square icon in most image editors. To crop the frame you simply place the tool in the part the image where you want to start the crop and drag it across to encompass the area you want to keep.
Cropping to remove an out-of-focus area in the foreground, created as a result of using a very long telephoto lens. The image was composed with a deliberate aim to change the aspect ratio from the camera’s 4:3 format to a widescreen format. Discarding such a large area did not change the photographer’s intentions.
Many editors provide settings for cropping to particular aspect ratios ““ the original ratio of the image, 3:2″¨4:3 and 16:9 being the most popular. 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.
There several valid reasons to crop images:
1. You can crop to change the aspect ratio of the image, for example to make a 3:2 aspect ratio image fit on a 16:9 aspect ratio screen.
2. Images are always cropped”¨ when you change the aspect”¨ ratio of a shot before taking it.”¨ In such cases, the cropped pixels are never recorded; instead the overall resolution of the image is less than the sensor is capable of.
3. You can crop to improve the way an image is framed by excluding unwanted elements that were inadvertently (or deliberately) included. Cropping out a branch that blew into the edge of the frame while you were taking the picture is an example.
4. Some cropping is usually required “¨to straighten slanting horizons. The straightening tools in most sophisticated editors can do this with minimal loss of pixels.
Cropping to straighten a horizon. The straightening tool in the image editor is circled in red. You simply drag the tool along any line you want to have parallel with the frame, either horizontal (as in this case) or vertical. Note how much of the image area is being discrarded.
5. Cropping may also be required when you don’t have a long enough lens to cover the main subject. Sports and wildlife photographers may use this strategy when they can’t get close enough to fill the frame with the subject.
6. Drastic cropping will occur when you change the orientation of the shot from landscape to portrait (or vice versa).
7. Images can also be cropped before printing to make them fit onto a particular sheet of printing paper or into a frame.
Most editors come with pre-sets for cropping images to fit particular situations. For example, images for display on widescreen TV sets will usually fill the screen if they are cropped to 1920 x 1200 pixels at 72 ppi (pixels per inch), although 4K TVs will require at least 3840 x 2160 pixels. Images destined for printing can be cropped to fit a particular output size at 300 ppi (the recommended resolution for printing).
The cropping pre-sets sub-menu in Photoshop, including a few special purpose pre-sets created by the photographer.
You can also create your own crop presets and save them in the more sophisticated image editors, as shown in the screen grab below. That way if you need to produce a slideshow for displaying on a TV screen, it’s easy to select the preset and crop and re-save all the images in a particular folder for this purpose.
Most editors will identify images that were taken vertically (in ‘portrait’ orientation) and display them correctly in the editing workspace. For those that don’t ““ and at other times when you want to change the orientation of a photo ““ there can be several ways to change an image’s orientation, both subtly and radically.
This creen grab shows some of the basic rotation and flipping options available in Photoshop.
The simplest way is to select the Image Rotation tool shown in the screen grab below, which is taken from Photoshop. Choosing the 180 degrees option flips the image upside-down, while flipping the canvas provides a mirror image of the original.
Selecting the Arbitrary mode lets you rotate the image by small increments.Another way to rotate an image is to click on Select>All and then swap to the Edit menu and choose Transform to open a sub-menu containing the rotation tools. This sub-menu contains a Rotate tool that lets you rotate the image freely. It’s great when you’re working with layers and if you only want to apply a small correction.
This screen grab shows the Rotate tool in the Transform sub-menu.
Excerpt from Photo Editing Pocket Guide