Use the tools in Preview to edit photos on your Mac

What software do you use when editing photos on your Apple Mac? How about Preview? The latest version of this app has some great features for editing your snapshots.

Preview is not the first application to spring to mind when thinking about photo editing, but it is more powerful than many people realise. Double click a file and it opens in Preview and you can view the contents. It is a useful general purpose file viewer, but many people do not explore beyond this obvious use.

The app is not simply a file viewer and it has powerful photo editing capabilities that are often good enough in many situations. It loads quickly and you don't need to search for it in the Applications folder because you just double click a .jpg file and Preview starts, loads the image and displays the contents.

It is easy to forget how powerful the tools in Preview are, so double click a photo or image on the disk drive to open it for editing.

Preview Tools

Go to the Tools menu and there are menu items to adjust the colour and the size, and rotate left and right. Rotating photos is useful when you take photos on a digital camera on its side, so the image comes out in portrait mode. When it is transferred to the computer the image might be displayed in landscape mode and you have to rotate it right or left to make it vertical again.

Preview on the Apple Mac

Resize photos

Select Adjust Size on the Tools menu. The top item, Fit into has a list of predefined sizes. The smaller ones, such as 640 x 480 and  800 x 600 pixels are useful for shrinking photographs for emailing, posting on the web, and sharing using social networks.

Resize photos with Preview on the Apple Mac

The width and the height of the photo can be set using a variety of units, such as mm, cm, inches, and pixels. This is useful if you want to print a photo and want to make it fit the paper. Find out the size of the paper, A4 is 210 x 297 mm for example, set the units to mm, and set the width and height to fit the paper.

Bear in mind that printers don’t always print right to the edge of the paper. They leave a margin, so you might want to allow 10 mm at each edge when resizing.

If you change the width then the height is automatically changed accordingly so as to keep the same aspect ratio (width:heuight), so if you make it twice as wide then the height is automatically set to twice too. Click the padlock to clear the Scale proportionally item. This enables the width and height to be changed separately. It distorts the image, but sometimes it can be useful, such as when you want a special effect.

Resample images

There is a Resample image tick box and this is an interesting feature that can be confusing at first. Suppose a photo is 1,000 pixels wide. If the resolution is set to 200 pixels/inch then the image will be five inches wide when printed on paper because 1000/200 = 5. If the resolution is set to 250 pixels/in then the image will be four inches wide when printed because 1000/250 = 4.

The resolution and the number of pixels wide or high an image is, determines its size when printed. To calculate the size on paper you divide the width in pixels by the resolution in pixels per inch.

It is therefore possible to set the size of an image when printing by changing the resolution. Suppose you wanted that 1,000 pixel image to be 10 inches wide on paper. You could set the resolution to 100 pixels per inch.

Suppose you want to print the image and you want it to be 10 inches wide, but you want to print at 200 pixels per inch. This is where the Resample image option comes in. The image would need to be 2,000 pixels wide, so that printing it at 200 pixels/inch would be 10 in (2000/200).

The Resample image option changes the number of pixels in the image appropriately. If there are not enough pixels, it inserts them (a best guess at what colour they might be), but if there are too many pixels then it removes the surplus ones.

Resampling adds or removes pixels. This affects the quality of the image and if pixels are added it makes it fuzzy. If pixels are removed, it doesn’t affect the sharpness so much, but pixels are discarded and cannot be put back in.

If you are resizing images to display on the screen, on the web, or in emails, tick the Resample image box and ignore the resolution. When printing images, try to get the right size without resampling. The best print resolution is 300 pixels/inch, but anywhere from 200 to 400 is fine.

If you cannot get the right size by adjusting the resolution, set it to 300, tick the Resample image box and then set the size. Never resample original images, always work on a copy. This is because if you shrink an image by resampling, and then enlarge it back to the original by resampling, it will look awful. It destroys the quality. Only resample once from a copy of the original.

Colour adjustment in Preview

Select Adjust Color on Preview’s Tools menu and a panel containing nine sliders, a chart and two buttons is displayed. These can be used to correct faults in the photo such as the wrong exposure making it too light or dark, if the focus is slightly off then it can be sharpened, the contrast and saturation can be adjusted, and so on.

Adjust the colour in Preview

The Temperature and Tint sliders change the colour of the image and they can be used to correct photos taken indoors under different lighting conditions.

Apply a Sepia tone in Preview

You can also use the controls to create special effects, such as this sepia tone.

The best way to become familiar with these controls and what effects they have is to experiment. Load a photo and drag the sliders around.

The chart at the top has three buttons – left, right and centre. Dragging the buttons to the left makes an image lighter and dragging them to the right makes it darker by increasing the contrast. The Auto Levels button is good at automatically selecting the right settings. Use it and then tweak the result by moving the sliders a tiny bit to the right or left.

Each image is different and some might need considerable tweaking of the sliders, but others might not. Experiment with them.



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