A digital picture should be changed in size to suit your output device. If you only intend to view the pictures on a computer monitor or display them on a Web site you can keep the resolution (number of pixels per inch) down to just 72ppi. The same picture printed out using a home inkjet printer needs to be between 170 and 240ppi and if it is going to end up reproduced in a book or magazine 300ppi resolution is essential. Photoshop has an option that lets you resize pictures with ease, providing you know what you are doing. And if you don't, you will after reading this small tutorial.
With the photograph open in Photoshop, click on Image from the menu and a drop down list appears. Select image size from this list.
This brings up a dialogue box with various dimensions and check boxes.
It's here where you can either resize or resample a picture.
Resizing allows you to adjust the number of pixels per inch and keeps the same amount of data by adjusting the dimensions to suit the new pixel setting.
Re-sampling keeps the same dimensions and removes or adds pixels to reach the new pixel setting you key in. Increasing the number of pixels is usually referred to as interpolation.
In this example the original picture is 432 pixels wide by 297 high. This gives an image size of roughly 6x4in when displayed at a resolution of 72ppi. There are two other boxes - constrain proportions and resample image. By clicking on the constrain proportions box it links the width and height measurements so that when one is changed the other changes automatically to keep the format the same. If this is not checked you will stretch or distort the image when adjusting one of the figures.
Turning resample Image on unlinks the resolution setting so the number of pixels will increase or decrease when you change the width or height. If resample image is turned off the link icon joins the resolution to the width and height boxes so that any changes to the dimensions changes the resolution, keeping the file size the same. Let's look at how this works in more detail.
Let's say I've decided I want a 10inch wide image instead of the original 6inch. If I don't have resample image turned on all that happens is that the resolution is reduced. You've basically stretched the same number of pixels used over a larger surface area, resulting in less pixels used per inch (43ppi instead of the original 72ppi) Notice the file size, indicated at the top of the box, remains at 376k.
If I want to print my result out at 240ppi I would turn on resample, set a new resolution from 72ppi to 240ppi and a new size from 6inch to 10inch. Notice the size at the top now jumps from 376k to 11.4M pixels. What we've done is add all the extra pixels to cope with the higher resolution and size required. This is called interpolation. The pixels that have been added are calculated by Photoshop and often, in such great jumps like this, the image quality is reduced dramatically. You can ensure the best possible result here by selectingBicubic interpolation from a choice of three resample image options.
If I now turn off constrain proportions I can adjust the height from 6.8inches to 8inches so that it fits the paper better. The problem is the image would look stretched which may be acceptable for some things, such as landscapes but not for portraits. Generally it's wise to keep this turned on.
When you resample an image using image editing software new pixels are created with colours and density based on values of existing pixels in the image. This is known as interpolation and the more sophisticated methods gather more information from the original image to ensure more accurate results.
Photoshop has three methods of interpolation - nearest neighbor, bilinear and bicubic.
Nearest Neighbor offers a fast method of resampling, but it only takes info from pixels at each side of the new one so the calculation is less precise. It's fine for use with illustrations containing non-anti-aliased edges but it results in jagged edges when scaling an image
Bilinear takes info from the pixels above and to the side of where the new pixel will appear and offers slightly better quality than the nearest neighbor option.
If you want the smoothest results you need to select Bicubic. This is the most precise method as it samples from all eight surrounding pixels resulting in the smoothest tonal gradations. This is the best option to use as you don't see the jagged edges, but it's slower to process and you do start to see a softening of image detail with large image scaling.
The image size dialogue box lets you choose the interpolation option before you resample.
You can also customise Photoshop by selecting theinterpolationmethodinthegeneralpreferencespaletteedit>preferences>general. Then whenever images are resampled using the image size or transformation options they are done so using your selected default interpolation method.