Resizing an image using Photoshop

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Resizing an image using Photoshop
Any digital image can be resized from its original dimensions off of the media card to suit your needs. The first thing to decide is what the end use for it will be - this determines the resolution in PPI (pixels per inch) that you should use.
For internet or monitor viewing the default resolution used is 72ppi. If the image is used for consumer level printing it should be at a minimum of around 180ppi or it might appear pixilated and grainy. If you plan to print on semi-gloss or especially on glossy photographic paper, 240-300ppi will produce a better image with added detail. 300ppi is about as high as you need to go unless you plan on using a commercial print shop. To output in larger sizes, the shop will usually need an even higher resolution (often 600ppi or 1200ppi), especially if the work is very large. Photoshop gives you the option to resize pictures easily.
Opening Image: To open the image with Photoshop:

1: Click FILE then OPEN and select the image you want to work on from the drop down box.

2: Then, select IMAGE from the top of the Photoshop menu bar, then IMAGE SIZE from the drop down list. This opens up a dialogue box with the images actual dimensions in pixels plus the default units of measurement and the images existing resolution. This box shows you several check boxes and other manipulation options. These are where you can either resize or resample the image as needed.
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.
RESAMPLING - keeps the same dimensions and removes or adds pixels to reach the new pixel setting that you’ve keyed in. Increasing the number of pixels is generally referred to as interpolation. If for example if the original image is 2048 pixels wide by 1532 pixels high it will give you an image of roughly 11.4 x 8.5 inches in size when displayed at a resolution of 180ppi.

There are also check boxes for CONSTRAIN PROPORTIONS and RESAMPLE IMAGE. Clicking on constrain proportions links the width and height measurements so when one is changed the other also changes automatically to keep the format in the same size ratio as the original file. If this box is not checked you end up stretching or distorting the image when adjusting one of the figures (width or height).

Turning Resample Image on unlinks the resolution (ppi) 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 (that looks like small chain link) joins the resolution to the width and height boxes so any changes to the image dimensions change the image resolution as well, keeping the file size the same.

To see this in more detail we start by using the above image dimensions and decide we want a 24 inch wide image instead of the original 11.4 inch. If Resample Image is not turned on all that happens is the image resolution is reduced – doing this basically stretches the existing number of pixels over a larger area, resulting in less pixels used per inch (113 ppi here versus the original 180 ppi). The actual file size remains the same as before.
One thing to remember, the pixel dimensions of any image determines its resolution. You cannot ad pixels to a small sized image (called upsampling) and expect to retain the sharpness and detail of before.
Resizing Image for web use : resizing an image to 72ppi with Photoshop.

1: Click FILE then OPEN and select the image you want to work on from the drop down list.

2: Then, select IMAGE from the top menu bar, and IMAGE SIZE from the drop down list.

3: In the DOCUMENT SIZE area, key in the new size you want the image to be. You have several measurement choices including: percent, inches, centimeters, millimeters, picas, points and columns. After doing so, be sure and also key in the new resolution (ppi) based on your needs as mentioned above. Then hit OK. You’ll notice that the physical size of the image on the screen will either decrease or increase compared to before and the number of pixels in the pixel dimension box will either drop or rise. Both depend on the resolution you chose as you are resampling the number of pixels in the image from the original file.

Interpolation Options:

When you resample an image new pixels are created and added to it based on the values of the existing pixels that in the image. This is called interpolation and Photoshop has three basic ways of doing it - nearest neighbor, bilinear and bicubic. The image size dialogue box lets you choose which of these options who want to use before resampling.

Nearest Neighbor is the fastest method of re-sampling, but only takes info from pixels at each side of the new one so the calculation is less precise and often results in jagged edges when you scale up the images size.
Bilinear takes info from the pixels above and to the side of where the new pixel will appear and gives slightly better quality than nearest neighbor does.
Bicubic gives you the smoothest results and is more precise. It samples from all eight surrounding pixels resulting in the smoothest tonal gradations. This is the best choice as you don't see the jagged edges, but it's also slower in processing and you can start to see a softening of images detail when upscaling.

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Please Note: Editing information from personal knowledge with some specific details from Photoshop CS’s built-in help submenu. Reference picture with kitten created by myself.

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