Resizing or Scaling an Image with Photoshop

It may help to read about Pixel Resolution if you are new to resizing images. Resizing or Scaling means changing the total number of Pixels in an image.

There are many occasions when you will want to resize an image. For example when you want to have an image professionally printed.

How to Resize

Select Image Size from the Photoshop menu and a dialogue box appears in which you can specify the new dimensions of the image. You can enter these in Pixel Dimensions (eg 1920 x 1080 pixels) or as a new Document Size (e.g. 8 X 10 inches at 300 Pixel Per Inch)

There is a dropdown box at the bottom of the dialogue where you should select the appropriate resampling method. It is important to choose the right one for the job...

Resampling methods (aka Interpolation methods)

Bicubic resampling is ideal for complex images such as photographs. For each pixel to be resampled it looks at the surrounding pixels to optimize what the new values should be. Bi-linear also does this but is generally not as effective as Bicubic which bases it's calculation on more data points.

So Bicubic is the way to go for photographs. There are 3 variants of the bicubic which are optimal for different situations:

Bicubic is best for small increases or decreases in size (say up to 50%)

Bicubic Smoother is better for greater up scaling as it introduces small amounts of Gaussian blur as it up scales in order to minimize potential artifacts and haloing.

Bicubic Sharper is best for down sampling as it introduces slight sharpening in order to maintain edge detail as the image shrinks. Visually it maintains the sharpness of the image

Nearest Neighbour is ideal for images containing only hard edges and patches of colour such as polygons, grids and line art.

Downscaling an Image

Imagine you have a 12 Mega Pixel image that you want to print at 10 X 7 Inches at 360 PPI. Natively the image will have pixel resolution of over 400 PPI at 10 X 7 inches so it would be beneficial to downscale it.

The amount of re scaling is quite small so I would select bicubic as the resampling method unless I thought the image could do with a bit of sharpening and then I would choose bi-cubic sharper.

If I was making a copy of the image for the internet where I might want a pixel resolution of just 72 PPI (at 10 X 7 Inches) I would use bicubic sharper since the amount of re scaling is large and I would want to maintain the sharp appearance of the image.

Up scaling Images

The key with up scaling an image is to do it in small steps rather than all in one go.

Lets say I have an image with a pixel resolution of just 100 PPI when sized at 20 X 30 Inches. If you have read my Pixel Resolution article you will probably want to increase the resolution to 180 PPI.

Lots of testing has lead me to conclude that the best way to achieve this is to upsize the image in steps between 5% and 10%. I use the bicubic smoother method and once the up scaling is complete consider whether some output sharpening is required.

After the 1st round of up scaling the image might be 110 PPI and after the 2nd round 121 PPI, then 133 PPI and so on until 180 PPI is achieved.

Alternatives and Plugins

There are a great deal of software options out there for up scaling images and I have tested quite a few. I have yet to find one that I feel clearly outperforms the method I have described above.