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The Image Size Box

The Image size box is your friend. While you can commit some horrendous crimes here, it is very powerful and a great place to pick up on some key ideas regarding resolution

The most powerful idea the image box demonstrated that it is the file size that is the critical factor - not the output size or resolution
TIP right click on the title bar to quicky access the image size box!
  • Open an image and choose Image > Image size
  • Uncheck 'resample image' ( we don't want photoshop to add or remove pixels)
  • Lower the resolution of the image. What happened to the other figures?
  • Increase the resolution.

The file size remains constant, the pixel dimensions stay the same, but the output size and resolution see saw up and down. It's like 6x2=12 as does 3x4 or 2x6!

So an image can be low res at a large out put size or hi-res at a small output size. This change does not destroy any information at all - it just swaps numbers around.

What am I getting at? That file size is the big kahuna! Quite often a bureau will say 'we don't care what you send, as long as it's 32meg' etc. It's the number of pixels that matter..  believe me this is a cool idea that is sometimes hard to appreciate!

Sampling Up vs Sampling Down

Sampling up is another way to describe the process of asking an application to add information to a file. Sampling down is the process of removing information.

We can do this in th image size box by checking 'resample image' and change the resolution, pixel dimensions, or output size of the image. The file size will update to show you what you will end up with.

Notice that 'bicubic' is selected by default - it is the most advanced form of 'interpolation' ( calculated guessing' ) that photoshop uses - it is also the slowest. With bicubic interpolation photoshop examines the ring of 8 pixels that surround each pixel and bases it's decision on how to colour new pixels. Experiment with the other two, bilinear and nearest neighbour - they a scary suckers!

A good exercise is to  open a file, go to Image > Image Size. With 'resample image' checked, halve the resolution. Notice the file size drop and apply the change. Zoom in and examine the image. Sampling the image down usually leaves the image in an acceptable state.

Again go to Image > Image Size. Return the file to it's original resolution. Zoom in and examine the image. Did you observe a loss of sharpness and saturation? There is always fixing to be done after sampling an image up.

The aim is to keep resampling to a minimum ( unless you are trying to create a special effect ). Each interpolation destroys the accuracy of the pixel information.

With low res web graphics the results of interpolation can often render the file useless. There's just not enough pixels to play with.

With all digital work it is critical to get the file size and resolution right at the start - know where you are headed and plan in advance.
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