Image resolution and Pixel dimensions explained

Someone asked a question today that got me to think that pixel settings is still not really understood. So I thought I’d explain what the Image Size resize/resample option does.

When you open an Image Size dialgue box you usually have three methods of adjustment. One to scale styles one to constrain proportions and one to resample.

Constrain proportions and scale styles are checked (ticked) by default. If you want to stretch the image so it looks squashed then you’d uncheck constrain proportions and adjust either the Width or Height dimensions depending whether you wanted it stretched across or up. This can be used subtly to make a landscape look more like a panoramic shot, but should be used with care.  If you want to keep the image the same proportions you keep these ticked and the aspect ratio remains the same.

Resample Image is the one that gets most people in a pickle. By default this will be set as default. If you leave it at default you will notice that the resolution setting is unconnected from the width and height in the Document size settings. Resolution is usually set at default at 72ppi (pixels per inch) If you change the resolution to, say 300ppi you will notice that the Top value Pixel Dimensions will increase dramatically. This is because you are about to resample the image. Doing this instructs Photoshop or similar image-editing program to add pixels (interpolate).

If the box is unchecked you will see it connects the Resolution settings with the Width and Height. If you now change the default 72ppi to 300ppi you will see that the Pixel dimensions stay the same, but the width and height decrease.

You are basically taking the 72 pixels in each inch and squashing them into 300 per inch. Computer screens are set at 72ppi so you do not need an image to be set to a resolution of 300ppi, although it doesn’t matter if the file is, because the computer will display at 72ppi anyway.

Now the person who asked the question had been told he needed the image to be 300ppi for a magazine submission. While it’s true that magazines and book publishers need 300ppi files the only important figure is the pixel dimensions, because this determine how big an image can be at 72ppi, 240ppi or 300ppi.

So what do you need to know? Any good magazine editor/picture buyer will know how many pixels he/seh needs for the image and will request the necessary…ie he/shh will request a 2400pixel image. As they are printing at 300ppi it means they have asked to use the image at 8in (2400pixels / 300), yet the same image on your computer may appear to be 33in (2400pixels / 72) if you change from 72ppi to 300ppi with the resample image unchecked the 2400 pixels will stay the same. If you check resample image when changing from 72 to 300ppi your 2400pixel image will increase to 10000pixels and will be a huge unecessary file.

Hope that’s clear…if not ask questions here

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