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Image resolution

July 15, 2007

Image resolution is easy isn’t it? If it’s going on press the image needs to be 300 dpi, if it’s only going to be viewed on Screen then 72 dpi is fine. Like I say, easy! That’s all there is to it.

Not quite. Recently, one of the designers where I work ordered a High Resolution image from one of the stock photo libraries and shrieked in terror when he opened it in photoshop only to discover it was 72 dpi. In fact it was plenty High Res enough for what he needed and was quite red faced when we explained it to him.

When referring to the resolution of an image, the number applies to how closely the pixels sit next to one another for a given length of image. Which is how we get dpi – Dots Per Inch.

Resolution of an image displayed on screen is referred to as Pixels Per Inch – ppi. A pixel, (short for picture element) is the smallest dot of colour an screen can show.

Think of resolution as pixel density – how closely the pixels are packed together. Increasing resolution means the pixels will be packed together more tightly, meaning a smaller physical size, but creating higher quality output. Dropping resolution means the pixels aren’t as closely packed together for a given length of image, creating a larger physical image, but a lower quality output.

The amount of ‘dots’ you need to make a good quality print depends on how it’s going to be printed. A home or commercial inkjet will be fine with something like 220 dpi, if it receives a 300dpi image it doesn’t matter and won’t make any difference apart from perhaps slowing things down a bit. But for top quality offset printing, 300 dpi is a realistic minimum. Always check with your print provider though.

So why was the 72 dpi image from the stock photo library High Res? Well, it boils down to image size and effective resolution.

In the picture below, you’d be forgiven for thinking the image is low resolution, but if you look at the numbers you can see it’s not. The Image size is 30 Mb, that’s big, secondly the pixel dimensions are pretty big at 2290 x 3466 and the physical dimensions say that it’s 32 cm wide. That’s over two feet wide! It’s unlikely to printed at it’s current physical size.

image-size-1.jpg

Clearly by the time the image has been scaled down in a page layout application it’s effective resolution will be what the current resolution is multiplied by the scale factor. For example, if the image were to be printed at a sixth of it’s actual size the effective resolution would be 72 x 6, which is 432 dpi.

There’s a lot more you can do with an image, using the Image Size dialog box in Photoshop, and I recommend looking at Photoshop’s help file, that like all Adobe help pages. is extremely well written.

Here’s something to ponder,

Image resolution doesn’t matter until the image is going to be printed. The same file can be measured at any resolution, it’s the pixel dimensions that matter most. The trick is in knowing how to change the resolution without changing the amount of pixels. So long as you don’t the Resample the Image, change the resolution as much as you like.

Alternatively, if you want a an application to work it all out for you and do a couple of other things as well, then you should try scaletron it has the advantage of being free.

DISCLAIMER: I have no connection with Rocketjam, though we are both members of PPF.

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