Children of the resolution...

I thought I'd write a post about digital images. Now, that might not sound like the most exciting post, but this is about the resolution of images that people receive from their photographer. Resolution might not always make sense, but it is important. I bring it up following a discussion in a photography group, which has thrown up some interesting stories.

Many photographers offer web resolution and print resolution images, either included in your package or they may have the print resolution images as an optional extra. But why have two copies of the same image? What difference does it really make?

Imagine a photographer provides a customer with a screen resolution image. The image dimensions are 25 by 16 inches. The customer tries to print from that, to the same image dimensions. The image comes out looking pixelated.

Now, the customer can actually print from that file, but as it has been saved to a screen resolution, it has fewer pixels per inch than an image saved at print resolution. Traditionally, a screen or web resolution is usually recommended at 72 pixels per inch (ppi) . Printing resolution is usually recommended at 300 ppi. The image has to be reduced in its physical dimensions to achieve a crisp printed image. In this case, the image would need to be reduced to 6x4 inches.

So why do photographers have a difference between "web/screen" and "print" resolution images? Why not just have everything as one image, good enough to print from at the same dimensions you can see on screen?

By reducing the resolution but maintaining the dimensions, this reduces the actual physical file size. So where a print resolution image might be 16MB in file size, it's screen resolution equivalent might be 4MB. That makes the image file smaller, easier to send via email, it's often faster at uploading to file-sharing websites, and on screen, you won't see a difference in quality. It's perfect for things like Facebook or Flickr. Remember too, that most email hosts have a limit on the maximum file attachment size, Hotmail allows 10MB attachments, while Yahoo and Gmail allow 20MB of attachments.

So want to see what it'll all look like? Here's a photo I took recently in Norway. One is saved at screen resolution (72 ppi), the other is saved at print resolution (300 ppi), both have exactly the same physical dimensions.

Now, they both look the same here. But what would it look like if we printed from them at the same size?

Well, a bit like this. The first is how the screen resolution image would look printed. The second, is how the print resolution would look printed. Both at identical physical dimensions. This is shown at actual size.

Admittedly, it looks like I've sampled the top image slightly further to the left, but the physical dimensions are exactly the same.

If we wanted to print from the screen resolution successfully, we'd have to reduce the physical dimensions. This is the size we'd have to reduce it to make it sharp again.

It can all be a little bit confusing. Screen resolution isn't a bad thing, and not receiving print resolution doesn't necessarily mean you can't print from the images. What it does mean is that you would need to reduce the physical size of the image to achieve a good result. So for the imaginary customer to have a nice print that measured 25 by 16 inches, the screen resolution image dimensions would need to be 104 by 66 inches.

If you're in any doubt, it's always worth checking with your photographer about your printing requirements.