The Difference Between Pixel Density On Paper and On The Screen

Someone asked on Quora ‘How come super quality pics can be zoomed up so close but only on the original picture?’

I answered –

I’m guessing you’re talking about being able to zoom in on a photograph that is shown on the web.

In order to understand why it is that you can’t zoom in on an image on the screen like you can with the original image, let’s start at the opposite end with a photograph on the side of a bus.

The camera that took the original photograph dictates the number of pixels in the file from which the image will be printed. The more pixels, the bigger they can print the photograph without it looking like a collection of dots.

But it depends on how far away the viewer is looking.

If I look at any photograph at all with a magnifying glass I can see the dots of ink. If I stand across the street I won’t see the individual dots.

If you look at that giant photograph on the side of a bus from across the street then it will look perfect. But if you get very close you will see all the individual dots of ink that it’s made up of and the image will ‘fall apart’.

The way to make a printed image look better quality is to print it with more dots per inch. The limit is the number of pixels in the camera that took the photo.

That’s for printed images, but it’s not the same for images on the screen. You can’t increase the pixel density of the screen. Only the screen manufacturer can do that.

So when I upload a photo to the web, I don’t need to upload a huge picture because when looking at the complete photo I can’t increase the quality with more pixels.

The only time I would upload an original image is if I want the viewer to be able to zoom in and see the detail. The downside of doing that is that the file size of the photo will be much bigger and take up more bandwidth and be slower to load. I put some examples of web-size photos and bigger photos in an article I wrote in response to people submitting their work for consideration.

Cambridge Flooding: Willow In The Water

This is a view across the river Cam. On the far side the river has burst its banks and reached almost up the boathouses.

Tamara and I walk along this side of the river sometimes, and we discuss how it is that the branches of the trees are trimmed off so neatly. Tamara says it must be people who trim them. I say that the trees on the Common are trimmed by cows, so perhaps some animals do that here. It’s unlikely though because there is no access for cows. So she is probably right.

(Photographed with an iPhone 8).