Image Resolution
Photo Resolution Explained

Image resolution, you see the term crop up in photography articles and hear it banded around the forums, but what exactly is it and why do you need to know?

Well if you want to make sure that your images are crisp and sharp when either viewing them on screen or printing them out, then knowing how resolution affects your images will ensure you get the best results.

Understanding what resolution is can be a little confusing, I know it took me a while to fully get to grips with it, so I have tried to simplify the whole process down and hope that by the end of the article you have a better understanding.

What is resolution?

When we talk about image resolution we are basically talking about the quality of an image. For instance:

If all the details were sharp and clear even in the smallest detail then we would say that the image had a high resolution.

If fine detail is blurry or appears as blocks of color then the resolution is low.

Digital Images are made up of tiny little blocks of color known as pixels (picture elements). There can be hundreds, thousands, even millions of these pixels in every image and the more pixels in an image the better the detail.

Pixels are arranged in a small grid format, the smaller the pixels the smoother the image appears. If the pixels are too large the picture breaks up and appears more like a mosaic.

The size of the pixels = print resolution. I.e. how big the image can be printed without seeing the blocks of pixels.

Printing and Image resolution

Low resolution = Large pixels = Limited detail

image resolution

High resolution = Small pixels = Smooth image with fine detail

image resolution

When printing an image the term used is PPI (pixels per inch).

i.e: 1 ppi = each pixel is 1 inch square, 1000 ppi = each pixel is 1/1000 inch square

Every digital image contains a finite number of pixels so if you print at a high ppi then those pixels are squeezed into a smaller area and so reduce the overall size of your print.

i.e: An image containing 1000 pixels printed at 1 ppi will produce a print 1000 inches wide whereas an image containing 1000 pixels printed at 1/1000 ppi will produce a print 1 inch wide.

So when printing your image you must balance the print size with the amount of pixels in your image to produce a sharp image. If printed too large then the pixels will be visible and the image will be poor quality.

Generally 300ppi is considered to be the optimal image resolution when printing.

image resolution - image size menu

Calculating Print Size In Photoshop.

  1. Open your image in Photoshop
  2. Go to image>image size
  3. Make sure the resample image box is NOT ticked
  4. Enter a desired resolution in the resolution box i.e. 300.

  5. The dimensions of the width and height of your image will change accordingly, indicating how big your image will print at.

Different Types of Resolution

Just to confuse things even more the term resolution can be applied to different areas of photography. Below is a brief description of each.

Scan resolution

When scanning an image using a flatbed scanner you want to make sure that you capture enough detail (pixels) to create a smooth reproduction, remember this should be a minimum of 300ppi.

To print same size as the original image, set scanner software to capture 300ppi.

If you want to print larger than the original you will have to use a higher resolution i.e. if you want to print double the original size then you should scan at 600ppi.

To work out the correct resolution

Divide the desired width of the print by the width of the original image, and then multiply it by 300.

i.e: A4 print required, A4 = 297mm (longest edge), Slide image = 36mm (longest edge)

297 / 36 = 8.25
8.25 x 300 = 2475

2475ppi will be the resolution required to scan at.

Image Resolution

Once a digital camera has captured an image it has no physical dimensions only pixels i.e. how many pixels are in the image (referred to as mega pixels), this is image resolution. This is why cameras are sold indicating their mega pixel capture rate. Remember the more pixels the bigger the print.

Screen Resolution

image resolution - display proerties

This is the maximum amount of pixels a monitor can display. A pixel on a monitor is different to an image pixel as they are independent of each other. 72ppi is generally the norm for monitors.

Each pixel is made up of three smaller pixels, red, blue and green, these are adjusted to create a single color

Just to complicate things a little further your monitor also uses a graphics card to display the correct image resolution on screen. You should always set your monitor to display the highest available resolution possible. To do this:

  1. Go into the control panel menu
  2. Choose display properties
  3. Change the screen resolution to the highest setting.

Printer Resolution

This is the resolution your printer software will instruct your printer to print at i.e. the quality of the print is determined.

Printer resolution is measured in dpi (dots per inch), i.e. the amount of dots of ink on the page.

image resolution - printer settings

Again the higher you set your printer software resolution to print at the better the quality of final print. Common resolutions are 360dpi, 720dpi, 1440dpi, 2880dpi and 5760dpi.

The higher the dpi the more ink is being used, the longer the image will take to print
and the better quality of paper that will be required to best display the image.