Cameras have a history of increasing the number of pixels with each new product release. We have seen digital cameras starting off around 1.5 to 2.5 million pixels (MPx) and now you can get up to 20 MPx readily and even more for big, and expensive, pro cameras.
Your Smartphone probably has something between 4 and 8 MPx even though the actual camera occupies a small corner of a hand sized technology packed package.
So what is a pixel and when does the number of MPx matter?
What is a pixel?
Digital pictures are made of small squares of photo information and each individual square is called a 'pixel'; it's a name derived from the concept of a 'picture cell'. These are the individual elements of the digital picture that are connected back to the separate parts of your camera's sensor that respond to the light falling on it.
Before we go on to the examples it is important to view this web page on a computer monitor as the size effects will not show on many smart phone screens, the size and resolution is too small.
Computer displays and resolution
This picture shows an image at a resolution of 200 x 150 px, that's 30,000px or 0.03 Mpx. You can clearly see the jaggedness imposed by the large blocks cause by the individual pixels that comprise the image.
This is the same image but now at an even lower resolution of 20x15 pixels, 300Px. It is now impossible to tell what the object is.
This picture has bee recorded at a resolution of 1,000 x 750 pixels, 0.75 Mpx; it is shown here as only 300 pixels wide. Click on the picture to see it at full size (use our browser's Back button to return here). On any monitor or projector (other than the highest professional equipment) it is practically impossible to discern the blocks.
For on-screen display the largest resolution that can be displayed is that of the display itself. In recent years that has come to mean 1,600 x 1,280, i.e. a fraction over 2MPx. In practice, on a website, the picture only occupies a proportion of this space and our example of 1,000 x 750 will normally display well.
Photo Printing and resolution
However, the picture changes when we look at photo printing. Prining is inherently a higher resolution process as the final printed image is an analogue object and the resolution is not limited by pixel size. The limitation is now the acuity of your eyes.
Many experiments have been undertaken and it is established that the resolution necessary for a good quality printed image is 300 dpi (that's dots per inch, or pixel). Really top quality prints will go further, but you stray into Pro territory here.
Given the 300 dpi and the standard sizes of photo printing, what resolution do you need from your images to get good quality prints through the process?
||Image PX width
||Image PX width
So, from this it is clear that if your camera has 10MPx then you can print up to 12 x 10 or A4 with good results.
Smartphone cameras are now 4 to 5 Mpx so should print up to 8 with excellent quality and be quite acceptable up to 10 x 8, quite remarkable for such a tiny device. But there's a lot more to know about Smartphone cameras; but that's for another time.
The effect of cropping on picture quality
Another factor to take into account is cropping. When you crop a picture you reduce the number of pixels that make up the image. To see the size of the effect, let’s do a simple calculation.
Let’s use some small numbers so the math is clear, say we start with an image that’s 400 by 300 and crop 10% from each edge. Our original image has 120,000 pixels.
Now let’s do that cropping. The width is reduced by two lots of 40 (10% each side) and, similarly the height is reduced by two lots of 30. Thus we have an image that has shrunk to 320 x 240 pixels, that’s 76,800 pixels.
The new image has only 64% of the pixels of the original image. Ouch!
So it really is important to fill your camera’s frame with your picture to get the best printed quality!