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Megapixels, PPI and DPI Explained

Every year camera manufacturers come out with new cameras with higher megapixels to entice you to buy their products, but are more megapixels better?

First you need to understand what a ‘pixel’ is and how it will benefit you, to make that decision for yourself.

Pixels and PPI

A pixel (when talking about images on a computer screen) is one small ‘square’ of virtual data in an image. When you look at your photo on a computer screen what you are really seeing is lots of tiny squares of information that make up the image you see. Each square pixel will contain a combination of the colors red, green and blue to make up the color of that pixel. You will often hear the acronym ‘PPI’ (pixels per inch) when people talk about computer display/device screens (ipad etc.) too. If you want to see what a pixel looks like, you just need to zoom right into your photo till you see the individual squares. A ‘pixelated’ image is one where you can see the individual square pixels that make up the image.

Just to confuse things further, some people will refer to PPI as DPI, (dots per inch). DPI refers to the resolution of a printer and how many ‘dots’ of ink you will print per inch. So, it is PPI when talking about something on a screen and DPI, when you are referring to how many dots of ink you will choose to print per inch of photography paper.

The ‘Mega’ in Megapixel

The ‘mega’ in megapixel means a million. So a 10-megapixel camera would output images with 10 million pixels of information and an 18-megapixel camera will have 18 million pixels.

A photo that is 4368 pixels in width and 2912 in height is a 12-megapixel photo (12.7 to be exact). To get the calculation megapixel number, you multiply 4368 by 2912, which gives you 12,719,616 pixels.

So how many megapixels do you need?

Now that you know what a megapixel is, let’s figure out how many you need for the size of image you will be printing. First you need to establish at what resolution you want to print your image. I would recommend a resolution of 300 PPI or higher for a good quality photo. A higher resolution usually gives you a more detailed and sharper image. If you are in Photoshop you can find the resolution and image size dialog box by clicking on image and then image size.

Let’s use the pixel dimensions from the 12-megapixel camera. 4368 pixels in width, divided by 300 PPI resolution, will give me a print width of 14.56 inches. If we divide the height of 2912 pixels by 300 PPI we get a height of 9.7 inches. In this case, our photo size at a resolution of 300PPI will be 14.56 inches, by 9.7 inches.

At this point many people get confused, because they think that the resolution of 300 PPI, refers to how many dots of ink the printer will place on the photo paper per inch. This is not the case. It may help to remember that PPI is for displays or different devices (ipad, laptop etc.) and DPI is for the printer. To get that chosen resolution of 300PPI, your printer may need to put down 3 colours to represent the colour of 1 pixel. In which case you may have 900 plus dots of ink per inch of paper to represent your 300 PPI that you saw on your screen.

Now that you know this, let’s calculate backwards to see how many megapixels you need in your camera. If you always print 10x8 inch photos at a resolution of 300PPI, then as long as your image is 3000 pixels by 2400 pixels you are fine (a 7.2-megapixel camera will give you those pixel dimensions).

So why do people choose more cameras with higher megapixels? Photographers often print much larger images, so they will need more megapixels to achieve the larger sized prints. More megapixels are also good if you think you will be cropping your images a lot. Each time you crop your image you discard some of the PPI and therefore the dimensions of your image get smaller.

Hopefully by now PPI, DPI and megapixels are a little less confusing and you can now make an informed choice on how many megapixels you really need!

But for now, forget about the megapixels and go out and do what you do best - create!

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Content copyright © 2014 by Ewa Sapinska. All rights reserved.
This content was written by Ewa Sapinska. If you wish to use this content in any manner, you need written permission. Contact Ewa Sapinska for details.


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