What is a Megapixel? – Print Quality and Digital Cameras

What is a Megapixel

Americans shoot more and more photographs each day thanks to the magic (and low cost) of digital photography. Twenty years ago you had to buy film (expensive), then take pictures, then pay to have that film processed and printed (more expensive!). This meant that, generally, people took fewer pictures – and more than a few rolls of film got left lying around with vacation photos undeveloped because of the inconvenience of the process.


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Now people take photographs with digital cameras all the time. The cameras have shrunk down to where they fit inside our phones, after all, and nearly everyone has one. Of course most of the phone cameras take low quality images – “low quality” in the sense that they are “low resolution”, and most use a fixed-focus plastic lens. Still, many are good enough that they look decent when printed at 4×6 inches, particularly as they’re primarily snapshot images. But what if you want a larger print, one to hang on the wall? You’re going to need more megapixels – but what is a megapixel, and why does it matter?

First things first – “pixel” is short for “picture element” – the “dots” that make up digital pictures. On the typical modern computer display, they’re one of approximately 16 million colors, and there are somewhere between 65 and 100 of them per inch of your display. A “megapixel” is approximately 1 million pixels. A full screen image, for most computers, is somewhere between .7 megapixels and 2 megapixels. Cameras available today range from around three megapixels all the way up to 22 megapixels. What this means is that if you only want to view images on your computer screen, 2 megapixels might be sufficient for your application. But what if you want prints – old style, hard-copy photo prints.

Printed images are made up of dots, too. They’re just much smaller dots and much closer together. The general guideline, to produce quality images, you need 300 pixels per inch of printed image. So a .7 megapixel screen image (1024×768) would be only 3.5 inches by 2.2 inches at “photographic quality”. This is the guideline suggested by magazine producers and photo service companies like Kodak. So when you set out to determine the number of megapixels you need, you’ll need to decide the largest print size you think you’ll want to make. This is important because in most cases you cannot increase the number of pixels your camera produces, so think ahead.
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As an example, say you think you’d like to be able to make “photo-quality” 8×10 inch prints from your images. This means your camera must be able to produce 10×300, or 3000 pixels on the long side, and 8×300, or 2400 pixels on the short side. This means you’ll need 3000×2400, or 7.2 megapixels, for a full-quality 8×10. Unfortunately, though, that’s not the whole story – the aspect ratio of the image recorded by the camera you choose may mean you need more than that basic 7.2 megapixels, due to cropping. An 8×10 has the ratio of 5::4, whereas most cameras are built around the standard 35mm film aspect ratio, which is 2::3. In practice, this means a full-frame 8×10 will require an 8.6 megapixel camera, or 3600×2400 – which would produce an 8×12 image, but we’re cropping an inch off of either side to fit on our print.

Furthermore, you may want to crop even more. Say you have taken a wonderful picture of a fountain in Paris, but the image is somewhat tarnished by the gaggle of tourists in the lower left hand corner. You can zoom in to crop that part out, but this reduces the number of pixels you have to print with. If you want to have this capability, and still want to make 8×10 photo quality prints, you’ll need even more megapixels. In this case, the sky is the limit, with each successive increase in pixel count adding more cropping ability while maintaining print quality.

What is a Megapixel
It’s important to note that all pixels are not equal. There are no-name “six megapixel” cameras available that do what’s called “interpolation”. Interpolation is the process of calculating an average value for pixels that fall between pixels that were actually recorded. So if we have an image that’s 1024 pixels on the long edge, we can interpolate that to 2048 pixels. This doesn’t add any information to our image, though, and results only in a smoother picture that’s not nearly as sharp. So while you are able to make a 5×7 inch print from the resulting image, it won’t look nearly as good as one shot with a non-interpolated 6 megapixel camera. You can avoid this by staying with known brands like Canon, Nikon, Olympus, Sony, Pentax, and similar, or by carefully reading the specifications looking for words like “interpolation” or “sensor pixels”. Be careful, though – it’s common for off-brand manufacturers to do everything in their power to tell the truth in such a way that you misinterpret it.

Another important factor to pay attention to is something called “viewing distance”. It’s the distance one would be expected to view a print from. Small prints that you can hold in your hand are designed for viewing distances as close as ten to fifteen inches. A 20 inch by 30 inch poster is meant to be viewed from several feet. This is a complex concept to master, but what it means is that the allowable resolution can be reduced for large images, since detail will be lost in distance anyway. Kodak’s guidelines suggest you can go as low as 100 pixels per inch at these sizes and still get prints that look good when viewed at appropriate distances. Remember, though, that if you hold the print close, or walk up to it on the wall, it won’t look as sharp and detailed as the 8×10 print you made from the same image.

The megapixel race continues apace in digital cameras, with new ones coming out every day with more and more pixels packed into a smaller and smaller space. Once you decide the image quality you need, based on maximum print size and cropping allowance you think you’ll need, you’ll be prepared to go shopping. If you’re serious about taking pictures, you should obtain sample images from the cameras you are considering – preferably by actually shooting images in the store, if you’re buying locally. Then take the images home and look at them on your computer, or go to your local photo processor and get a couple of prints made, to see what they look like. Only then, when you’re satisfied with the outcome, should you put your hard-earned money on the counter. If you follow this guide, though, your new camera should bring you solid value and lasting enjoyment. Good luck and happy shooting!

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