What is a Megapixel?
A megapixel simply means 1 million pixels. But what is a pixel? A pixel is a single point (or dot) in a graphic image. All graphic images are made up of thousands of tiny dots. Your computer screen, your digital images, both are made up of thousands (or millions) of little dots. If your camera is 8 megapixels, it means that any pictures it takes (on its highest quality setting) will consist of 8 million of these pixels. For example, the Canon Rebel XT on its highest setting, takes pictures which are 3456x2304 pixels. If you multiply this, you get 7,962,264 pixels, which would make this an 8 megapixel camera. If your camera does not have enough megapixels, you may find printing a quality 8x10 difficult. Generally, 5 megapixels is enough to print good quality 8x10s. But, just because one camera has a certain amount of megapixels doesn't mean that it will take better pictures with one with a lower amount. There are many factors which affect this, including build quality, type of camera, etc. So, don't just jump and buy a lower priced 10mp over a higher priced 8mp for example just because it has more megapixels. Do your research, find reviews, and see what one is really worth it.
How many megapixels is enough?
Obviously, if you're just planning on printing small prints of your photos, or just planning on viewing your photos on a computer, you can get by with a lower number of megapixels. But, if you want to print quality 8x10s, you will need to step up your game. First, let's look at what makes a quality photographic print. For a quality print, you'd ideally want to start with a photograph which contains at least 240 DPI (or dots (pixels) per inch), 300 DPI would be even better, but 240 will get you by. So, with this knowledge, we know that for a 4x6 print at 200 DPI, we would need an image size of 960x1440 pixels or better. For a 5x7, we would need 1200x1680, and for an 8x10 we would need 1920x2400 or greater. Now, most 5 megapixel cameras produce an image of around 2592x1944, which when printed at 240dpi comes out to a 10.8" x 8.1" print. So, with a 5 megapixel you would be in the clear, but what if you want to crop your image? If you cropped even a little bit off, you'd be altering the DPI drastically, and your image would no longer look that great. Hence the need for more megapixels.
Why isn't more megapixels always better?
As I mentioned earlier, a camera's megapixel rating is merely a rating of how many pixels it will contain in the largest image that it can produce. Camera companies keep touting more and more megapixels, to the point where even some point and shoot cameras have well over 10 megapixels. But does this mean that they will take pictures of the same or better quality that their higher priced lower megapixel counterparts can? Of course not. By continuously upping the megapixels which cameras can output, the camera companies are not paying as much attention to quality as they should. The camera companies know that the average consumer will see a 10 megapixel camera as automatically better than an 8 megapixel camera. Buy why? The higher megapixel camera merely contains more pixels and a higher resolution in the photos it takes, but its not necessarily any sharper than its lower megapixel counterpart. Just because you are cramming more pixels into a photo does not mean that the pixels are sharp enough for there to be any discernable increase in image quality. So, while your pictures may be getting larger, they might not even be sharp enough to be printed at this larger size, merely because there was an increase in pixels, but not an increase in quality. In theory, more megapixels would mean a nicer photo, but in actuality it may just mean a terrible photo composed of more dots.
Once you get past 4 megapixels or so, the resolution stops mattering as much, and optical quality comes more into play. This is why a 8 megapixel digital SLR camera will take better pictures than a 10 or 12 megapixel point and shoot digital. Digital SLRs use extremely advanced components and optics. In fact, the old outdated 3 megapixel Canon EOS-D30 will take pictures that rival some of the cheaper 8 megapixel digitals. Another fact worth mentioning is that digital SLRs have larger sensors than your typical point and shoot. These larger sensors produce much less noise than their point and shoot rivals, leading to a much cleaner shot. This allows for higher ISO settings with less noise. There are very few point and shoot cameras which can come anywhere near ISO 1600 with even remotely acceptable noise.
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