People buying cameras tend to fixate on the size of the image, specifically, the number of megapixels. But how many do you actually need?
Well, that depends on a number of things: whether you want to display the picture on a computer monitor or as a print, how large it will be, the subject, and how fussy you are.
The standard resolution for computer monitors is 72 dpi. That means you should have 72 pixels across every inch of screen. If you want a picture to display 3" x 2", then that's 216 x 144 pìxels (3 x 72 = 216: 2 x 72 = 144).
For a photo on a standard Blogger blog, you only have enough width for 400 pixels. Even if it's a landscape photo, 600 pixels high, that's still a measly 0.24 megapixels. When I add a picture to an SMS message, it's slightly bigger than that. In other words, any mobile phone that has a camera has enough pixels for your blog.
If you want to fill the entire monitor, (perhaps for a Powerpoint presentation) the commonest monitor size today is 1024 x 600 pixels, which is still only 0.6 mega-pixels.
But if you print out a photograph at 72 dpi, it will look dreadful. The standard resolution for prints is 300 dpi. A postcard-sized print is 6" x 4", or 1,800 x 1,200 pixels = 2.16 mega-pixels. A standard 10" x 8" print (a bit smaller than letter-size) is 3,000 x 2,400 pixels = 7.2 mega pixels. Obviously a 10 or 12 megapixel camera will do that comfortably. But a full-sized exhibition print, 24" x 16" would be a whopping 7,200 x 4,800 pixels. That's 34 megapixels.
If you find an affordable 34 megapixel camera, please let me know. Until somebody makes one, it's probably best to stick to print sizes that won't disappoint you.
Fortunately, that's not the end of the story. You can increase the number of pixels in your image with Photoshop, or with the free program GIMP. It doesn't give as good a result as having the extra pixels inside the camera, but it's better than sticking with not enough pixels. The details will look a little blurry, though. Even the best inflation algorithm in the world can't produce details that fell between the cracks of your camera detector
300 dpi is the standard, but that doesn't mean it's carved in stone. Some subjects, where there is no fine detail, look fine at much lower definition. Sunsets and clouds, or anything out of focus, for example. A few, like water, may even look better at lower resolution, since the effect is much like soft focus. Print the skin under my eyes at 100 dpi, please!
And then there are subjects which look much better at 400 dpi, or even higher resolution. Long grass and animal fur are particularly demanding. It's not they they look bad at 300 dpi: more that they look better at 400 dpi, or even higher. (In much the same way as a print from a 35 mm negative looks OK, but one from a larger negative looks better. There's a simple reason why Ansel Adams carried a heavy plate camera and tripod around Yosemite.)
And finally, there's the question of how fussy you are. Some people don't mind blurry prints, just as some people don't mind a sea-horizon tilted by ten degrees. But if you're intending to sell your photographs, I strongly suggest that you ought to be pickier than your customers.
So if your photos are only going on the web, the number of pixels in the camera doesn't really matter. If you only want small prints, a modest camera will do. But if you have ambitions of exhibiting your work, you want every pixel you can afford.
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