What's in a megapixel?

I found this table buried deep in a Pentax manual but it does give you a good idea of the size of image you can expect to create with a given camera megapixel resolution. I've added the couple of lines to add context of common screen displays & web sites.

Megapixels	Size	Print/Display
14.6	4672×3104	A2 paper	14"×17"
10	3872×2592	A3 paper	10"×12"
6	3008×2000	A4 paper	8"×10"
3.1	2048x1536	Google+ (free)	6"x8" LR5b smart preview
2	1824×1216	A5 paper	5"×7"
0.5	800x600	Web Pages	Older computer screen

For those that like there information graphically, Design 215 has a great megapixels chart. (also shown below)

Finally here is a great little megapixel calculator that will back calculate a lot of useful information, given your image size in pixels. It will calculate such measures as aspect ratio, file size in various formats,  approx. number of photos that will fit on your memory card at that resolution and suitable print sizes. It is worth noting that the 300dpi requirement so often claimed by otherwise knowledgeable photo sites, is a little misleading. The dpi (dots per inch) is actually a measure that dates back to traditional printing whereas modern ink jet are more truly measured in ppi (pixels per inch). Each pixel normally being produced using 4 tiny dots of CMYK colour, Cyan, Magenta, Yellow, Black, in a 2 by 2 dot pattern. Frequently these dots overlaps on purpose. So at the simplest level 300ppi is equivalent to 150dpi. A lot of common ink jet "photo" printers normally only have a standard print density equivalent to 120 or 190 dpi, but with good results. Some of the more expensive ones may offer a High Res, setting that will be equivalent to 200-300dpi. So if you are a keen home printer you might be able to "push" the maximum size a little.

Remember that megapixels are not a measure of the true picture quality, which relies on a number of other factors, exposure, focus, composition, points of interests, colour balance etc. Megapixel is purely just a measure of size.

Small and fast = )

Digital Photos can be quite large, especially as camera manufacturers are continually upping the megapixel count, often without improving the image quality. If you just want the photos for a website, blog, facebook or to email to relatives, you usually don’t need to send the full image. In fact you can do your bit for more responsible use of band width but optimizing the size of your image. Even if you have a reasonably fast net connection you will probably find that the conversion time is more than rewarded in upload time once you are processing more than one or two photos
Image Optimizer is really neat service (with a free downloadable version, that can do bulk conversions). I have previously posted about windows live & picassa’s ability to export & email smaller version of photos.
Here is a useful table for common sizes, you can click on the size descriptor to see an example of that image size.

Suggested Pixels	Max Width	Max Height
Small Web Photo	640	360
Blog Photo	400	300
Email	800	600
Flickr/Photobucket	800	600
Normal Print 10 by 15cm (4 by 6”)	1200	800

Remember which ever approach you use, keep the original photos and just use the converted files for the upload or email (then delete them)