How many pixels in a megapixel?
I am trying to find out the definition of a megapixel? Some references on the web place it at 1 million pixels, and other places say it is equal to 2^20 = 1,048,576 pixels.
Just to satisfy normal human curiousity? Or is there something where it matters in which way megapixels are counted?
About a million.
I think that in general due to rounding — and more importantly, other real world factors which mean that megapixels only relate loosely to actual resolving power — it doesn't really matter if "megapixels" is binary or decimal. It is a useful term because it happens to be in the range where we get human-useful small numbers with the digital cameras (so far). It's rarely used to mean a precise value — one 16-megapixel camera will likely generate photos with a slightly different size than one from another brand.
For the same basic reason, "kilopixel" isn't a real word, because there's no particular case where it would be useful.
Overall, a lot of us coming to photography from a tech background, be it programmer, engineer, or otherwise, have a tendency to look for precision. When it comes to exposure, anything under a third of a stop is unlikely to be a big deal, and when it comes to pixels, a similar basic rule makes sense: until we're talking about doubling or halving the number, don't sweat it.
I originally posted this as a comment to another question, but I think it answers this one.
My thermoimager is 20kPixels, and Basler racer series are 2-16 kPixel . That is as useful as MPixels. Or in Baslers case it is more useful than the usual MP measure as they dont mix up 2 dimensions in an unrecoverable manner.
And, in your example, those instruments are in a range where a factor of about 1000 makes for a nice human scale.
yes and MPs is a human scale, thus M= 1.000.000. it is not a measure about a digital unit , like kbytes and Mbytes in the computer. the pixels are on our real world domain, so decimal numbers are correct. The colour depth of hte digitaized signal, however, is and exists only in the binary domain and there k= x1024 M= x1024^2 are correct.
and the threshold where it matters as much as 1 Mp whether you use x1000 or x1024 as a base is at 20.5MP. So a 21MP camera would be a 20MP camera in base 1024. No way the manufacturers would advertise it as a 20MP camera, though, so I bet they agree, haha. Not that it matters if it really has a few more or less pixels though. a factor 2-4 is what really matters
Right, exactly. At 20.5MP, even though the difference in rounding is One Million, that amount isn't so important that we care about the details. I completely agree about it only really mattering when you're getting to significant factors. Above 20 we should probably round to the nearest 5, and once cameras are routinely in the 40+ mpix range, rounding to the nearest 10 would be reasonable.
it is not a matter of rounding from 20.5 up or down. See my last comment on the other answer