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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.

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Just to satisfy normal human curiousity? Or is there something where it matters in which way megapixels are counted? –  Esa Paulasto Jun 27 '13 at 15:47
    
Nothing really matters , so all questions are of pure curiousity ;p /nihilist pov –  Michael Nielsen Jun 28 '13 at 9:12

7 Answers 7

up vote 13 down vote accepted

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.

I originally posted this as a comment to another question, but I think it answers this one.

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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. –  Michael Nielsen Jun 26 '13 at 21:24
    
I stand corrected. However, it's not a term in general photography. –  mattdm Jun 26 '13 at 22:35
    
And, in your example, those instruments are in a range where a factor of about 1000 makes for a nice human scale. –  mattdm Jun 26 '13 at 22:49
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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 –  Michael Nielsen Jun 26 '13 at 23:31
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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. –  mattdm Jun 27 '13 at 2:54

A megapixel is defined as 1 million pixels, not 2^20.

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what he said / points at MikeW –  Michael Nielsen Jun 26 '13 at 19:55
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Right. It absolutely does not matter. Photography today, particularly in Internet circles, is plagued by excess precision. The difference between 9.7, 10, and 10.2 is insignificant. We say "f/22" and don't worry that it really should round to 23 (which is good), but people talk about "ISO 51200", when we really should just say ISO 50k. Same thing applies here. –  mattdm Jun 26 '13 at 22:41
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I agree that in practice it doesnt matter, but as the topic here is taxonomy , it matters a lot if you use the decimal k/M or the "digital" k/M. It is not a mater of rounding off to 21MP or 20MP, it is that a sensor with 21MP real world Megas would have be be noted as a 20MP camera with base 1024 per kilo (and this is "rounded". that is not insignificant. So you see, rounding and definition is not the same thing. It has no practical influence on the images, ofc, but that it not the topic. –  Michael Nielsen Jun 27 '13 at 8:31
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But the fact that it doesn't matter is the important part. The usefulness is in describing a certain class of resolution, not really in specifying the exact number of pixels. –  mattdm Jun 27 '13 at 12:20
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-1; I don't think an uncited sentence in a Wikipedia article is good evidence, particularly when (as @mattdm's answer) it doesn't really matter. –  Reid Jun 28 '13 at 17:26

It depends how you count but almost every company multiplies the number of photosites and divide by one million. They rarely make the distinction if those photosites are next to each other or layered. For this reason, a 45 MP Sigma SD1 makes an image which has the same resolution as a 15 MP Canon 50D.

They sometimes quote two numbers, effective megapixels and actual. Effective are the ones that make into final maximum resolution images and which may be a little less than the actual ones which are how many are on the sensor. Some of these may be masked out to read the back levels and others lost because of the imaging area of the lens.

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It depends on whether you're selling or buying. When you're writing marketing literature, you want the Mpix number to be as high as possible. That means you use 106 for "mega". When it's to your advantage to make the number look small, you use 220, which is 1,048,576.

In reality, a 5% difference in the total number of pixels is pretty much irrelevant. Note that the linear resolution goes with the square root of the total number of pixels, so 5% more pixels is only 2.5% more linearl resolution. You won't be able to notice that difference even in two prints at the proper size you get to compare side by side.

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In computing, when talking about kilobytes, and megabytes, the terms kilo and mega have traditionally been modified, letting kilo=2^10, and mega=2^20.

This has led to confusion, because hard disk manufacturers would use megabyte to indicate 1 million bytes instead of 2^20 (resulting in more impressive numbers).

This has led to the definition of two new terms, Kibibyte and Mebibyte, meaning 2^10 and 2^20.

But when talking about something different than bytes, kilo and mega should still refer to their original meanings, one thousand, and one million.

Thus a megapixel should be 1 million pixels. But this can often be an approximation. E.g. my 18 megapixel Canon EOS 7D 'only' has 17.9 million pixels.

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To answer you have to understand what a pixel is.

Wikipedia :

In digital imaging, a pixel, or pel (picture element) is a physical point in a raster image, or the smallest addressable element in a display device.

So Mega being a unit prefix, it simply means 1'000'000. Knowing that, 12 Megapixels means simply 12'000'000.

This said, when your camera constructor displays 'Around 14.3 Megapixels' in the data sheet, it's a simplification to avoid writting stuff like : 14,204,928 pixels.

This value being calculated from the resolution of the pictures you're taking : 4352 x 3264 pixels = 14,204,928 pixels.

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I would like to also say that each "pixel" in a DSLR is actually only a portion of a pixel. So, the sensor itself has say, sensitive light elements for an R and a G and a B, and maybe another G. Now, these three, or four, together should form a single pixel but it don't. They interpolate it and make the four, count as four pixels.

Or something similar (http://en.wikipedia.org/wiki/Bayer_filter)

This means that your 20 megapixel camera might actually be a real 5 megapixel camera but it's interpolated up using algorithmic magic.

Same applies to the LCD screen. A "1 million dot" screen only has 300kish pixels. Sadly.

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This underestimates the quality of those interpolation algorithms. If it were simply a matter of 4:1, the practice would probably have stopped as sensor density increased and file size correspondingly went up. But in fact, the interpolation really does contribute to extra resolution. It's not as good as 1:1, but it's also not 4:1. –  mattdm Jun 27 '13 at 23:05

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