Why *less* MP is better

Started Dec 17, 2005 | Discussions thread
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Lee Jay Forum Pro • Posts: 52,092
Why *less* MP is better

I was recently called "rude" for trying to teach someone about this so I thought I'd start a thread to provide information to those that might not understand this topic as well as they'd like to.

Let's start out with print size. You really only need about 1MP to get a pretty decent (200 ppi) 4x6 print. Most people who own compact cameras hardly ever print bigger than that. More on this later.

If the sensor in the 20D were simply cut down from it's current size (22.5x15mm) to the size of the sensor in the Pro 1 (8.8x6.6mm), it would be about 1.4MP (1370x1028). This would provide sharp 4x6 prints with high-ISO performance and dynamic range equal to the 20D, namely very usable ISO 3200 and quite good ISO 1600 with about 9 stops of dynamic range at ISO 100.

However, it's not unreasonable to expect owners of compact cameras to occasionally grab a great shot that they want to print bigger than 4x6. You need about 2.5MP for a very good 8x10 (180 ppi). If you were to put enough pixels in the Pro 1 for an 8x10, you'd have 1800x1350 which would provide about 1/2 stop lower performance than the 20D in high-ISO noise and dynamic range given the same basic sensor technology.

I actually think this is about where we should stop for the smaller, less advanced cameras. However, the "prosumer" market is likely to be more demanding of print size.

I think a very reasonable stopping point for that market is a 13x19 print. This is the largest you can get from the wide ink-jet printers and the largest most owners of these cameras are likely to ever print. To get a pretty good 13x19, you need about 5-6MP. Such a camera in 4x3 would be about 2850x2138 (150 ppi). Such a camera would be 1 to 1 1/2 stops behind the 20D in high-ISO performance and dynamic range. That would provide usable ISO 1600 and pretty good ISO 800 with very clean ISO 400.

Some proposals for the Pro 1 replacement suggest up to 12.8MP. That's about 4131x3098. Such small pixels with the current sensor size would cost about 2-stops versus the 20D. That means usable ISO 800, pretty good ISO 400 and clean ISO 200. That's assuming it's even possible to manufacture 2.1 micron pixels in a CMOS process as good as that used on the 20D.

To me, the tradeoff isn't worth it. The biggest problem with these compact cameras already is dynamic range with high-ISO noise coming in a close second. I think the "prosumer" buyers should be looking for a Pro 1 with a fast, sharp lens with IS and about 5-6 MP with usable ISO 1600 (all technologically possible, it seems). That would provide the ability to shoot good 13x19 prints in low-light or with good dynamic range in bright light, all with a camera the size of the Pro 1 - not the size of the Sony R1.

I hope I've been able to show that MP aren't the only indicator of image quality (far from it, in fact). As a matter of fact, in many cases, more MP means lower image quality , not higher, as a result of the loss of dynamic range even at low-ISO. Noise is a little bit different and there are some mitigating factors I didn't want to go into so I did the above analysis at the pixel level.

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Lee Jay
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