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05-07-2010, 05:12 PM   #1
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Question about megapixels

I was reading a review of the Canon T2i on dpreview and in a comparison to the K-x it says:
The difference between the raw output of the EOS 550D and the Pentax K-x is pretty much the same as we've seen in the JPEGs. Both cameras are capable of excellent resolution, but the EOS 550D just edges ahead thanks to its greater pixel count. However, the difference is so subtle as to be almost irrelevant to normal photography at print sizes smaller than A3.
When I looked up A3 it said 11.7 x 16.5 inches which is a huge picture! My question is why isn't megapixel difference between 12 and 18 greater or more noticeable. I have read many megapixel myth articles so I'm not really trying to bring up that argument. It's more of a sub-question. I just couldn't believe it took a picture that big to see the differences (I believe it. I just didn't understand why.)

05-07-2010, 05:58 PM   #2
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Because most printers can't print more than 300 pixels per inch anyhow. The K-x already has almost 3000x4500 pixels, which at 300 pixel per inch works out to a print size of 10x15". So any more pixels than that would be simply thrown away in the process of printing at sizes 10x15" or smaller.
05-07-2010, 06:05 PM   #3
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A camera is more than just resolution. But to make it simple:

The printer's resolution is 250 dpi. It makes no difference if you give it a file with resolution greater than that.

11.7 X 16.5 inches at 250 dpi is 2925 X 4125 dots.

K-x is 4288 X 2848, roughly 11.7 X 16.5 inches at 250 dpi.

T2i is 5184 X 3456.

In fact, with large prints, you can get away with less than 250 dpi due to the greater viewing distance (compared to smaller prints).
05-07-2010, 08:36 PM   #4
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QuoteOriginally posted by justtakingpics Quote
My question is why isn't megapixel difference between 12 and 18 greater or more noticeable.
Note that the 1.5 factor translates to "only" 22% more resolution in each dimension.

12 MP is plenty and you only need more if want to
  • print 300dpi larger than 9.5x14.3.
  • have more headroom for cropping (can be used as a "software tele converter").
  • have finer-grained noise (however, noise is one of the fortes of the K-x).


05-08-2010, 10:39 AM   #5
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Great answers above, but here's another one:

The human eye is only capable of seeing so much. And when you see an incredible image on a website that fills the screen, that's only like 12 inches wide at 72dpi.

Which can be taken with the cheapest point and shoot.
05-10-2010, 06:38 AM   #6
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QuoteOriginally posted by justtakingpics Quote
I was reading a review of the Canon T2i on dpreview and in a comparison to the K-x it says:
The difference between the raw output of the EOS 550D and the Pentax K-x is pretty much the same as we've seen in the JPEGs. Both cameras are capable of excellent resolution, but the EOS 550D just edges ahead thanks to its greater pixel count. However, the difference is so subtle as to be almost irrelevant to normal photography at print sizes smaller than A3.
When I looked up A3 it said 11.7 x 16.5 inches which is a huge picture! My question is why isn't megapixel difference between 12 and 18 greater or more noticeable. I have read many megapixel myth articles so I'm not really trying to bring up that argument. It's more of a sub-question. I just couldn't believe it took a picture that big to see the differences (I believe it. I just didn't understand why.)
Because per pixel sharpness may be higher and also 12 to 18 megapixels is "only" 50% more which (sqrt 1.5) would be 22% more at each side. And eventually the glass isn't sharp enough anyway.
05-10-2010, 04:06 PM   #7
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QuoteOriginally posted by aliquis Quote
Because per pixel sharpness may be higher
I guess you are referring to different strengths of AA filters. Everything being equal, a 12MP sensor will definitely have more contrast between pixels (but also less resolution).

QuoteOriginally posted by aliquis Quote
The contrast between two pixels for the 12MP and also 12 to 18 megapixels is "only" 50% more which (sqrt 1.5) would be 22% more at each side.
I think it is good practice not to repeat things which have already been pointed out in a thread.

QuoteOriginally posted by aliquis Quote
And eventually the glass isn't sharp enough anyway.
On what basis are you stating this? It is certainly not true for good glass. Furthermore, MTF curves are multiplied in an optical chain. An improvement to a link which is not the weakest link will still result in an overall improvement.

Please be careful in making assertions about things you are not quite on top of. I don't mean this personally, it happens a lot in the forum, but I hope that asking individuals to be more careful with their assertions will make the forum a more reliable source of information.
05-13-2010, 10:09 AM   #8
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Another thing to consider it that with digital photography, it is very easy to crop and common to crop photos to as much as 100% and make prints. The poor mans telephoto lens. Whether or not an 18 megapixel camera does a better job of this is a question I can't answer because I'm too poor to afford one.

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