From DXOMark: More pixels offsets noise3

Started May 30, 2009 | Discussions thread
natureman Veteran Member • Posts: 3,979
Re: Terms - not too confusing

ejmartin wrote:

natureman wrote:

Bogdan_M wrote:

These two things are the same. Image level for him was normalisation
(by downsampling with preblur) and then comparing the normalized
images at 100% (because once normalized they have the same
magnification).

How can you consider that the same magnification? If one image is
downsampled (or upsampled), and the other is not, to match the sizes
of the images, the magnifications are not the same.

Two FF images, one 12MP and the other 24MP, start off their life both
the same size -- 24mm x 36mm. When viewed at 100% on a monitor, the
24MP image is magnified by a factor sqrt[2] 1.4 times more than the
12MP image. By downsampling the 24MP image to 12MP, one restores
equal magnification to both images (for the purpose of viewing at
100% on a monitor).

I'm just going to throw a few things out here and see what responses there are:

Should magnification be based on sensor size, or megapixels?

If sensor size is what matters when it comes to magnification, why is it that the camera manufacturers state that image size depends on the amount of pixels?

Image editing and viewing programs have settings for percentages of magnification. Two images from different cameras with different sized sensors (with the same aspect ratio) but with the same amount of pixels would be the same size if magnified by the same percentage in a computer program.

In other words, two cameras with 12mp each (and the same aspect ratio), with one having an APS-C sensor and the other a FF sensor, would display at the same size if both were magnified to the same percentage of magnification in a computer program.

For instance, according to Canon a 5D has a maximum (100%) image size of 4368 x 2912 pixels, while a 450D has a maximum (100%) image size of 4272 x 2848 pixels. They're not exactly the same size because the total pixels amounts vary a a bit between the cameras, but they are very close even though the sensor sizes are quite different.

I don't recall ever seeing a camera company, a website, or a computer program basing image sizes from a camera on sensor size. It's always based on the amount of pixels.

That will do for now.

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tko
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