Too many megapixels? Six is enough for an 8x10.

Started Apr 26, 2009 | Discussions thread
Daniel Browning Senior Member • Posts: 1,058
[2/2] Noise power is a function of spatial frequency

[...continued from part 1]

Unequal sensor sizes.

It's always necessary to consider the impact of sensor size. The most common form of this mistake goes like this:

1. Digicams have more noise than DSLR.
2. Digicams have smaller pixels than DSLR.
3. Therefore smaller pixels cause more noise.

The logical error is that correlation is not causation. It can be corrected by substituting "sensor size" for "pixel size". It is not the small pixels that cause the noise, but small sensors.

A digicam-sized sensor with super-large pixels (0.24 MP) is never going to be superior to a FF35 sensor with super-tiny pixels (24 MP).

Unequal processing.

The most common mistakes here are to rely on in-camera processing (JPEG). Another is to trust that any given raw converter will treat two different cameras the same way, when in fact none of the commercial ones do. For example, most converters use different amounts of noise reduction for different cameras, even when noise reduction is set to "off".

Furthermore, even if a raw converter is used that can be proven to be totally equal (e.g. dcraw), the method it uses might be better suited to one type of sensor (e.g. strong OLPF, less aliases) more than another (e.g. weak OLPF, more aliases).

One way to workaround this type of inequality is to examine and measure the raw data itself before conversion, such as with IRIS, Rawnalyze, dcraw, etc.

It's important to be aware of inequalities that stem from processing.

Unequal expectations.

If one expects that a camera that has 50% higher resolution should be able to print 50% larger without any change in the visibility of noise, despite the same low light conditions, then that would be unequal expectations. On the other hand, if one only expects to it be at least print the same size and the same noise for the same low light, then that would be equal expectations. Such output size conditions are arbitrary and in any case does not support the "small pixels are noisier" position.

Unequal technology.

If you compare a 5-year-old camera to a 1-year-old camera, it will not be surprising to find the new one is better than the old one. But in one sense, it will never be possible to compare any two cameras with completely equal technology, because even unit-to-unit manufacturing tolerances of the same unit will cause there to be inequalities. It's common to find one Canon 20D with less noise than another Canon 20D, even if absolutely everything else is the same. Units vary.

I don't think that means we should give up on testing altogether, just that we should be aware of this potential factor.

So that summarizes the reasons why I think the myth has become so popular. Here is some more information about pixel density:

http://forums.dpreview.com/forums/read.asp?forum=1034& ;message=31584345
http://forums.dpreview.com/forums/read.asp?forum=1032&amp ;message=16107908
http://forums.dpreview.com/forums/read.asp?forum=1029&amp ;message=21440105
http://forums.dpreview.com/forums/read.asp?forum=1029&amp ;message=23296470
http://forums.dpreview.com/forums/read.asp?forum=1019&amp ;message=31512159
http://forums.dpreview.com/forums/read.asp?forum=1018&amp ;message=30211624

A paper presented by G. Agranov at 2007 International Image Sensor Workshop demonstrated that pixels sizes between 5.6 and 1.7 microns all give the same low light performance.

http://www.imagesensors.org/Past%20Workshops/2007%20Workshop/2007%20Papers/079%20Agranov%20et%20al.pdf

Eric Fossum said that FWC tends to increase with smaller pixels: "smaller pixels have greater depth (per unit area) and saturate 'later in time'". ( http://forums.dpreview.com/forums/read.asp?forum=1000&message=30017021 )

So the question might arise: what should be considered with regard to pixel density? There are at least three things to consider:

  • File size and workflow

  • Magnification value

  • Out-of-camera JPEG

File size is an obvious one. Magnification is what causes telephoto (wildlife, sports, etc.) and macro shooters to often prefer high pixel density bodies (1.6X) over FF35.

Out-of-camera JPEGs are affected by pixel density because manufacturers have responded to the throngs of misguided 100% crop comparisons by adding stronger noise reduction. If JPEG is important to you and you can't get the parameters to match your needs, then it becomes an important factor.

Higher pixel densities require bigger files, slower workflow, longer processing times, higher magnification for telephoto/macro. For me this is not a factor, but it may be important to some shooters. Lower pixel densities result in smaller files, faster workflow, and lower magnification.

I'm sorry this post is so long, I did not have time to make it shorter.

Noise scales with spatial frequency.

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Daniel

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