Pixel density - can the playing field be leveled???

Started Jun 6, 2009 | Discussions thread
Daniel Browning Senior Member • Posts: 1,058
[2/6] Myth busted: small pixels bad, 4 legs good - part 2
1

[Part 2 out of 6.]

Although it's not necessary, it is always possible to resample any two images (e.g. large pixel and small pixel images) to the same resolution for comparison. This would make it possible to compare 100% crops and draw conclusions about the spatial frequencies under analysis. However, sometimes the ability to resample is called into question, such as a blog post by Phil Askey at DPReview.com:

http://blog.dpreview.com/editorial/2008/11/downsampling-to.html

However, it was thoroughly debunked:

http://forums.dpreview.com/forums/read.asp?forum=1018&message=30190836

http://forums.dpreview.com/forums/read.asp?forum=1000&message=30176643

http://forums.dpreview.com/forums/read.asp?forum=1031&message=31560647

There is ample proof that resampling works in practice as well as in theory. Given that fact, as long as small pixel sensors have proportionately higher noise and higher spatial frequencies, it will always be possible to resample the image and get lower noise power at lower spatial frequencies, so that the image is the same as that created by large pixel sensors. For example:

http://forums.dpreview.com/forums/read.asp?forum=1018&message=30211624

http://forums.dpreview.com/forums/read.asp?forum=1018&message=30190836

Again, in cases where it's not necessary to resample the image, it is best not to, since the resolution often has a highly beneficial impact on the image.

Another way to think about it is performance per detail. Say one small but important detail in an image is an eye. A large pixel sensor has a certain performance ”per eye”, so that over the area of the eye there is a certain noise power, dynamic range, etc. A small pixel sensor, too, has a certian performance per eye, again over the same area, only there are many more pixels. The noise power per pixel is higher, but since each pixel contributes a smaller portion of the eye, the noise power per eye is the same.

Here are some very good references about noise and spatial frequency:

http://luminous-landscape.com/forum/index.php?showtopic=29801&st=20&p=241562&#entry241 562

http://forums.dpreview.com/forums/read.asp?forum=1000&message=30394220

http://forums.dpreview.com/forums/read.asp?forum=1034&message=31584345

http://forums.dpreview.com/forums/read.asp?forum=1000&message=30394220

Here are some images comparing small pixels and large pixels at low analog gain and high analog gain:

http://forums.dpreview.com/forums/read.asp?forum=1019&message=31512159

This error may have roots in the fact that the standard engineering measurements for sensor characteristics such as noise is necessarily at the level of the pixel. Sensitivity is measured in photoelectrons per lux second per pixel. Read noise is measured in RMS e- or ADU per pixel. Dynamic range is measured in stops or dB per pixel. There is nothing wrong with per-pixel measurements per se, but they cannot be compared with different pixel sizes without understanding the difference in spatial frequency.

Image sensor performance, like MTF, cannot be compared without understanding differences in spatial frequency.

[Continued in part 3.]

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Daniel

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