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

Started Jun 6, 2009 | Discussions thread
Daniel Browning Senior Member • Posts: 1,058
[5/6] Myth busted: small pixels bad, 4 legs good - part 5

[Part 5 out of 6.]

Other considerations

There are at least three things to consider with regard to pixel size

  • File size and workflow

  • Magnification value

  • In-camera processing (JPEG, etc.)

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

Out-of-camera JPEGs are affected by pixel density because manufacturers have added stronger noise reduction which may not be desired and may be difficult to tune with in-camera settings.

Higher pixel densities may require bigger files, slower workflow, and longer processing times. Lower pixel densities may result in smaller files, faster workflow, and shorter processing times. This is an area where there are many possible software solutions for having most of the benefit of smaller pixels without the size/speed downsides. REDCODE is a good example.

Finally, here is the math on a comparison of dynamic range between the LX3 and 5D2.

Compare the 2-micron pixels of the LX3 (10.7 stops DR) with the immensely larger 6.4 micron pixels of the 5D2 (11.1 stops DR). Going by the per-pixel numbers, it seems that the smaller LX3 pixels have less dynamic range. But remember that the LX3-sized pixel samples a much, much higher spatial frequency.

At the same spatial frequency, the scaled LX3 pixels have 12.3 stops of dynamic range, 1.2 stops greater.

5D2 maximum signal: 52,300 e- LX3 maximum signal: 9,000 e-
5D2 read noise at base ISO: 23.5 e- LX3 read noise at base ISO: 5.6 e-
5D2 per-pixel DR at base ISO: 11.1 stops (log_2(52300/23.5))
LX3 per-pixel DR at base ISO: 10.7 stops (log_2(9000/5.6))
LX3 scaled maximum signal: 92200 (9000 e- * (6.4µm/2.0µm)^2)
LX3 scaled read noise at base ISO: 17.92 (sqrt(5.6 e-^2 * ((6.4µm/2.0µm)^2)))
LX3 scaled DR at base ISO: 12.3 stops (log_2(92200/17.92))

Now you might be wondering: when does the law of diminishing returns kick in? Small pixels have to get worse at some point, even if they aren't now. It's a tricky question.

First, there's diffraction, lens aberrations, and mechanical issues such as collimation, alignment, backfocus, tilt, manufacturing tolerances, etc. Diminishing returns have definitely kicked in here already.

For diffraction, if anyone is shooting 5 micron pixels at f/32 because they really need DOF (e.g. macro), they are not going to get any benefit from smaller pixels: the returns will be close to 0%. Even f/11 will have returns diminished slightly by smaller-than-5-micron pixels.

Lens aberrations can be an issue too. Usually even the worst lenses will have pretty good performance in the center, stopped down. But their corners wide open will sometimes not benefit very much from smaller pixels, so the returns in those mushy corners may be 0-5% due to aberrations.

And there's the mechanical issues. If your collimation is not perfect, but it's good enough for large pixels, then it will have to be better to get the full return of even smaller pixels. This relates to manufacturing tolerances of everything in the image chain: the higher the resolution, the more difficult it is to get full return from that additional resolution. Even things like tripods have to be more steady to prevent diminishing returns.

So essentially the diminishing returns depend on the circumstances, but the higher the resolution, the more often the returns will be diminished. Some people don't need more resolution than they currently are getting, so as long as the improvement from smaller pixels is 0% or more, that's OK for them.

[Continued in part 6.]

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