Hi,
To begin with:
Thinks about two horizontal rows of pixels with a line nearly parallel to pixels rows. If the pixels are small with spacing between them the line could either affect the upper row or the lower one. So the line would at one point flip from one row to next one. This is called the staircase effect some times. Now if the pixels are large, the line line would affect both rows simultaneously at a stage, so it would glide from one row to another. A thin line would still flip. I'm not good at illustration but you could try it with a ruler and checked paper.
As long a lens can project a line that is thinner than the spacing between the sensors this problem will arise.
Now, what is resolution? Resolution is the ability to tell small thing at small distances apart. To be able to do that you need some contrast (like a dark line between two bright lines). MTF just measures the contrast between the dark and light part of a line pair. The thinner the line pairs are the lower the contrast will be. Ultimately MTF and resolution will be limited by diffraction.
Now, let's consider the Sigma with a pixel spacing of about 0.08 mm. It would have about 63 lp/mm. At this frequency simpler lenses will transfer really little contrast. Most MTF charts cover 10, 20 and 40 lp/mm, so we are essentially left with very little data. A perfect lens may have something like 65% contrast at f/8. This says essentially that with a lower quality lens a line pair would be mostly gray, with a truly excellent lens they would be dark gray and very light gray.
So a good lens would show jaggies while a less good lens would smear out the lines.
A Swedish monthly that tests both cameras and lenses has discovered that they need to find optimal apertures for resolution tests on new cameras. Earlier they could just use top notch optics stopped down to f/8 but this is clearly not the case any longer. For 4/3 cameras and also the Canon 50D they need about the best lens in the inventory, and stop it down to f/5.6 resolution worsens by f/8. Now, the Swedish monthly is using Imatest and what they actually measure is the resolution where half the contrast is lost, and I think this also takes some sharpening into account. But there is no doubt that sensor pitches are close to be limited by lens performance.
You will find absurdly high resolution figures for lenses like 200 lp/mm, but those figures are only possible at large apertures an at smallish contrast.
This figure that I have measured myself illustrates the issue.
http://83.177.178.7/ekr/images/stories/difractionlimit.gif2000 Lines/PH corresponds to about 63 lp/h. You can see that resolution is diminishing when stopping down beyond f/8. Better lenses would shift the peak to the left. Higher resolution would push the peak upwards. Note that the Alpha 100 (at 10 MP) doesn't resolve much higher than the KonicMinolta 7D (at 6 MP) past f/11 or so. This limitation is by laws of physics, it applies to Leica and Zeiss, too. For optimum sharpness you cannot stop down!
Best regards
Erik
Thank you Erik for helping understand these things in a plain fashion as I am not well versed in the scientific terms.
The possibility of the line on a single pixel is something that comes to my mind.
So if the SD14 had higher resolution, would we be seeing those jaggy lines (also see it on the thicker lines going diagonally up) ?
I am not sure of #2 being a factor, it is beyond my observations and understanding of the relation.
#3, I would like to know if the 50D has a lower RAW setting...ONLY THEN can we see if the AA is doing the job...As I suspect the line would be a blurry darker tone of NOTHING if it were the case. But I cannot say for sure. :-)....
Because we have to calculate the measure of how it is viewed.
