Originally posted by regken You have aroused my curiosity. Is there a way of looking at the Lp/mm of a lens and be able to tell if it is capable of out resolving a sensor with a specific MPixel count?
Funny to see the word "arouse" in this context. As a non native speaker, I thought it would have some reserved other meaning
Well,
if you know the Lp/mm of your lens (line pairs per mm, or lines per mm) then you know how many lines the lens resolves with 5% (or 10%) contrast (which is not much -- black and white both become very similiar shades of gray here!). You better know the lens' MTF which tells you how many lines for 70%, 50% or 20% contrast... Also, some tests quote the MTF50 figure (i.e., the figure for 50% contrast). But MTF50 is not normally the figure defined as resolution.
Anyway, assume you know the lens' Lp/mm figure for some accepted contrast figure.
Multiply by 2, then multiply by your sensor height in mm, to obtain the corresponding pixel row count.
Example:
Lens: 150 Lp/mm
Pixel rows: 150 x 2 x 15.6 = 4680
K20D rows: 3104
So, this lens would still yield some low (but acceptable) contrast between pixels.
Zeiss claims that their K mount lenses resolve about 300 Lp/mm. Such figures are normally obtained using laser beams and not normally being publshed. Review magazines cannot do this.
You can inverse the computation.
For a 14.6 MPixel sensor, you get 99.4 Lp/mm (also known as Nyquist frequency).
The better the contrast at 100 Lp/mm for a given lens, the better the image. Contrast figures beyond 100 Lp/mm don't matter for the K20D.
Most tests I see quote about 1300 Lp/IH (per image height) for the K20D and use the DFA 100mm Macro. This is below 1550 Lp/IH (82 rather than 99 Lp/mm) this lens should easily deliver. I think those tests use the MTF50 value and contrast with the DFA 100mm Macro at 100 Lp/mm is slightly below 50%. Because contrast can be boosted by the firmware, MTF50-based resolution values aren't very interesting and Pentax regularly looses here against the competition despite them having less pixels...