Producing and comparing MTF's is not really as easy as it may seem. Different manufacturers use different contexts, sensors are often tested in a different context than lenses, and direct apples to apples comparisons can be difficult.
That said, the way Canon MTF charts work is fairly strait forward. The 10lp/mm MTFs are intended to measure sharpness at a moderate level of contrast. That is supposed to correspond to MTF50. MTF50 is used because it has been demonstrated to closely determine whether an optical system can produce results that the average human viewer will perceive as "sharp, with good contrast". MTF50 is not exactly a resolution test, it aims to measure how IQ will be perceived. These days, as the resolving power of sensors and lenses has increased, 10lp/mm may increasingly become more similar to an MTF80 than MTF50 (based on the separation of airy discs at wider apertures). With a diffraction-limited lens at f/8, 10lp/mm better represents MTF50...but still, it is not entirely the same.
The 30lp/mm MTFs are intended to measure resolving power. That corresponds better to MTF10, however it has the same problem as 10lp/mm and MTF50. Both lenses and sensors resolve much more today than they did a decade ago. MTF10 is really a resolution test at the Rayleigh Criterion. Rayleigh is effectively a systems minimum resolving power according to the following:
The Rayleigh criterion is the generally accepted criterion for the
minimum resolvable detail - the imaging process is said to be
diffraction-limited when the first diffraction minimum of the image of
one source point coincides with the maximum of another.
MTF at Rayleigh is usually performed at a low contrast, ~9% (for vision, 10% in most imaging test cases). A high line pair count is best to determine resolving power and maximum resolution. These days, with such high resolution sensors, the use of a 30lp/mm test chart is probably insufficient to effectively determine what an imaging system is really, truly capable of resolving. Canon's MTF's measure how accurately that 30lp/mm test chard is reproduced, and as sensor and lens resolving power increases, the accuracy at which the test chart is reproduced certainly increases. A resolution of 30lp/mm might correspond better to MTF20, I am not really sure.
Theoretically, ignoring some of the complexities of bayer arrays and pixel interpolation...a modern sensor in luminance-only resolution is capable of resolving much finer detail than 30lp/mm (assuming a perfect lens at a wide enough aperture.) The Canon 1D X is capable of a luminance spatial resolution of ~72lp/mm, the 5D III ~80lp/mm. The Canon 7D is capable of 116lp/mm. The prototype 7D Mark II, currently rumored to have a 24.1mp APS-C sensor, would in theory be capable of resolving as much 135lp/mm. With a near-perfect f/4 lens, these sensors could resolve considerably more than 30lp/mm...AT MTF50!
For a little math, just using a bit of the theory, here is what the 1D X, 5D III, and 7D II could theoretically resolve with the exceptionally good EF 600mm f/4 L IS II lens (which, for all intents and purposes...at least based on Canon's current MTFs, might as well be perfect).
Quick Fact: Total System Resolution is based on the total system blur, which is the Root Mean Square of the resolutions of each independent component. As such, the resolving power of the combined system is always going to be lower than the highest resolution of the best component. Improving the resolving power of the poorest component is the best way to improve resolution as a whole.
To truly derive the system resolution of an imaging system, you would need to account for each factor...individual lens elements, pockets of air between lens elements, the effect of each layer of the filter stack (IR cut, low pass, etc.) on top of the sensor, etc. That is generally impractical for something like this. So, we'll assume that, at a lenses optimal aperture, it is effectively behaving as a purely diffraction-limited lens, and thus capable of the theoretical maximum resolution at that aperture. The EF 600/4 II performs optimally at it's widest aperture, so we'll assume that it is capable of 173lp/mm (which is the theoretical max for MTF50). We'll assume that for the three cameras, they are capable of 68% of the physical spatial resolution (the numbers I mentioned above: 72lp/mm, 80lp/mm, 135lp/mm). I say 68% to account for the differences in spatial resolution between green and red/blue pixels, as well as the sparse sampling rate.
So, the math:
TSB = sqrt(LR^2 + SR^2)
Where:
- TSB = Total System Blur; Size of "blur" in microns
- LR = Lens Resolution; Diffraction limited "blur" in microns
- SR = Sensor resolution; Pixel pitch in microns times 0.68...approximate blur for bayer
For the 1D X, system blur is:
TSB = sqrt(2.89µm^2 + 9.174µm^2)
= sqrt(8.3521µm + 84.1623µm)
= sqrt(92.5143µm)
= 9.6184µm
The blur circle size is 9.6 microns. In terms of spatial resolution in units of line pairs per millimeter, that is:
SR = 1l/(TSBµm / 1000um/mm) / 2l/lp
Where:
- SR = Spatial Resolution, in line pairs per millimeter
- TSB = Total System Blur, diameter of blur circle in microns
- Additional conversions to reduce microns to millimeters (as lines per millimeter), and from lines to line pairs
So, system spatial resolution:
SR = 1l/(9.6um / 1000um/mm) / 2l/lp
= 1l/(0.0096mm) / 2l/lp
= 104.l7/mm / 2l/lp
= 52.085p/mm
The maximum resolving power of the 1D X and the EF 600/4 II is 52.085p/mm, assuming the lens is diffraction limited at an MTF of 50%. That is nearly twice the 30lp/mm Canon measures, and five times the 10lp/mm measures. If we run the same numbers for the forthcoming 7D II:
SR = 1/(sqrt(2.89^2 + 4.9^2) / 1000) / 2
= 1/(sqrt(32.3621) / 1000) / 2
= 1/(0.0057) / 2
= 175.4 / 2
= 87.7lp/mm
If the next 7D is released with a 24.1mp sensor, it'll be capable of resolving approximately 87 line pairs per millimeter, at MTF50. At MTF10, maximum resolving power is FAR higher than at MTF50 (theoretically, while diffraction-limited f/4 resolving power is ~173lp/mm, at MTF10 it is 373lp/mm! So a 30lp/mm test chart is still generally inadequate to measure maximum resolving power of any modern camera with a modern lens.)
Finally, all of the math above assumes a diffraction-limited lens. In your test cases, you are testing with the lenses at very wide apertures. MOST lenses do not perform optimally when wide open like that. For most lenses, the sweet spot is around f/4, maybe slightly wider, often closer to f/5.6. A lens must be diffraction limited in order to have a moderately good idea of how well it is able to perform. At wide apertures, such as f/2.8 and, to a significantly worse degree, f/1.8...lenses become aberration limited. Optical aberrations occur in a variety of corms, each with their own mathematical contributor to the PSF of the lens as a whole. It is far more difficult to know how well a lens is performing when it is aberration limited than when diffraction limited (as when diffraction limited, diffraction is by far the primary contributor to the PSF.)
Resolution in an aberration-limited context can be significantly worse than in a diffraction limited context. Especially at ultra-fast apertures like f/1.8. I did a visual test of the effects of aberrations and diffraction on IQ (it is in a thread somewhere here), and at f/1.4 through f/2.8, IQ was WORSE than at f/22!! So, if you are testing lenses wide open, do not be surprised if resolution (which in your case looks like l/ph, or lines per picture height) is much lower than you would prefer it to be.
