Originally Posted by
rnclark
A few things that might clarify this thread for people. Things are not always so black and white. First there isn't a simple sharp cutoff of line pairs per mm (lpm), and there is still perceived detail beyond MTF50. There is detail that people do perceive in images close to MTF0 (that's 0% contrast for those not familiar with MTF). Next, while putting on a 1.4x TC does reduce light by a stop, the magnification is only 1.4x so the resolution loss due to increased diffraction is only 1.4x. Depending on the quality of the TC, the loss could be a little greater, but beyond that, it will not significantly change the angular resolution of the main lens, which is what we are concerned with. In general if the use of a 1.4x TC meant a 2x loss in resolution, most here would not use TCs. Clearly this is not the case.
So now let's look at the 7D case. At the 4.3 micron pixel spacing, MTF (contrast) is down to about 20% (about 35% at f/4) pixel to pixel with perfect optics (diffraction limited). The blur filter will reduce that a a little more. But as has been discussed on BPN with macro photography working at f/16 and slower with some cameras, the mere fact that there is a hint of contrast means that the contrast can be boosted. and some of the image quality restored using tools like unsharp mask. If one uses more sophisticated tools, like Richardson-Lucy image deconvolution, more can be recovered. The cost is generally more noise, and of course if pushed too far, one gets algorithm artifacts.
So, I find the premise of this test/comparison flawed and the specifics of the reasons the test came out this way did incorrect.
I feel your tone in your response earlier this evening to Alan was a little harsh. Please try and tone it down.
Some notes on that response:
Note bicubic is a cubic spline in 2 dimensions. Every resampling algorithm in my experience does well on some subjects and not so well on others. Cubic spline generally does a good job.
And too simple math is not the answer to refute what others are trying to tell you. Lenses don't deliver, for example, 50 lpm and lower. It is a slow fall-off. You should quote MTF 0% if you want to talk limited by optics. And note it is also possible to detect isolated subjects that are much smaller than the diffraction disk; such subjects also impact image perception.
It is the basic fact that lenses roll off slowly in MTF why cameras like the 7D can squeeze out a little more resolution, and do it with a 1.4x TC, and why citing single MTF values is incomplete.
Roger
