ErikKaffehr
Well-known member
Hi Jerome,
Let's look at a practical expirement by Jim Kasson:
Leica 90mm f/2 Apo Summicron-M ASPH at f/2. Jim shot 41 expsures at around three meter distance utilizing the full rnage of his Stackshot rail and evaluated MTF for each exposure.
Done that he plotted MTF for red, green and blue channels. You can see that MTF has three peaks. Those represent different planes of focus.
I would say B peaks about 2.62 and G about 3.93, so we have something 1.3 mm shift of focus.
Next image is same lens stopped down to f/4. Here I would say the blue curve peaks at 1.31 while the green one is probable centered about 2.40. So the shift been the color planes here is around 1.1 mm.
So the optimal color planes were 1.3 mm apart at f/2, and 1.1 mm at f/4. That is a small improvement.
The plane of optimal focus has also shifted something like 1.5 mm., by stopping down.
The main effect of stopping down was that the peaks got broader. That reduces the visible effect of the color plane shift. That broadening is caused by the increased depth of field but also by diffraction when stopping down beyond optimal aperture.
Here is the corresponding graph for the Otus at f/1.4:
And at f/4:
The aberration is the shift of best focus colour planes. It causes colour fringing and the amount of the fringes we see is aperture dependent.
Best regards
Erik
Let's look at a practical expirement by Jim Kasson:
Leica 90mm f/2 Apo Summicron-M ASPH at f/2. Jim shot 41 expsures at around three meter distance utilizing the full rnage of his Stackshot rail and evaluated MTF for each exposure.
Done that he plotted MTF for red, green and blue channels. You can see that MTF has three peaks. Those represent different planes of focus.
I would say B peaks about 2.62 and G about 3.93, so we have something 1.3 mm shift of focus.
Next image is same lens stopped down to f/4. Here I would say the blue curve peaks at 1.31 while the green one is probable centered about 2.40. So the shift been the color planes here is around 1.1 mm.
So the optimal color planes were 1.3 mm apart at f/2, and 1.1 mm at f/4. That is a small improvement.
The plane of optimal focus has also shifted something like 1.5 mm., by stopping down.
The main effect of stopping down was that the peaks got broader. That reduces the visible effect of the color plane shift. That broadening is caused by the increased depth of field but also by diffraction when stopping down beyond optimal aperture.
Here is the corresponding graph for the Otus at f/1.4:
And at f/4:
The aberration is the shift of best focus colour planes. It causes colour fringing and the amount of the fringes we see is aperture dependent.
Best regards
Erik
I don't understand how you can write at the same time "does not vary with aperture" and "stopping down reduces the visible effects". Maybe you are confusing longitudinal and lateral chromatic aberration? In any case, longitudinal chromatic aberration varies with aperture and lateral chromatic aberration does not: https://en.wikipedia.org/wiki/Chromatic_aberration#Types