09-07-2011, 04:17 PM
[quote name='Bjoern' timestamp='1314645558' post='11147']
I had the same experience with the Tammy and Pentax. The Pentax is round about the same in speed if something a tiny bit quicker, BUT it is a lot better to find the focus. I never touched the [Tamron] again.[/quote]
It just occurred to me, hmm after years of photography, that longitudinal and in some cases lateral chromatic aberrations, as well as spherical aberrations and low contrast, would tend to interfere with fast, accurate autofocusing. Meaning that optically superior lenses would tend to "autofocus better". Regardless or in addition to issues involving the distance the lens elements must move, the mass of the elements, the "gearing" of a screw drive, the power of the driving motor, speed of controlling electronics and the sophistication of the controlling algorithms, loss of light in translucent mirror, lens aperture, etc etc.
As a most simplistic example, have a to-me-worthless old Sigma 80-200 push-pull zoom. Built nicely and compactly, with wonderful handling. But so much longitudinal color aberration that it becomes a philosophical question, what is "the focus point", when you look at a magnified image in the viewfinder. Do you mean the focus point where things are looking reddish around the edges, or the point where they're bluish? Are you focusing on something bluish, that's comes out best where the fringes are reddish? Does God exist? etc. All questions to be pondered by all too mortal autofocus firmware.
I had the same experience with the Tammy and Pentax. The Pentax is round about the same in speed if something a tiny bit quicker, BUT it is a lot better to find the focus. I never touched the [Tamron] again.[/quote]
It just occurred to me, hmm after years of photography, that longitudinal and in some cases lateral chromatic aberrations, as well as spherical aberrations and low contrast, would tend to interfere with fast, accurate autofocusing. Meaning that optically superior lenses would tend to "autofocus better". Regardless or in addition to issues involving the distance the lens elements must move, the mass of the elements, the "gearing" of a screw drive, the power of the driving motor, speed of controlling electronics and the sophistication of the controlling algorithms, loss of light in translucent mirror, lens aperture, etc etc.
As a most simplistic example, have a to-me-worthless old Sigma 80-200 push-pull zoom. Built nicely and compactly, with wonderful handling. But so much longitudinal color aberration that it becomes a philosophical question, what is "the focus point", when you look at a magnified image in the viewfinder. Do you mean the focus point where things are looking reddish around the edges, or the point where they're bluish? Are you focusing on something bluish, that's comes out best where the fringes are reddish? Does God exist? etc. All questions to be pondered by all too mortal autofocus firmware.