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 p.8 #20 · Sigma 35mm f/1.4 DG HSM - AF | |
OK guys, for what it is worth to some of you... I emailed CPS due to the MA shift I was getting between lighting sources. Not with distance, but light. Both copies I have tried of the Sigma 35 have performed identical... Great at any distance, MFD to infinity... 0MA 90% of the time on my 5D3... But sometimes indoors, requires around +8 to be sharp again.
I got a call back from CPS just now in regards to my email about the focus shift. I was told that behavior is perfectly normal for any camera body to exhibit, because the white balance temperature has a direct relation to the AF performance of the lens. Since the AF system is using light to achieve focus, the color temperature is very critical and can definitely cause shift between lighting sources. Fast lenses are more noticeable, obviously, due to the very shallow depth of field. f/2.8 lenses and up will not show nearly as well. The main point here is to MA in conditions you will be shooting with and adjust as required. One of the main reasons for the ability to MA, and not just to fix lens calibration issues
But he did stress to me that it is perfectly normal to see this type of behavior, especially indoors with complex color temperatures.
Now I know others and myself included have not seen this behavior as much (or at all) with Canon lenses. Here is my theory (comparing the 35L to the Sigma 35).
1.) The Sigma 35 is a MUCH higher resolving lens wide open. Therefor it is easier to see when it does miss, because you don't get that incredible detail you would when it does nail the focus. The 35L is not nearly as sharp, so a little focus tolerance is not as easily detectable.
2.) Being a third party, reverse engineered lens... The Sigma, while precise in focus is not exactly as accurate as the Canon would be. In a simple explanation, think of it as simple MA values... Canon's lenses may be calibrated to hit within +/- 2 values shot to shot in ideal conditions. The Sigma, may be +/- 3 values to critical focus. Mix that with my theory in #1, the Sigma may appear to miss critical focus a bit more often, because the Sigma has slightly looser tolerance with focus along with being much sharper, so it is much more obvious to visually see the difference in slightly focus.
3.) The lens coatings may also play a part in this... Canon knows exactly what light is required to ideally operate the AF system, therefore, lens coatings are optimized to allow the necessary light rays into the AF module. The Sigma, may be *slightly* different (which is why some see a slightly different color cast compared to Canon lenses), and in turn, may not be 100% ideal for the Canon AF system to utilize to get the most accurate AF shot to shot.
Now the three explanations above are just my own personal theories on the subject. This is not obviously accounting for distance related MA issues, but only the shift in various light sources.
In conclusion, the point is that all lenses will exhibit this behavior. But depending on one or a combination of the (3) factors above, may be more or less obvious to the user. So this shift we are experiencing with the Sigma (to me) appears to be no fault at all with the lens directly (not defective). But in reality is this... In the end, it is an extremely sharp, high quality, but still a reverse engineered lens. So it will never be as accurate and precise as a Canon optic, no matter what.
That being said, WHEN adjusted for the lighting conditions... The Sigma is a stellar performer. It focuses as good as any Canon for me with incredible, sharp results. No exaggeration here by any means. Compared to the Sigma 30 I had years ago, this one blows it away with the AF performance. No comparison. It is a whole new level, a Sigma "L" if you will.
Hope this helps clarify the shift and possible reasoning why it may be more noticeable. Thanks to Canon NJ for the information to help ease my mind with this shifting!
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