This is the third in a series of tests on the Sony 100-400 f/4.5-5.6 GM zoom. The series starts here.
I have found the AF accuracy of the Sony a7RII to be outstanding, except in the case of two lenses. One is the Batis 85, whose longitudinal chromatic aberration (LoCA) seemed to cause the AF mechanism to sometimes optimize the red plane focusing, rather than my preferred green plane. And the other is the Sony 12-24/4, which has some difficulties at some f-stops and focusing modes. Today’s post is about testing the AF accuracy of the camera with the 100-400 attached.
I used this test protocol:
- Sony 100-400, set to 400 mm
- a7RII, firmware 4.0
- f/5.6, f/8, and f/11
- ISO 200
- Prefocus set to on
- AF-S
- Flexible spot, S, M, L; Center; Zone (5 cases in all)
- Release priority: focus
- A-mode exposure; all shutter speeds at each aperture were identical
- EFCS
- Wescott LED panels set to 5500 K.
- Electronic shutter
- 31 exposures (yes, 31; I lost a fight with the intervalometer) 4 seconds apart
- Target distance about 6 m.
- Target centered
- ARW exported as TIFF mosaiced file in dcraw (document mode)
- TIFFs cropped and raw channels selected in Matlab program
- MTF50 of cropped TIFFs measured with MTF Mapper
- Data assembled in Matlab
- Data plotted in Excel, for one of the green raw planes.
Here’s the center third of the frame in both height and width (one-ninth in area):
The first thing I did was do a run using Flexible Spot with the large spot option, and the best of four exposures at each aperture using manual focusing — it is so painful to manually focus this lens that the thought of making 3 series of 31 images each using manual focus is something I can’t even comprehend.
The three horizontal locations are, moving from left to right, f/5.6, f/8, and f/11. The vertical axis is the MTF50 value in cycles per picture height. Sigma is statistician-speak for the standard deviation. If the distribution is Gaussian, about 70% of the results will lie between the mean + sigma and the mean – sigma lines. The yellow line is the manually-focused case. You can see that it was marginally better than the best AF image at f/5.6, fractionally worse at f/8, and more so at f/11. Except for the worst capture at f/5.6, the spacing of all the lines is quite tight. This is excellent AF performance with what should be a very easy target.
I increased the exposure for the rest of the captures, so the numbers aren’t directly comparable.
A repeat of the above run, less the manual focusing:
Making the spot a bit smaller:
Smaller yet:
Using the Center AF option:
And the Zone option:
The particular option chosen makes little difference. This is different from many cameras I tested.
With this target, the AF performance of the Sony 100-400 zoom is excellent. This is a good thing since the manual focusing experience is so painful.
I demoed the 100-400 and an a9 for a few weeks, shooting football and non-sports events, against my usual Canon kit, including a 1DXII and 200-400 for sports. The 100-400 easily held its own against the 200-400 in respect to overall image sharpness and focus tracking ability on the a9, except for one major problem. As soon as I zoomed the 100-400, the a9 would completely lose focus tracking and end up focused on something way off of the intended subject. At first I didn’t really catch what was happening and just thought the camera was having trouble. But sure enough, when I specifically tested it, it happened consistently. Just to be clear, this happened with focus set to AF-C and with AF tracking of a moving subject (or stationary too). The camera & lens would track the subject just fine as long as the lens wasn’t zoomed. As soon as it was zoomed, focus would jump off the intended subject. I only had an a9 to try, so not sure if this would also happen with an a7RII, or any of the other cameras. I no longer remember the firmware version of the camera and lens, but since it wasn’t my gear, I didn’t check if there were updates available. In any case, a friend demoing the same equipment (but different copies) had the exact same experience. I thought the lens was impressively sharp and enjoyed using it, except for the inability to AF track while zooming, which is potentially a deal breaker for my requirements, if not fixed by Sony.
I’ve not seen that, or looked for it. That behavior would be consistent with the way that the a7x and a9 cameras reset things when you touch the zoom ring. I think you ought to be able to zoom in magnified view, but you can’t.
thanks,
Jim
I haven’t used the Sony cameras much with zooms and wasn’t aware of there being a reset when a lens is zoomed. I agree, you’d think it would be preferable to be able to zoom while in magnified focus.
During the demo period I also had access to the 70-200/2.8 but didn’t specifically test it for AF during zooming. That said, while I think the same problem happened with this lens, it didn’t seem to be as significant. At least it wasn’t an obvious thing in use. If it happened, the camera seemed to recover much more effectively than what I experienced with the 100-400. Given that many will use the 100-400 for action, whether sports or wildlife, I imagine there will be the desire to be able to zoom while tracking a subject and sequence shooting during zoom pulls.
I never really gave it a second thought coming from long-time Canon DSLR use because there it simply works as expected. A lot of my 200-400 use is shooting extended sequences during zoom pulls, usually with great AF tracking. The 100-400 problem reminded me of reports of the Samsung NX1 mirrorless back a couple years ago. That camera required a firmware update in order to enable AF during zoom…