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
- 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
- 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:
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.