One of the most discussed features of the Canon EOS 5D Mark IV is Image Microadjust. This uses the slight difference in perspective between the left and right-facing halves of the split 'dual' pixels to fine-tune the effective focus point of the images.
Like everyone else, we were interested to see what degree of refocusability this gave.
If you're wondering: 'will this let me correct which eye my portrait is focused on?' The answer is a resounding 'no'. Indeed, even if the question is: 'can I shift the focus back from the eye lashes to get the iris sharp,' the answer isn't much more positive.
Dual Pixel Image Microadjustment
We set up the 5D Mark IV with EF 35mm F1. 4L II USM at F1. 4, set up at approximately 25x focal length distance from our LensAlign target. The Dual Pixel Raw file was then processed in Digital Photo Professional (DPP) to see how much the maximum backward and forward adjustments could move focus.
+5 (Max backward adjustment)
0 (No adjustment)
-5 (Max forward adjustment)
AF (Lens) Microadjustment
For comparison, here's the amount of adjustment that can be achieved using AF microadjustment - the traditional method for calibrating your lens to your body to correct back/front-focus issues. The rollover starts at +1 as this is the degree of adjustment needed by this lens on this body.
+20
+10
+3
+2
+1
0
-1
-2
-3
-10
-20
Real-world difference
To demonstrate the real-world impact image microadjust might have on a traditional head-shot portrait, we shot Carey with an EF 85mm F1. 8 at F1. 8.
This portrait was very slightly front focused, so we tested the degree to which it can be refocused, backwards. For each of the adjustments, 'Strength' was set to 10 to maximize the input from one set of pixels.
+5 (Max backward adjustment)
0 (No adjustment)
-5 (Max forward adjustment)
Interestingly, it appears the images become noticeably softer when you apply forward or backward adjustment, which may mask some of the advantages of the focus shift (there's a chance that slightly better-looking results will be possible if you apply higher levels of sharpening to the microadjusted images). However, the degree of correction we're seeing is so small (on the order of -2 to +2 in terms of traditional AF microadjustment) that we wonder whether it's worth the effort of incorporating DPP into your workflow, especially given its slow performance even on a fast computer. Or the doubling in file size.
Overall, traditional 'AF (lens) microadjustment' is a much more powerful tool for achieving pinpoint sharpness and ensuring any particular lens is properly calibrated to your body. It appears the Dual Pixel CMOS design's primary value is what it was designed to do - easy-to-use and accurate Dual Pixel AF - rather than a means of correcting slight focus error. We hope to see more capable implementations in the future as Canon iterates on the technology.
. dpreview.com2016-8-31 20:03