Autofocus Accuracy and Consistency

In my initial testing indoors I shot a series of objects at different distances,
cycled through them to get at least 20 images in total. I repeated these
tests using various selectable AF points across the frame to simulate my
typical usage. My inital findings were that straight out of the box, (i.e.
without any AF-miscroadjustments) the lens was consistenly nailing focus.
I had no concerns. After almost a year of happy shooting I estimate the
lens delivers a healthy keeper rate of upwards of 90%. 

But feelings are one thing, facts are another though so lets delve into this
in a more measured fashion...

Setup:
I set up a camera with cable release on sturdy tripod. 
 Exposure Mode: Manual
 Quality: RAW
 AF mode: Single shot
 AF type: Single point AF
 Drive mode: Single Shooting 


Methodology:
step 1) defocus to foreground, 
step 2) release, 
step 3) defocus to background, 
Step 4) release
...repeat steps 1-4 until 20 AF-driven images are taken

step 41) Switch lens to manual focus
step 42) Turn on live view with 10x magnification
step 43) Take reference shot for comparison

I ran the test for 6 scenarios:
1) Center AF-point at f/5.6
2) Center AF-point at f/1.4
3) Off Axis AF-point at f/5.6
4) Off Axis AF-point at f/1.4
5) Low light with Off-Axis, cross-type AF-point at f/2
5) Very Low light with Off-Axis, line-type AF-point at f/2

Analysis of Results:
1) The first test is very easy for most lenses. Cross-Type AF point,
reasonable depth of field, midtone subject, good contrast for AF system to
use. Subject was in full shade. The AF was 100% consistent

2) The second test is a little more tricky. Subject was in full shade.
Cross-Type AF point and good contrast for AF system, however this is,  
shallow depth of field, dark subject, and the textured background is
brighter than the subject and there is a bright downpipe in the foreground to the right of the cross-type point. Here we missed 2 images out of the 20, both front-focused slightly. AF was 90% consistent

3) The third test was relatively easy as well. While it is only a line-type AF
point, the subject is in full sun and has very high contrast. AF system
should have no problem measuring the subject distance and there is
plenty of depth of field as well. The AF was 100% consistent

4) The fourth test again was a bit more tricky. Subject was in full shade.
Line-type AF point, moderate subject contrast, shallow depth of field. Here
again we had 2 images which were marginally front-focused, so AF consistency was 90%.

5) The fifth test was conducted on the 80D, which as of making of this
video has the tightest pixel pitch available in DSLRs. Ambient light was
5.3Ev (9.3 stops darker than full sun) - this is 2/3 of a stop darker than the
DXO "low light" lens tests. Subject was backlit, but direct sun was flagged
off (so no direct sunlight into lens) Subject dark-to-midtone. Off-Axis,
Cross-type AF point. Shallow depth of field. AF was spot on with 55% of
shots. 30% were marginal misses while the remaining 15% were clear
misses. Of all the inconsistent shots 25% were front-focused, which
means the remaining 75% of shots were backfocused. With a small backward calibration the AF accuracy and consistency would be improved.

6) The sixth test was conducted on the 6D at even lowere light levels. 3.3
Ev. (11.3 stops darker than full sun) Off-Axis, non-cross-type AF-point was used. Subject is slighly darker than midtone subject, but with good contrast for AF system. AF was 75% consistent with 20% being marginal misses and only one shot being clealy misfocused.

Conclusions:
i) Consistency was excellent in ideal scenarios
ii) In trickier scenarios consistency remained at a level I'm happy to work with.
iii) On the ultra demanding 80D, in low light conditions accuracy dropped but was mostly still in the range where AFMA calibration could prove useful. (the near-misses were predominantly in the same direction)
iv) Fairly consistent performance with the 6D in low light.

Update:
I did a followup test with the 80D after doing AFMA calibration which resulted in a more consistent keeper rate:

General Comments:
When using autofocus high contrast elements on your subject acquire focus more reliably.
If your subject is brighter than the surrounding then the camera will grab focus more consistently
Brighter objects near the selected AF point can fool the AF system if the lens is focusing from a very defocused state.
Back-light is tricky for an AF system. If there is flare introduced by back-light then AF system is more likely to miss focus.


 

Amphibian Crisis

I remember recreational outings as a kid, when we'd go to ponds and streams, camping areas and picnic sites with friends and family. We kids would play in the streams and catch frogs and tadpoles for fun and releasing them at the end of the day. I remember how fascinated I was by these animals, how dramatically they physically changed during their life cycle and that all the transitional forms could be captured in the same stream. It was easy as their numbers were countless. Who knew that in our lifetime things would change so drastically?

Sadly, today the sound of croaking frogs is fast disappearing. Habitat loss, climate change, Increases in UV-B, environmental pollution, invasive predators (introduced by humans for sport fishing) and spread of diseases ... all these factors brought on by accelerating human activity are decimating amphibian populations that previously were able to thrive in healthy aquatic ecosystems and isolated niches. The situation is dire and I fear that many species will be lost before even being discovered. I am reminded of the movie Avatar where Sigourney Weaver's character (Dr. Grace Augustine) says: "The wealth of this world isn't in the ground - it's all around us."

So why are amphibians so important?
Amphibian skin is both permeable to water, and can perform gaseous exchange, so they are incredibly sensitive to any changes in their surroundings. They also spend parts of their life cycle in both aquatic and terrestrial environments and this makes them invaluable ecological indicators. So an abundance of amphibians therefore represents a healthy ecosystem. Conversely their rapid decline points to deteriorating environmental conditions and a host of associated problems. The Amphibian Crisis is a proverbial alarm going off and we need to wake up!

Symbolism: The frog in silhouette represents the idea of loss of the amphibians (only the shadow of a memory remains). The green represents the natural world, while the red represents the destructive impact of man.

So what can we do?
It is important to tighten restrictions and regulations surrounding air and water quality, and around the management of solid waste. By getting involved in the political process through activism we can ensure that policies that are already in place are not eroded by the corrupt few for their own short-term financial gains at the expense of the environment at large. 

I will end on a quote by one of my favorite human beings ever to have graced the small screen:
 “The truth is: the natural world is changing. And we are totally dependent on that world. It provides our food, water and air. It is the most precious thing we have and we need to defend it.” ― David Attenborough