a7 + 55 f/1.8 vs 35 f/2.8 point-to-point AF accuracy?

Started Jul 2, 2014 | Discussions thread
viking79
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Re: a7 + 55 f/1.8 vs 35 f/2.8 point-to-point AF accuracy?
In reply to RonFrank, Jul 2, 2014

RonFrank wrote:

The A7 is a mirrorless camera with the older AF performance. The AF accuracy is the biggest short coming of the A7 and mirrorless in general. The A6000 changed this but the A7 and all Nex models have iffy AF especially in low light.

If I was considering an A7 I would wait. The next version of the A7 will likely have fast PDAF.

The current version already has fast PDAF focus, but it only works in high contrast situations.  It will focus nearly instantly if it uses it.

My opinion is that mirrorless cameras greatest strength is AF accuracy.  They are extremely accurate.  The downside is they are dumb and don't know what is background or what is foreground.

The user needs to understand how CDAF works, in that it looks for sharpest/highest contrast edge in the image (video feed from the sensor) and focuses there.  If you use single center point focus and the focus point overlaps the background and subject is not high contrast but the background is, the camera will certainly focus on the background as it does not understand (very well) the depth to the image.  Often they will focus on sharp edges in out of focus highlights too.

Multi-point can be bad because it looks for highest contrast subject, so it will often grab the wrong thing.  It can sort of judge distance based on algorithms of where the lens was more focused, etc, but it isn't great at this.  PDAF has the advantage of being able to calculate how out of focus it is (Panasonic uses out of focus characteristics of the lens to do this with CDAF).

The A7 is an extremely accurately focusing camera.  On my test chart it will almost always do better than I can manually.  I have to bracket 4 or 5 focus shots to get as good as it will (or use max magnification zoom).

In all the shots from OP it looks like the camera went for something it thought was higher contrast.  Setting the focus point manually and making sure it doesn't overlap the background is your best bet for accurate.  For people, eye detect focus is excellent, but if the small eye box overlaps a hair highlight or the background, it will often pick the hair or background instead of the eye.

Eric

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