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

Started Jul 2, 2014 | Discussions thread
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Angrymagpie Contributing Member • Posts: 665
a7 + 55 f/1.8 vs 35 f/2.8 point-to-point AF accuracy?

I went to the store to get a7 today. After going through the literature, I decided to go for the 35 f/2.8 because I primary shoot street and I thought 35 f/2.8 would offer more flexibility. Unfortunately (or fortunately?) the store did not have the lens in stock, so I ended up playing with the a7 + 55 f/1.8 combo for a bit. Because I frequently find myself in lowlight situation, I thought about getting the 55 f/1.8 instead for that extra stops of lowlight performance. I tried to shoot point-to-point with AF as fast as I could manage at the store, and for some reason, the AF, whilst snappy most of the time, tend to miss focus from time to time. Sometimes (maybe 20-30%) it would choose odd things to focus on.

I remember reading that AF performance of the 55mm f/1.8 can be a bit slower than the 35mm f/2.8?

I wonder if that's the case? And what about accuracy?

Here are some test shots I took. They are atrocious as I was simply testing the AF performance of the camera. Hopefully they can at least provide sufficient info to illustrate what I meant:

Normally AF works quite well with 55mm, but it does slow down a bit when I tried to focus on things up close

Sometimes it just doesn't seem to be too accurate (?), i tried focusing on this guy, but it didn't work

I had better luck focusing on them at the same location

Didn't do so well with this one

Sometimes it focuses on strange spot... like here: instead of the face, the AF went with the hand

I wonder if these are considered as normal?

All the images are unprocessed, converted from RAW

By the way, I guess I had an unrealistic expectation, but I find high ISO to be a bit nosier than I had hoped. Here's one shot example at ISO6400, something I'd be using quite a lot with the camera.

If the AF performance is indeed better on the 35mm f/2.8 than with the 55mm f/1.8, then I have one more reason to go for the former instead.

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