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

Started Jul 2, 2014 | Discussions thread
viking79 Forum Pro • Posts: 14,147
Re: a7 + 55 f/1.8 vs 35 f/2.8 point-to-point AF accuracy?

Angrymagpie wrote:

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

It worked fine, I bet it grabbed the edge of the glass under the blue store front in the background, which has a nice sharp edge to it. Move the focus point to the bottom of the guys shirt and it would have done fine (maybe grabbed escalator)

Below, I can't tell what it focused on at the small size, but the escalator or trash can look like very attractive targets to the CDAF system.

I had better luck focusing on them at the same location

Sure, because she takes up a large percentage of the frame since she is closer, and has a nice bag strap to focus easily on.

Didn't do so well with this one

Same as before the escalator is appealing for the focus system, or maybe the plants, etc. His watch would have been a good target here.

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

I imagine the hand near the computer is appealing. Use center point and focus near is mouse hand shoulder or tip the camera at a 45 degree angle to focus on his glasses (they don't like horizontal edges).

I wonder if these are considered as normal?

Yes, it looks for high contrast edges. I like to use flexible point and adjust size based on what the subject is.

Face detect works at smaller apertures (at large apertures, like f/1.8) it likes to focus on hair-lights (if subject is back lit, hair is very high contrast). At 55mm f/5.6 or something this is not noticeable, but at f/1.8 the eyes will be out of focus.

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.

You gain maybe 2/3 to 1 stop flexibility over A6000 with A7, an extra 1/3 stop for A7R.

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.

Yes, only because you won't notice focus differences as much (deeper depth of field), but it will probably have a greater tendency to grab the background since the subjects tend to be smaller at wider field of view.

I hope this helps, I think it is a misalignment of your expectations of the focus system and how it performs vs an accuracy issue.

For precise focusing use flexible spot so you can select what it is focusing on. Choose a spot size just small enough to not overlap the background (or try to minimize it). Use Eye AF to focus on faces, or face detect (just be aware of issue focusing on hair highlights or backgrounds that are in the face box). Learn to place flexible spot on high contrast edge.

The A6000 might be better at judging what is near and far, but not all samples look good at this. An SLR with a many point AF system might be better at picking the closest subject, but you can still trick those as well.

If in doubt, focus on the chest/collar, it is usually in line with the eyes. This is what I do at weddings or something if the camera is wanting to grab the background. You should do a visual inspection of the EVF for focus. It is high resolution so you can usually see when it focuses on the background. If you work in moderate to low light, the A7 focus system might become frustrating to you. It may or may not be the camera for you.  Remember, a single candle can easily provide enough contrast for the system to focus.  You just have to know where to focus and position the light source so it makes contrast.  In some situations, like an event, I don't have liberty to easily adjust ambient light on the fly for a candid shot


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