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

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Angrymagpie
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a7 + 55 f/1.8 vs 35 f/2.8 point-to-point AF accuracy?
5 months ago

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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RonFrank
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Re: a7 + 55 f/1.8 vs 35 f/2.8 point-to-point AF accuracy?
In reply to Angrymagpie, 5 months ago

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.

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sean lancaster
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Re: a7 + 55 f/1.8 vs 35 f/2.8 point-to-point AF accuracy?
In reply to Angrymagpie, 5 months ago

Try setting the AF point to center (or to the place you'd like) and then it will focus where you want it to focus instead of guessing.

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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, 5 months ago

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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viking79
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Re: a7 + 55 f/1.8 vs 35 f/2.8 point-to-point AF accuracy?
In reply to Angrymagpie, 5 months ago

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

Eric

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Dave Sanders
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Re: a7 + 55 f/1.8 vs 35 f/2.8 point-to-point AF accuracy?
In reply to RonFrank, 5 months ago

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.

Incorrect. AF accuracy is the biggest ADVANTAGE of the A7 and mirroless in general. AF speed on the A7 isn't the greatest in low light but other mirrorless cameras, such as my EM1, are faster than, well, anything.

The A6000 changed this but the A7 and all Nex models have iffy AF especially in low light.

Again, this speaks to speed, not accuracy.

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

The A7 already has PDAF...

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HPS3
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Re: a7 + 55 f/1.8 vs 35 f/2.8 point-to-point AF accuracy?
In reply to Dave Sanders, 5 months ago

What AF settings were you using?

I find myself using the small flexible point most of the time and the A7 and 55 usually nail focus. I also like the face detect on the A7.

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Angrymagpie
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Re: a7 + 55 f/1.8 vs 35 f/2.8 point-to-point AF accuracy?
In reply to viking79, 5 months ago

viking79 wrote:

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 see. I woke up this morning feeling convinced that I'd get the 35mm f/2.8 over the 55mm f/1.8 for sure. Now I am doubting again! A greater tendency to grab the background? Sounds bad.

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.

I think so. Thanks for clearing this up for me

Unfortunately manually setting the focus point is not something that I envision myself doing too much since I mostly shoot street where I often find myself in situation where I wouldn't have time to do that.

That's why I was hoping that I could count on my camera to distinguish subject(s) & background well. I think the 55mm f/1.8 did fine. I guess I was just not used to the much shallower DOF

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

Eric

Thanks Eric, there are truly some wonderful advices. I'd give them a try for sure!

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Angrymagpie
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Re: a7 + 55 f/1.8 vs 35 f/2.8 point-to-point AF accuracy?
In reply to HPS3, 5 months ago

HPS3 wrote:

What AF settings were you using?

I find myself using the small flexible point most of the time and the A7 and 55 usually nail focus. I also like the face detect on the A7.

The AF was on Multi I believe. Flexible point works well but since I mostly shoot street I doubt I'd have enough time to manually adjust the focus point often. But I will try with AF centre to see if it works better. Thanks for the advice!

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philip pj
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Re: a7 + 55 f/1.8 vs 35 f/2.8 point-to-point AF accuracy?
In reply to Angrymagpie, 5 months ago

With such a complex camera with so many focus related settings, it is best not to rush to judgment. Yes, yes, it is like stopping a raging bull on the web. It is true nonetheless.

You can read up on optimising AF around the place. In half decent light, most issues are user-related.

Much of it comes down to matching the AF spot to desired subject size, and aiming for surrogates for problem targets...FWIW, I use centre spot small to medium size mostly or flex spot, and try holding the cameras very still. It's not luck, there is a bit to learn.

On waiting for the next model:

I've never agreed with this argument. I have thousands of images that could not be shot with a next gen A7 series camera. Anyone advising you to wait for the next release is suggesting you waste your present photographic opportunities.

Your 6400 noise problem scarcely exists. If you prefer to remove the film grain-like noise quality, take 2 seconds to move a slider, really, it's a photographic decision - noise is not something to be totally eradicated. Few images at this ISO won't benefit from a little judicious NR. Appealing noise gives the viewer's eye something to grab hold of. Like grain, it can be a gritty feature.

Of the 35 and 55 buy the one you think best for your work.

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Angrymagpie
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Re: a7 + 55 f/1.8 vs 35 f/2.8 point-to-point AF accuracy?
In reply to Angrymagpie, 5 months ago

Thanks for all the advices. Just wanted to give an update: I eventually went for the 35mm f/2.8

And although I've only taken less than 100 frames with it (day time condition), I am very happy with its performance (AF & image quality-wise). And I am very glad that I went for 35 f/2.8 rather than the 55 f/1.8 I had in mind originally - even with the 35 I find the camera to be a little bit unwieldy sometimes. The 55 is just way too enormous for my purpose (If only the lens hood is not so big).

I missed some shots I wanted because I haven't gotten used to the much shallower DOV

I was kicking myself for missing this shot, for instance. I saw an old lady looking longingly at an old photograph (perhaps her family member) on a bus. I quickly changed my focus setting to centre and snapped this. I thought I'd nailed the focus on the photo she's looking at for sure. But then it turned out that the AF preferred her dress, and the shallow DOV blurred the photo she's looking at instead. Ah!

I had a lot more time to manually refocus with this shot, as I was not afraid of my subject noticing me (though I was slightly worried that it might want to attack me). And the detail is fantastic.

Here's 100% crop, both files are unprocessed RAW converted to JPEG

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