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Z8 - Weird AF issue when using the flash

SCoombs
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p.1 #1 · p.1 #1 · Z8 - Weird AF issue when using the flash

I just received the 40mm/2 today so went to test it on my Z8. I was not expecting perfection and indeed on initial testing I found it to be a bit soft wide open but to sharpen up reasonably well around 2.8 and especially 3.5. Later in the evening I went to do some more tests - this time with the flash since it had gotten too dark - and suddenly I was having a huge trouble getting sharp photos. I noticed it especially when trying to photograph people in glasses, which can sometimes be a problem but which the Z8 hasn't really had a big problem with for me.

At first I attributed this to the softness wide open so stopped down again, but this didn't improve things. In fact, I seemed to find f/4 to be extremely poor - not simply soft, but essentially out of focus. I did a lot of experimentation with aperture, shutter speed, IBIS, etc. and found a lot of inconsistency. 9/10 shots was quite poor. Putting my 70-180 on - which has been extremely sharp for me - I did see improvement, but was a bit surprised to get a much larger ratio of poor shots than I have seen from this lens before.

Eventually I started to suspect something was off with the camera's ability to focus in the lighting and started playing around with ways to improve matters. It was getting a better success rate in brighter rooms than dimmer. I tried playing with the setting which adjusts the viewfinder/viewscreen for ease of viewing when using a flash because the camera uses the EVF feed for AF, but I didn't have much success with that.

Then I found something very unusual. If I take the flash off of manual and put it in TTL and take ISO off of manual and set that to auto, the camera started nailing almost every shot. The viewfinder was not any brighter, for what it's worth, Also, it didn't matter what ISO I "suggested" to the camera - i.e., if you set the ISO to auto but then, while keeping it in auto, select an ISO value it seems to treat that value as a kind of soft cap and only increases beyond that if necessary. With the flash, if I set the ISO to auto-3200, my photos are ISO 3200 but the EVF and final photo have about the same brightness as if I set it to auto-400, but the photo will be ISO 400. My point here is that either way, the camera's focus was working much, much better with these settings. Even the eye-detect box was noticeably more secure and seemed to adjust at a much faster rate as the subject moved around - and it stayed mostly green, whereas with all the same settings except for the flash and ISO being on manual I'd get a lot of yellow for the eye box.

Of note is that this all applies even when my images were well exposed in 100% manual. I am fairly certain I was setting the exposure correctly with the flash/ISO in manual - the AF was just working far less reliably.

Any idea what is going on here? In particular, I'd really prefer to be able to use manual flash, but it seems I'm getting almost unusable AF performance on the 40mm and extremely inconsistent performance on at least one other lens when doing so.

Nov 03, 2023 at 09:11 PM

snapsy
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p.1 #2 · p.1 #2 · Z8 - Weird AF issue when using the flash

Excellent write-up and troubleshooting effort. Based on what you're describing it seems clear TTL and auto-ISO are changing the sensor feed that the AF system is seeing and relying upon. Normally the AF system uses the same feed you see in LV, so that for example if the feed is very dark then AF system suffers along with it. But in your scenario the apparent LV brightness isn't changing with your ISO changes, which tells me the LV feed's brightness is perhaps being rendered independently of the sensor feed and what the AF system is seeing.

There's likely a few steps involved in drilling down further to figure this out. First, what is your view mode set to? You described some experimentation of this in your post but I'm not sure what setting you were using for the TTL/ISO experiment. Here's the manual page reference:

https://onlinemanual.nikonimglib.com/z8/en/csmd_view_mode_photo_lv_guid-a583e41c-e946-ee33-51c2-0bbc8f7a2cc1_209.html

Nov 04, 2023 at 03:28 AM

sungphoto
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p.1 #3 · p.1 #3 · Z8 - Weird AF issue when using the flash

I’ve shot the z8 a ton with both on camera speedlites in low light to studio strobes, in ttl and manual, and in both manual and auto iso and haven’t experienced issues like this with the 40 f2 or any other lens I’ve used.

What are your focus settings? Perhaps posting some of your shots might help?

Nov 04, 2023 at 06:24 AM

SCoombs
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p.1 #4 · p.1 #4 · Z8 - Weird AF issue when using the flash

I'm currently experimenting and and will probably be able to post some shots later, but I've discovered something else extremely strange that I haven't noticed with any other lenses before: if I use a wide area box - small, large, or the 1x1 custom I often use - focus accuracy will be very spotty so that even if the eye detection box rests comfortably green on someone's eye the actual photo is usually a bit out of focus. However, if I use the full frame AF it will grab the eye perfectly every time and the photos are tack sharp. I am also having the same kind of success using single point AF.

I have played around and verified that this is true even when the wide-area box is fully on top of the subject so that there is no question of it somehow getting distracted by the background.

Nov 04, 2023 at 01:42 PM

snapsy
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p.1 #5 · p.1 #5 · Z8 - Weird AF issue when using the flash

SCoombs wrote:
I'm currently experimenting and and will probably be able to post some shots later, but I've discovered something else extremely strange that I haven't noticed with any other lenses before: if I use a wide area box - small, large, or the 1x1 custom I often use - focus accuracy will be very spotty so that even if the eye detection box rests comfortably green on someone's eye the actual photo is usually a bit out of focus. However, if I use the full frame AF it will grab the eye perfectly every time and the photos are tack sharp....Show more →

Is this on the 40mm? Thom Hogan reports that it has inconsistent performance across the frame - perhaps that's tripping up the AF system. Maybe see if the variability you're seeing is related to where the subject is in the frame.

Nov 04, 2023 at 01:45 PM

SCoombs
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p.1 #6 · p.1 #6 · Z8 - Weird AF issue when using the flash

snapsy wrote:
Is this on the 40mm? Thom Hogan reports that it has inconsistent performance across the frame - perhaps that's tripping up the AF system. Maybe see if the variability you're seeing is related to where the subject is in the frame.

Yes, I read his review last night after starting to suspect after a couple hundred photos that the results seemed inconsistent at different distances and noticed that he speculated on the same thing. Since then I have tried to unscientifically test this and haven't really found a strong pattern that stands out in terms of subject distance. I am also not sure why, even if indeed it has inconsistent focus at different distances, the full auto-area AF would be so much more consistent over the wide area boxes when it comes to subject detection.

Nov 04, 2023 at 02:31 PM

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p.1 #7 · p.1 #7 · Z8 - Weird AF issue when using the flash

Unfortunately most of the more obvious examples got deleted before people asked for examples and this morning I have mainly been outside. I'll play more with the flash later and may post more examples then. In any event, here are a few:

Here is one with flash, clearly soft - I think this one the softness can even be seen at a smaller viewing size. It was taken with a 1x1 custom wide area box put on the eye, and the subject detection triggered on that same eye and is what drove the AF :

This one was taken seconds later. Everything was the same except that I used the Auto-area AF. It's clearly much sharper:

Here is another comparison I took back-to-back, with flash. The difference is a lot more subtle here; as I said, it's not that it's never sharp with the wide-area box, but that it's much less consistent. Still, I think there's a noticeable difference, especially if you look at the larger size. I admit this one is getting closer to nit-picking territory, but I was trying to give you some quick examples I could easily download. Wide-area 1x1 custom:

Vs. Auto-Area AF:

One more for now to demonstrate how successful it's being when using Auto-area in more challenging situations like this lower light action shot. I had her do this a few times and had a good hit rate with Auto-are AF. With Wide area, keeping the box on her perfectly and with the subject detection box being on her eye perfectly the whole time, shots were generally not in focus.

Nov 04, 2023 at 04:29 PM

sungphoto
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p.1 #8 · p.1 #8 · Z8 - Weird AF issue when using the flash

Try limiting your variables and keep your exposure triangle the same, while also keeping a high enough shutter speed to keep motion blur out of the equation
. Maybe try back button focus (though I’ve stopped using it on my z8), perhaps it’s a technique issue and you’re subtly changing your AF right before the shot? Or maybe bring it into Nikon or your local camera store, as this seems either like a full on malfunction or perhaps your technique.

Nov 04, 2023 at 05:33 PM

SCoombs
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p.1 #9 · p.1 #9 · Z8 - Weird AF issue when using the flash

sungphoto wrote:
Try limiting your variables and keep your exposure triangle the same, while also keeping a high enough shutter speed to keep motion blur out of the equation
. Maybe try back button focus (though I’ve stopped using it on my z8), perhaps it’s a technique issue and you’re subtly changing your AF right before the shot? Or maybe bring it into Nikon or your local camera store, as this seems either like a full on malfunction or perhaps your technique.

For the shots I posted, I did keep everything the same other than AF choice.

As far as technique goes, anything is possible but I'm usually shooting wildlife and birds in flight at 500mm and successfully handholding that stuff at very low shutter speeds, so I have a hard time thinking my technique at 40mm is at fault. All of these are taken with BBF, which I always use, and AF-C so really there shouldn't be any question of the focus changing. Also, with the flash motion blur shouldn't be a real concern, though I have gone up to 1/800 a few times, which has generally worked well other than the banding on the background you can get with the Z8/9 from the high speed sync.

On the other hand, I have a hard time imagining what technical fault could yield this behavior.

Nov 04, 2023 at 07:00 PM

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p.1 #10 · p.1 #10 · Z8 - Weird AF issue when using the flash

SCoombs wrote:
For the shots I posted, I did keep everything the same other than AF choice.

As far as technique goes, anything is possible but I'm usually shooting wildlife and birds in flight at 500mm and successfully handholding that stuff at very low shutter speeds, so I have a hard time thinking my technique at 40mm is at fault. All of these are taken with BBF, which I always use, and AF-C so really there shouldn't be any question of the focus changing. Also, with the flash motion blur shouldn't be a real concern, though I have gone up to 1/800 a...Show more →

To be honest, I'm boggled by your issues. It does sound like you're trying to manage too many variables trying to track it down, maybe take the flash out of the equation and stick with manual iso, then slowly add other variables rather than trying to juggle 3 things at the same time. I'm wondering if you just didn't mount the lens properly or you got a dud, as those are all images that would be totally doable on a previous gen camera as well as the z8

Nov 04, 2023 at 10:46 PM

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p.1 #11 · p.1 #11 · Z8 - Weird AF issue when using the flash

I think you are drawing wrong conclusions. Look at the soft one above, there are two hairs crossing in front of her left iris. The focus is on those two hairs. The next picture down the hairs are not directly in front of her eye and is sharper. I see the same issue with our cat. But if you are looking for 100% perfection, this is the wrong lens to use. I see no such issues with the Z8. You may be seeing it more with flash, because a flash will light up anything that could distract the focus.

Nov 04, 2023 at 10:57 PM

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p.1 #12 · p.1 #12 · Z8 - Weird AF issue when using the flash

sungphoto wrote:
To be honest, I'm boggled by your issues. It does sound like you're trying to manage too many variables trying to track it down, maybe take the flash out of the equation and stick with manual iso, then slowly add other variables rather than trying to juggle 3 things at the same time. I'm wondering if you just didn't mount the lens properly or you got a dud, as those are all images that would be totally doable on a previous gen camera as well as the z8

In the examples posted above I kept everything the same between shots other than the AF mode because at that time the AF mode is what I had honed in on. Remember that troubleshooting is a progressive process in which a person tests one thing, learns from that, and then adjusts testing parameters to account for what was learned.

For instance, I initially noticed the problems in question with the flash. Today, I went outdoors to do a lot of testing without the flash in order to try to isolate the flash as a potential cause. I also shot indoors without the flash. Upon finding similar behavior with and without the flash, I began to hone in on other things. Eventually I somehow noticed a significant difference with the use of the auto-area AF vs. the wide-area AF modes and started trying to test in a way that would isolate that.

You're seeing my reports from various stages of the troubleshooting process, but that doesn't mean that everything is being done simultaneously.

Nov 04, 2023 at 11:40 PM

SCoombs
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p.1 #13 · p.1 #13 · Z8 - Weird AF issue when using the flash

bcaslis wrote:
I think you are drawing wrong conclusions. Look at the soft one above, there are two hairs crossing in front of her left iris. The focus is on those two hairs. The next picture down the hairs are not directly in front of her eye and is sharper. I see the same issue with our cat. But if you are looking for 100% perfection, this is the wrong lens to use. I see no such issues with the Z8. You may be seeing it more with flash, because a flash will light up anything that could distract the focus.

You're correct that there are hairs that could possibly explain the phenomenon in the examples posted here, but bear in mind that these are only two out of many dozens - more probably hundreds - of tests I've done. I grabbed two that were easy to quickly download from the camera, but they're not the only such cases. In fact, in most of these tests I have specifically tried to make sure that things like stray hairs were not in the way. Regardless, what I'm working with is dozens if not hundreds of comparisons which have followed the same trends. Individual photos here and there from this corpus might be flawed because of a stray hair or some other problem, but I'm looking at an overall trend built on situations that are more of the time without these sorts of questions.

The other thing is that I have not noticed the Z8 being particularly susceptible to missing focus over stray hairs with other lenses I've used. In fact, I haven't mentioned it here but I've also very specifically been testing back and forth with eyeglasses because I have noticed that with this 40 f/2 I am almost always getting focus on the lens rather than the eye for people with eyeglasses, whereas I've shot portraits of people with eye glasses with my 70-180 Z lens and it's very consistently grabbed the iris rather than the eyeglasses.

In other words, even if the hairs did cause the missed focus, it's not the way the Z8 seems to typically perform and I am wondering whether or not the issue is A) the design of the lens, such that any copy of this lens would behave in the same wa or B) a flaw with this particular copy of the lens. I am willing to accept A for this price point if that's the explanation, but I want to try to be sure of that so that I can replace the lens with another sample if that's actually the issue - and this whole thing is compounded by the fact that through a long process of relatively iterative troubleshooting I've arrived at the very, very strong trend that the auto-area AF mode is working very reliably with this lens whereas the wide-area modes are operating differently - which is NOT a trend I have noticed in other lenses.

To hopefully illustrate all of this, here's another example from today:

This photo was taken using auto-area AF with eyes that are very badly obfuscated by stray hairs, yet to me the focus on the eyes is excellent. With a more expensive lens they'd be sharper, but I think it's really impressive for what it is. This is what I normally see out of my Z8 no matter what mode I'm in. Yet I had another, taken minutes earlier in the same room of the same person using the wide-area mode with totally clear eyes but which was a out of focus.

At this point the trend is pretty strong: wide-area AF is in focus <50% of the time, whereas auto-area AF is in focus most of the time. This is with this lens only, as far as I am aware.

Regarding the flash, I'm not sure I follow the suggestion that it is lighting up things which distract the focus. By the time the flash has fired, the focus is already acquired.

Nov 05, 2023 at 12:07 AM

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p.1 #14 · p.1 #14 · Z8 - Weird AF issue when using the flash

Actually, something from Thom Hogan's article just came to mind. He writes this:

"In most circumstances, I don't see the change in focus or hear it during autofocus"

I wonder whether other owners of this lens hear it during autofocus? I ask because I most definitely hear the lens autofocusing. There is an absolutely distinct "robotic" sound when focus is missed and it has to hunt back the other way, and even when just trying to hold focus on a subject with AF-C one can hear the motor running.

Nov 05, 2023 at 12:18 AM

sungphoto
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p.1 #15 · p.1 #15 · Z8 - Weird AF issue when using the flash

SCoombs wrote:
You're correct that there are hairs that could possibly explain the phenomenon in the examples posted here, but bear in mind that these are only two out of many dozens - more probably hundreds - of tests I've done. I grabbed two that were easy to quickly download from the camera, but they're not the only such cases. In fact, in most of these tests I have specifically tried to make sure that things like stray hairs were not in the way. Regardless, what I'm working with is dozens if not hundreds of comparisons which have followed the same trends. Individual photos here and there from this corpus might be flawed because of a stray hair or some other problem, but I'm looking at an overall trend built on situations that are more of the time without these sorts of questions.

The other thing is that I have not noticed the Z8 being particularly susceptible to missing focus over stray hairs with other lenses I've used. In fact, I haven't mentioned it here but I've also very specifically been testing back and forth with eyeglasses because I have noticed that with this 40 f/2 I am almost always getting focus on the lens rather than the eye for people with eyeglasses, whereas I've shot portraits of people with eye glasses with my 70-180 Z lens and it's very consistently grabbed the iris rather than the eyeglasses.

In other words, even if the hairs did cause the missed focus, it's not the way the Z8 seems to typically perform and I am wondering whether or not the issue is A) the design of the lens, such that any copy of this lens would behave in the same wa or B) a flaw with this particular copy of the lens. I am willing to accept A for this price point if that's the explanation, but I want to try to be sure of that so that I can replace the lens with another sample if that's actually the issue - and this whole thing is compounded by the fact that through a long process of relatively iterative troubleshooting I've arrived at the very, very strong trend that the auto-area AF mode is working very reliably with this lens whereas the wide-area modes are operating differently - which is NOT a trend I have noticed in other lenses.

To hopefully illustrate all of this, here's another example from today:

https://live.staticflickr.com/65535/53309862309_1fc61db559_o.jpg

This photo was taken using auto-area AF with eyes that are very badly obfuscated by stray hairs, yet to me the focus on the eyes is excellent. With a more expensive lens they'd be sharper, but I think it's really impressive for what it is. This is what I normally see out of my Z8 no matter what mode I'm in. Yet I had another, taken minutes earlier in the same room of the same person using the wide-area mode with totally clear eyes but which was a out of focus.

At this point the trend is pretty strong: wide-area AF is in focus <50% of the time, whereas auto-area AF is in focus most of the time. This is with this lens only, as far as I am aware.

Regarding the flash, I'm not sure I follow the suggestion that it is lighting up things which distract the focus. By the time the flash has fired, the focus is already acquired.
...Show more →

I don’t think the flash itself is altering your focus. It kind of seems like a technique issue, but you won’t be able to ascertain if it’s the camera or user error unless you remove some variables in troubleshooting. Not sure why you’d use HSS and auto iso at maxed at 3200. Try a subject that isn’t moving to determine if it’s an actual technical issue.

Nov 05, 2023 at 11:27 AM

SCoombs
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p.1 #16 · p.1 #16 · Z8 - Weird AF issue when using the flash

sungphoto wrote:
I don’t think the flash itself is altering your focus. It kind of seems like a technique issue, but you won’t be able to ascertain if it’s the camera or user error unless you remove some variables in troubleshooting. Not sure why you’d use HSS and auto iso at maxed at 3200. Try a subject that isn’t moving to determine if it’s an actual technical issue.

I can hand hold a heavy 500mm lens at 1/30 and get sharp photos of twitchy wildlife. It's not a technique issue. I regularly shoot twitchy kids at 85 or 200mm in all kinds of lighting. I have countless photos under my belt on the Z system at 35 mm. I can't see how suddenly my technique could be at fault because of this one short lens.

I continue to be confused and a little frustrated to have the idea raised that I'm looking at too many variables. I'm confused because I really don't see where I've said anything to suggest that I'm looking at a bunch of variables at once. I'm a little frustrated because I've tried to repeatedly tried to explicitly explain the ways in which I am focusing on a single issue at a time. For instance, I've said at least twice that I have kept every last factor constant during comparisons besides the one factor that I was trying to analyze. You've also now twice said something to suggest that I'm blaming the camera body when I've been very specific in saying several times that the camera body does not behave this way with other lenses and I am trying to establish if there's something about the lens which is causing a problem - not the camera body.

Nov 05, 2023 at 11:44 AM

sungphoto
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p.1 #17 · p.1 #17 · Z8 - Weird AF issue when using the flash

Cool, just trying to steer you right as your posts are kind of all over the place. At this point, if technique isn’t the issue I’d recommend just returning the lens or getting body checked out, perhaps you don’t have the latest firmware to use the 40 f2.

Nov 05, 2023 at 07:11 PM

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p.1 #18 · p.1 #18 · Z8 - Weird AF issue when using the flash

I've now done a controlled test of about 200 shots and reached what I would consider a conclusion.

I set the camera in full manual with the shutter at 1/400 to try to avoid motion blur. I then set the aperture to 4. I chose three different settings:

* Indoors with decent natural window light, so lower light but still very workable

* Outdoors in full light

* Indoors with room lights only, so more challenging light

In each case I manually set the ISO for the proper exposure. I also changed the aperture to 2.8 for the final scenario to maintain a realistic situation and a workable light level.

I had three different people pose and made sure their eyes were clear.

With the camera in AF-C set at 5 fps I took 10 shots of each person in these settings using Wide-area small with subject detection, auto-area AF with subject detection, and single point AF.

I did this with both the 40mm f/2 and the 70-180 f/2.8, a lens I've previously used a lot with great success.

I then reviewed the images. (I started to try to compile them all to post or to make a composite image of them all but thisbwas a gargantuan task with 200 photos so I stopped. I may postna video later where I browse through them all). What I found was the following:

* With both lenses the Single point AF was the most reliable, producing a sharp and in focus eye in almost all cases.

* With both lenses auto-area AF was second.

* With both lenses wide-area AF was far behind the other two.

* There was not a considerable difference between the two with single point AF.

* With auto-area AF the 70-180 was, not surpringly, better. It hit between 8-9 shots out of ten in focus for each round. Some were tack sharp, others not but still good. The 40mm was closer to hitting 7-8, with again some tack sharp and others just good.

* With wide-area both produced pretty poor results in the indoor scenarios, but the 70-180 was better. They were both under 50% for what I'd call acceptably sharp. Outdoors both were better but still surpringly inconsistent.

My take:

I think the 40mm seems to be working well. It's AF seems less strong than my other lens, but this is unsurprising and is acceptable. I suspect that what happened is that because its AF is not as robust as the other lens' that when I first started testing Indoors in the evening it was just missing in a way similar to how my other lens would in that scenario, but to a greater degree because its AF is a bit weaker.

I probably noticed it for the first time because I was putting a lot more into testing it because I had read I'm reviews that the lens was a bit softer than some others and because I saw this myself on my initial tests which prompted me to play more with it. Because it was later I wound up trying it in a darker setting than I normally work, exacerbating things further.

I am a bit surprised to find the wide-area operating noticeably worse with subject detection. On the one hand I can believe it because I've seen lots of wildlife photographers talking in discussions of the Z9 firmware update about how they've previously found auto-area to be more reliable at eye-AF for animals, so this would be consistent. On the other hand, I can't fathom why the focus algorithms would work less well in a situation where they've got a more limited area to consider than in situations where they're working on the whole frame - and I'd think the subject detection would be the same algorithms regardless of the AF mode!

Either way, I'm pretty sure I'm satisfied with the 40mm now, but have a newfound curiosity to explore exactly what's going on with the AF systems in this camera.

Nov 06, 2023 at 09:50 AM


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