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Video - Our SLT/mirrorless experiences

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Photosopher View Drop Down
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Post Options Post Options   Quote Photosopher Quote  Post ReplyReply Direct Link To This Post Posted: 13 March 2019 at 13:36
Originally posted by jkp1 jkp1 wrote:

A remark to the reference to lens database by Photosopher....:
There are "no" info about lens quality related to video usage in the lens database, if, then only by coincidence somewhere hidden in text. (ie Focus breathing, aperture breathing, change in focus during zoom, estimates of T-values, af or aperture noise, things like that...)


Yes it's mostly stills talk.
 



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Post Options Post Options   Quote jkp1 Quote  Post ReplyReply Direct Link To This Post Posted: 13 March 2019 at 15:50
Oh yes, in lens database a simple checkbox telling "tested for video" or just "video" and then in the review text people could write their findings. Unstructured, yep, but if that checkbox was searchable, it would do great.
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Post Options Post Options   Quote windhorse Quote  Post ReplyReply Direct Link To This Post Posted: 13 March 2019 at 16:53
Originally posted by jkp1 jkp1 wrote:

Oh yes, in lens database a simple checkbox telling "tested for video" or just "video" and then in the review text people could write their findings. Unstructured, yep, but if that checkbox was searchable, it would do great.


Hey JKP, that's seditious, since as you know very well real cameras and lenses are made for taking photos and the video thing is just an add on to convince yuppies like me to buy one
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Post Options Post Options   Quote windhorse Quote  Post ReplyReply Direct Link To This Post Posted: 13 March 2019 at 17:49
Desire to avoid looking stupid is a great motivater, so is going for a walk. Both brought me to the conclusion this morning that most likely the problem I am concerned about is caused by the camera working exactly as it should in AF-C center focus mode, while I thought it was doing AF-C face detection, which it was not.

The reason for the focus hunting was probably a combination of shallow depth of field due to low light, moving hands in the center of the frame with the camera set in center autofocus mode, and perhaps a tad due to the way contrast focus works by going past the focus point then coming back to it again.

Looking at the footage more carefully than I had done before, it looks to me like the hunting corresponded to hand movements in the center of the frame, though I am not 100 percent sure about this and maybe some other factors were also involved.

What I am quite sure about is the camera was set to center focus, which means, according to this excellent "Before the Coffee" blog, face detection was not working:

This is an article on the focus characteristics of the Sony A7Rii. If you don’t own this camera you might want to skip this read. Face Detection in S,M,L, Center or Zone. This is the least desirable set with Face Detection. When S, M, L, Center or Zone focus areas are assigned, the Face Detection square stays GRAY and inactive when outside of the assigned focus area. If you take a picture the focus will be established in the chosen focus area, not the Face Detection box. The Face Detection box becomes WHITE and active only when the face detection box and the face of the subject is within the assigned focus area. The outline of the box begins as white, when focus is achieved, it turns green. Until you understand what is going on, it is easy to get out of focus images in this setup – you believe face detection is active but it is actually the focus area chosen. It results in faces out of focus.

Quite clearly, the problem was caused by doing the shoot early in the morning BEFORE THE COFFEE!!

This also explains why I did see the white face detection box in the viewfinder, coming and going on the face of the subject rather than constant, but did not see the green face detection confirmation box, which if I had been more aware (ie had got my coffee) should have alerted me straight away that the face detection focus was not focussing the camera on the face of the subject.

Due to previous experience I was using center focus rather than using the wide focus area setting, since it is less playful, though as "Before the Coffee" points out wide focus is "the best setup when faces are the subject of your photo".

So looks like mystery solved - all I have to do is learn more about how to make sure when face detection is working or not, how to tell if the camera autofocussing system is in contrast or phase detect mode, and where to get good coffee very early in the morning.

However I do have one more question: Do I understand right there is no way to HOLD the green face detection confirmation box on the LCD screen of the A7RII as a continuing confirmation that the camera is focussing on the face of the subject?

My A7RII displays a white box when a suggested face is found, switches to a green box to confirm the face is focussed on, then disappears. I need to check if this is because the camera works differently in video (M) mode than in photography (M) mode, and if this is the same with later models... after coffee!

Edited by windhorse - 13 March 2019 at 18:10
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Post Options Post Options   Quote pegelli Quote  Post ReplyReply Direct Link To This Post Posted: 13 March 2019 at 18:57
Hey guys, we have a dedicated thread for video discussions, created after repeated requests from several members, but then after three pages nobody contributed anymore . Currently the last post is from 2013.

Video - Our SLT/mirrorless experiences

I think this discussion would fit very well in there, I can move it there and hopefully that will be a restart of Video discussions between interested members. @windhorse, let me know if you like that idea.

Dyxum is basically a "stills" site and we only have enough volunteers to manage that. However we're not blind to the fact that most newer Sony cameras can be used for both stills and video, hence this dedicated thread to have it all in one place and easily found by others. I'll make it a sticky so it doesn't sink to the bottom and people remember to keep posting their video questions (and hopefully answers) there.

Mind the bandwidth of others, don't link pictures larger then 1024 wide or 960 pix high, see here
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Post Options Post Options   Quote stiuskr Quote  Post ReplyReply Direct Link To This Post Posted: 13 March 2019 at 19:04
I've been tricked by the floating white box before (shooting stills) and went ahead and tripped the shutter. I think in order to get the best out of Face Detection you need to use Wide autofocus in order to not restrict it in any way.
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Post Options Post Options   Quote windhorse Quote  Post ReplyReply Direct Link To This Post Posted: 13 March 2019 at 19:53
Originally posted by pegelli pegelli wrote:

Hey guys, we have a dedicated thread for video discussions, created after repeated requests from several members, but then after three pages nobody contributed anymore... I think this discussion would fit very well in there,I can move it there and hopefully that will be a restart of Video discussions between interested members. @windhorse, let me know if you like that idea.


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Post Options Post Options   Quote Canopus Quote  Post ReplyReply Direct Link To This Post Posted: 13 March 2019 at 20:38
I’d like to set annoying hype aside as well as advises like "be professional". I’d like to consider technical side of questions only.
First we have to consider fundamental difference between phase and contrast detection auto focus (PDAF and CDAF).

A) Pure CDAF

1. CDAF does measure contrast of the object we are focusing at. It is very important to understand that only contrast is measured. To successfully complete contrast focusing the system has to find “global maximum” of contrast function. The contrast function may have numerous local maximums. But they are not what we are looking for. Let’s simplify our task and imagine that we have only one maximum of contrast function – the GLOBAL one.

2. Having one single initial measure of the contrast, it is impossible to understand where to shift the lens to achieve better contrast and therefore better focus. That is why first shifting of the lens is, in general, random decision. The probability of success (shifting to correct direction) is 50%.

3. Imagine that (lucky case) system made first shift of the lens in to proper direction. Thus, the contrast of second measure is better than that of first one. However, CDAF system does not know whether or not global maximum is found. Thus, the second shift and one more contrast measurement is needed. The mathematics tells that one needs to have at least THREE measurements (therefore two lens shifting) to draw the decision whether or not global maximum of (contrast) function is found. There is nothing to do with “cutting age technology” or “know how”. This is just mathematics. Do not read further If you believe that one could overcome math using “cutting age technology”.

Thus, the very best scenario suggests that:
- that the contrast function has only one (global) maximum (see 1),
- we were lucky with initial lens shift (see 2),
- that we were lucky with scenario 3 (and performed two lens shifting only).

B) Comparison with pure PDAF
Phase detection auto focus is described in Wikipedia very well. In short, it measures “phase difference” function. It requires one single measurement to calculate lens shift to achieve proper focusing.
PDAF does not measure contrast. PDAF is irrelevant to contrast but to distance only. Nothing more. Nothing else.


C) Contrast change impact

Let’s consider “real word” scenario like picturing/filming head interview. Model is not moving. So distance has not change.
But person is speaking (lips are moving), winking, turning head right and left, smiling, etc…
All these factors led to contrast function change and therefore may push CDAF to search proper focus. It means that camera has to execute procedure described in p.3(see above) every time its happen.
That is why many CDAF cameras users are often claiming “hunting”, especially when sharp wide open lens is used.

Oh, yes… One can note, that Sony’s cameras do have “on-sensor PDAF” :-)
Answer: Have you ever used “on-sensor PDAF” itself, without CDAF? Do you even have a possibility to turn off CDAF and use “on-sensor PDAF” only in mirrorless cameras? So, what I suggest? I suggest that performance and reliability of dedicated PDAF and “on-sensor PDAF” are absolutely/tremendously different. These are sufficiently different technologies with certain similarity in names.
"On-sensor PDAF" is designed and used:
a) to overcome initial lens shift problem (see p.2 above) and make the firs shift in to proper direction
b) to organize powerful hype campaign around “on-sensor PDAF”. It is very exciting to listen something about “on-sensor PDAF”. Consumers start automatically associate “on-sensor PDAF” with real PDAF (including its focusing capabilities) but millions times smaller, that is now integrated directly in to sensor! Thanks to cutting age technology developed by Sony ! :-) Sorry, but this is just hype, nothing else.

Conclusions:
1. Mirrorless (CDAF) autofocus could be used more or less successfully to capture still (static) objects only. Still (or static) means objects that do not change distance and CONTRAST.
2. "Breasing" and/or "hunting" are unavoidable in CDAF systems
3. Real world focusing process time of SLR/SLT systems approximately 2-3 times shorter than that inherent to mirrorless cameras (at least for Sony family).

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Post Options Post Options   Quote windhorse Quote  Post ReplyReply Direct Link To This Post Posted: 13 March 2019 at 20:51
Thanks, this is a very good explanation of why focus hunting happens. Does it mean that for Sony E-mount cameras CDAF and PDAF are as inseperable as milk and water in a good cup of tea, whereas in other cameras PDAF may be different?
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Post Options Post Options   Quote QuietOC Quote  Post ReplyReply Direct Link To This Post Posted: 13 March 2019 at 21:15
Originally posted by Canopus Canopus wrote:


Answer: Have you ever used “on-sensor PDAF” itself, without CDAF?

Yes, all the time. Adapted lenses on A7 cameras can only use PDAF or CDAF. AF-C with native lenses may skip the contrast-detect step too.

Contrast-detect on E-mount doesn't use a global value. It measures contrast in specified areas of various sizes. I would often like to maximize global contrast, but that is not even an available option.
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Post Options Post Options   Quote windhorse Quote  Post ReplyReply Direct Link To This Post Posted: 13 March 2019 at 21:23
Originally posted by QuietOC QuietOC wrote:


AF-C with native lenses may skip the contrast-detect step too.


Is it possible or desirable to control or at least to know when this happens?
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Post Options Post Options   Quote Photosopher Quote  Post ReplyReply Direct Link To This Post Posted: 13 March 2019 at 22:18
Originally posted by windhorse windhorse wrote:

Thanks, this is a very good explanation of why focus hunting happens. Does it mean that for Sony E-mount cameras CDAF and PDAF are as inseperable as milk and water in a good cup of tea, whereas in other cameras PDAF may be different?


I can't recall if any E-Mount allows user to select PD or CD only. But you can get phase detect only by adapting A-Mount lenses with LA-EA4 adapter. The LA-EA4 adapter is better for video, even with SSM MK2 lenses. The LA-EA3 is fine for stills with many MK1 or MK2 lenses, but it doesn't perform well for video AF with anything that I've experienced.

It depends on the Sony camera you're talking about. Early E-Mount didn't have any phase detect. Early A-Mount didn't have any contrast detect. I think a99 with AF-D was first attempt at hybrid. a99II hybrid depends upon lens used, and to some degree the hybrid can be defeated with menu selection, or by removing the SLT mirror.

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Post Options Post Options   Quote Photosopher Quote  Post ReplyReply Direct Link To This Post Posted: 13 March 2019 at 22:24
Originally posted by windhorse windhorse wrote:

Originally posted by QuietOC QuietOC wrote:


AF-C with native lenses may skip the contrast-detect step too.


Is it possible or desirable to control or at least to know when this happens?


I can't answer that question specifically. You'll have to check your specific model of camera and the specific firmware version it runs, along with the specific firmware of whatever lens you're using. It's hard to keep track of it all, and video AF doesn't always work the same as stills AF.

I know the 18-105/G has had multiple FW updates, the last two of which claim better AF-C performance in video. I can confirm the improvement for a7RIII with latest FW.
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Post Options Post Options   Quote Photosopher Quote  Post ReplyReply Direct Link To This Post Posted: 13 March 2019 at 23:09
I just checked an interesting combo, and you might be interested because it works so counterintuitively, and illustrates how different camera/lens combinations will act completely different from one another, and different for stills vs video.

a6300 + LA-EA3 adapter + ZA24/2 for A-Mount.

Stills:
Allows AF-C with Face Detect (no Eye-AF).
Allows all AF selector modes.
Will shoot Hi+ FPS, but locks on first frame. Will track on Lo FPS.

Video:
Allows only AF-S, and only when stills shutter button is pressed. Otherwise no AF at all.
No other AF modes are available.
Pressing stills shutter button to engage AF is reasonable speed, obviously CDAF, very accurate, and confirms Face Detect with green box. It will not track AF-C by keeping shutter button pressed.
Releasing stills shutter button disengages all AF, and reverts to manual focus only.
Pressing stills shutter button engages AF for new target, with new AF-S lock on, but no tracking.
All this while video is recording.

This method sounds pretty good for the video shooter who just wants an initial lock on, with no ability to hunt after that.

The behavior changes with native E-Mount lenses, or with LA-EA4 adapter.

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