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video using a6100 - autofocus issues


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hello all, I am currently creating my youtube videos using face detection autofocus.  my face is in focus well, but the lights in the background that I want to be in bokeh are in constant motion, slightly in and out of focus and I can't figure out why.  It's subtle but it bothers me.  I'm using the 15-50mm kit lens, typically auto ISO & auto aperture, manual shutter speed to prevent flicker.  link to my video to see what I mean

I have also experienced really bad autofocus shake (face detection turned off) when filming a landscape video with a waterfall, it was rapidly focusing between the waterfall and the background to the point where the footage was completely unusable.  

if you know what is going wrong here and how I can solve these issues, please help me out.  thanks so much!

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In your video, you are making a lot of small movements like leaning forward to grab your phone, leaning back a little to read the question, putting the phone back down then you settle yourself back to the original position to reply to the question.  Rinse and repeat for every question.  Each time you move forward or backward, the camera automatically tries to keep your face in focus and it seems to do that very well.  The problem is, the camera and the background are stationary but you are not and so the distance between yourself and the camera and the distance between yourself and the background changes every time you move and these changes are showing up with the background seemingly going in and out of focus.  If the background was non-descriptive with no distinct feature this wouldn't pose much of a problem but in your case, those lights changing focus every time you move is very distracting.

Since you only move very little back and forth, it might be a good idea to use manual focus and take advantage of the depth of field to keep yourself in acceptable focus as you move back and forth.  Download the DOF Calculator for your Android phone or something equivalent for iPhone to calculate the depth of field.  For example, if you are 4ft away from the camera with the lens set at f8.0, 24mm focal length, 11.96in in front of you and 23.86in behind you will be in acceptable focus.  As long as you are within that 35in space you will be in acceptable focus.  You can play with the aperture and the focal length so you can work within the space of your place then set the camera and lens to the values you have selected and give it a go.

In the landscape video, you are most likely shooting at or near infinity.  Again, manually focus on the nearest subject of interest (most likely the cascading waterfall) and let the depth of field take care of the background which is probably at infinity focus will be in focus anyway.

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  • 4 weeks later...
On 4/22/2022 at 10:25 PM, tadwil said:

In your video, you are making a lot of small movements like leaning forward to grab your phone, leaning back a little to read the question, putting the phone back down then you settle yourself back to the original position to reply to the question.  Rinse and repeat for every question.  Each time you move forward or backward, the camera automatically tries to keep your face in focus and it seems to do that very well.  The problem is, the camera and the background are stationary but you are not and so the distance between yourself and the camera and the distance between yourself and the background changes every time you move and these changes are showing up with the background seemingly going in and out of focus.  If the background was non-descriptive with no distinct feature this wouldn't pose much of a problem but in your case, those lights changing focus every time you move is very distracting.

Since you only move very little back and forth, it might be a good idea to use manual focus and take advantage of the depth of field to keep yourself in acceptable focus as you move back and forth.  Download the DOF Calculator for your Android phone or something equivalent for iPhone to calculate the depth of field.  For example, if you are 4ft away from the camera with the lens set at f8.0, 24mm focal length, 11.96in in front of you and 23.86in behind you will be in acceptable focus.  As long as you are within that 35in space you will be in acceptable focus.  You can play with the aperture and the focal length so you can work within the space of your place then set the camera and lens to the values you have selected and give it a go.

In the landscape video, you are most likely shooting at or near infinity.  Again, manually focus on the nearest subject of interest (most likely the cascading waterfall) and let the depth of field take care of the background which is probably at infinity focus will be in focus anyway.

that makes a lot of sense, thank you so much for your reply!

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