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Focusing issue on A7RII

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Caprice View Drop Down
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Post Options Post Options   Quote Caprice Quote  Post ReplyReply Direct Link To This Post Topic: Focusing issue on A7RII
    Posted: 04 August 2017 at 09:38
Hi,

Recently I was out on a shoot and I was photographing a vehicle passing by me as I was stationary on the side of the road.
While I wanted the front of the vehicle to be in focus while the rest of the vehicle to show motion, it didn't seem to matter how far I had the flexi focus point set, the front of the vehicle would show the motion while the rear would be in focus.
[IMG]DSC09207 by Greg Forster, on Flickr[/IMG]

The green brackets are roughly where I had my flexi focus spot set to.

The rest of the camera settings are:
Manual settings
Continuous focus
Flexi spot focus set far to the left of screen
Continuous shooting on Hi
F10
1/160th as the bus was traveling an estimated 40 mph
Focal length: 33ml on a mid range G lens
ISO 160

Is the camera focus happening too slow and therefore the bus has already moved pass the focus point by the time the shutter reacts to the camera focusing?
Does anyone have a proven solution for this problem?

Cheers,
Greg


 



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Miranda F View Drop Down
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Post Options Post Options   Quote Miranda F Quote  Post ReplyReply Direct Link To This Post Posted: 04 August 2017 at 10:48
I know the bus is moving in a straight line at constant speed in reality, but in the image it isn't. The back of the bus is at least twice as far away as the front, and its direction of movement is nearer axial. The front bumper you have outlined in green is moving across the field of view at high speed and is therefore inevitably more blurred than the back. It's geometry!

To see this, do the same thing again but use a very fast shutter speed to avoid any motion blur, and small aperture to put it all in focus (high ISO is fine for this). Then set the drive mode to take two or three pictures at once.
*Don't* pan your camera when you take the picture.

Superimpose the images on your PC and look at the 'blur' you see. It will be mostly at the front.

Now do the same thing again, but this time look at the front left hand corner of the bus and pan the camera to keep this in the same spot in your VF. You should find the front sharp now and the back blurred.

Panning is very difficult to do effectively with a wide angle lens. You'd be better off with a longer lens, stand further away and use DOF to get the effect you want rather than motion blur. Or just accept all the bus sharp and the background blurred.

In lower light, you could also use flash with rear curtain sync to get a blur trail.

Edited by Miranda F - 04 August 2017 at 10:51
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Post Options Post Options   Quote maewpa Quote  Post ReplyReply Direct Link To This Post Posted: 04 August 2017 at 12:58
Yes I can't work out how you expect to freeze the action at one place and not the other unless they are traveling at different speeds. What was your theory here? I often pan at 1/125s as a desperation technique to save shots of moving animals in the dark and the blur is on the parts that move contrary to the pan. I wouldn't expect to freeze action at that speed with 1/125 s without panning. If the rear is in focus were you in fact moving to track the bus (so panning?) if so perhaps you simply weren't able to move yourself fast enough given how close it was? That will give similar results.

Just throwing in another idea.

Having said that I think focus speed could be another issue here, the A7RII is no speed king and this is a tough test
Paul aka maewpa
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artuk View Drop Down
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Post Options Post Options   Quote artuk Quote  Post ReplyReply Direct Link To This Post Posted: 04 August 2017 at 14:13
Originally posted by maewpa maewpa wrote:

Yes I can't work out how you expect to freeze the action at one place and not the other unless they are traveling at different speeds. What was your theory here? I often pan at 1/125s as a desperation technique to save shots of moving animals in the dark and the blur is on the parts that move contrary to the pan. I wouldn't expect to freeze action at that speed with 1/125 s without panning. If the rear is in focus were you in fact moving to track the bus (so panning?) if so perhaps you simply weren't able to move yourself fast enough given how close it was? That will give similar results.

Just throwing in another idea.

Having said that I think focus speed could be another issue here, the A7RII is no speed king and this is a tough test


I was totally confused by the wording of freezing the action at the front but wanting motion blur at the back - it's not possible to have sharpness and blur on an object moving in the same direction at a constant speed!

Where you do get different blur, as you say, is when you pan and parts of the subject are moving in a different direction to the pan - so for example you might get a car or motorbike "sharp" but the wheels which are going around will be blurred; or in your example an animal body may be clear where the legs will be blurred.

As said above, I suspect the lack of apparent critical focus is not a result of mis-focusing, but the effect of motion blur of an object moving at 30mph with a 1/125s shutter speed. That other parts appear less blurred may just be a result of different angular movement through the frame.
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Post Options Post Options   Quote sybersitizen Quote  Post ReplyReply Direct Link To This Post Posted: 04 August 2017 at 16:05
Originally posted by Caprice Caprice wrote:

Is the camera focus happening too slow and therefore the bus has already moved pass the focus point by the time the shutter reacts to the camera focusing? Does anyone have a proven solution for this problem?

We're looking at motion blur. There can be no analysis of an alleged focus problem until you first solve the problem of motion blur.
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Post Options Post Options   Quote Caprice Quote  Post ReplyReply Direct Link To This Post Posted: 04 August 2017 at 22:27
Thanks for the response guys.

I'm fine with motion blur as I want the bus to look like it's traveling. I don't expect the whole bus to be completely sharp but am wanting the front section of the bus to be without motion blur.
As you know, if the shutter speed is too fast, the wheels will look stationary and therefore the bus itself will look stationary and defeat the purpose of the shot.

I understand the difference between motion blur and focus blur and may have used the wrong description in my opening.
As you have described, the focus is fine, I just need to work out how I get the motion blur where I want it.

Because of the circumstances, I was on the verge of the road while tall grass and a muddy spoon drain prevented me from getting further away from the subject.

@Miranda: "Now do the same thing again, but this time look at the front left hand corner of the bus and pan the camera to keep this in the same spot in your VF. You should find the front sharp now and the back blurred." When I do this, would this still work with the focus point set to the left of the camera or am I better off with the focus point central as that will theoretically be where my eye will be looking as I pan the camera with the front of the bus? I could always add a lot more negative space and crop in post production perhaps?   
 



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Post Options Post Options   Quote artuk Quote  Post ReplyReply Direct Link To This Post Posted: 04 August 2017 at 22:43
Originally posted by Caprice Caprice wrote:

Thanks for the response guys.

I'm fine with motion blur as I want the bus to look like it's traveling. I don't expect the whole bus to be completely sharp but am wanting the front section of the bus to be without motion blur.


If the front section of the bus is sharp, then probably the entire bus will be sharp, since the entire bus is moving in the same direction at the same speed. Either the entire bus is blurred, or the entire bus is sharp - I am confused how part of the bus could be sharp and another part blurred, with the exception of the wheels.
Art
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Post Options Post Options   Quote sybersitizen Quote  Post ReplyReply Direct Link To This Post Posted: 04 August 2017 at 23:32
Originally posted by artuk artuk wrote:

If the front section of the bus is sharp, then probably the entire bus will be sharp, since the entire bus is moving in the same direction at the same speed. Either the entire bus is blurred, or the entire bus is sharp - I am confused how part of the bus could be sharp and another part blurred, with the exception of the wheels.

As mentioned earlier ... with a long vehicle shot at that angle, the front (near the camera) will show more apparent motion than the rear (far from the camera). Still, there should be - and probably is - some blur even at the rear. I would like to see a full size version of the shot, but the provided link doesn't show me one.
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Post Options Post Options   Quote Caprice Quote  Post ReplyReply Direct Link To This Post Posted: 05 August 2017 at 01:11
Originally posted by sybersitizen sybersitizen wrote:

Originally posted by artuk artuk wrote:

If the front section of the bus is sharp, then probably the entire bus will be sharp, since the entire bus is moving in the same direction at the same speed. Either the entire bus is blurred, or the entire bus is sharp - I am confused how part of the bus could be sharp and another part blurred, with the exception of the wheels.

As mentioned earlier ... with a long vehicle shot at that angle, the front (near the camera) will show more apparent motion than the rear (far from the camera). Still, there should be - and probably is - some blur even at the rear. I would like to see a full size version of the shot, but the provided link doesn't show me one.


When you say "a full size version", what do you mean sybersitizen? Are you looking for a higher res pic or an uncropped pic? I can zoom in and show you a detailed shot of the rear if you're looking for a close up of the area in focus?
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Caprice View Drop Down
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Post Options Post Options   Quote Caprice Quote  Post ReplyReply Direct Link To This Post Posted: 05 August 2017 at 01:19
This is the original version as shot.

[IMG]DSC09207-2 by Greg Forster, on Flickr[/IMG]

Here's a crop of the rear end that shows no motion blur.

[IMG]DSC09207-3 by Greg Forster, on Flickr[/IMG]
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Post Options Post Options   Quote QuietOC Quote  Post ReplyReply Direct Link To This Post Posted: 05 August 2017 at 01:26
It is hard to get a sharp panning shot from that angle.


ISO 100 1/60 s f/22 180 mm

Here's one I took a few week ago that looks a bit like what you are going for. I think this worked because the car was turning right.
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Caprice View Drop Down
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Post Options Post Options   Quote Caprice Quote  Post ReplyReply Direct Link To This Post Posted: 05 August 2017 at 01:36
Great shot QuietOC!

Yes, that's what I was after with the bus. Front sharp and rear showing motion as to be expected.

What was your setting for this shot? I'm fine if you don't want to give away trade secrets!
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Post Options Post Options   Quote sybersitizen Quote  Post ReplyReply Direct Link To This Post Posted: 05 August 2017 at 01:45
Alrighty then. This section ...



... compared to this section ...



... indicates that you were panning at the (slower) apparent motion speed of the rear of the bus near the center of the frame, not the (faster) apparent motion speed of the front of the bus near the edge of the frame. So you were able to freeze the back end, but not the front end.

Because of your shooting angle and location, there would be no way to freeze both ends unless you used a faster shutter speed.

BTW, what was the focal length?
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Caprice View Drop Down
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Post Options Post Options   Quote Caprice Quote  Post ReplyReply Direct Link To This Post Posted: 05 August 2017 at 01:46

This is one I shot at night with my old A850!
Again, it's not the same panning shot which is why it has worked out well.
[IMG]Avalon Burnout Comp 2013 by Greg Forster, on Flickr[/IMG]

This is one I did with my old A850. I knew how to capture this type of image with this old camera but haven't been able to successfully convert the theory to the A7Rii!
[IMG]Convoy for kids 2015 by Greg Forster, on Flickr[/IMG]
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