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Thread started 21 Oct 2012 (Sunday) 06:39
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7D Focus :-) Why did this happen?

 
Arte ­ Automobilistica
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Oct 21, 2012 06:39 |  #1

Hi

After reading far too much on the 7D's AF "problems" (yes I did read the manual etc etc...) I took it for a test yesterday using, among others, 50mm (w/o filter) and centre single AF point expansion. In the photo the AF point (centre) was just above the green sticker and the f/stop was 5.6.

Why is only specifically the interior of the car in focus?

Thanks


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mridley
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Oct 21, 2012 06:44 |  #2

Arte Automobilistica wrote in post #15149237 (external link)
Hi

After reading far too much on the 7D's AF "problems" (yes I did read the manual etc etc...) I took it for a test yesterday using, among others, 50mm (w/o filter) and centre single AF point expansion. In the photo the AF point (centre) was between the number and the green sticker and the f/stop was 5.6.

Why is the interior of the car in focus?

Thanks

Post the picture, you will get a better response then..




  
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Lowner
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Oct 21, 2012 06:45 |  #3

Can we see the picture?


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SMP_Homer
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Oct 21, 2012 08:13 |  #4

if the F point was above the green sticker - that's just a white area - AF sensor may have had issue locking to something for focus
helps to focus on something the sensor can use... like the stripe that runs front to back on the side, or the '37' on the door


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alquimista
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Oct 21, 2012 08:18 |  #5

SMP_Homer wrote in post #15149394 (external link)
if the F point was above the green sticker - that's just a white area - AF sensor may have had issue locking to something for focus
helps to focus on something the sensor can use... like the stripe that runs front to back on the side, or the '37' on the door

agree, I had that issue before


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TheBigDog
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Oct 21, 2012 08:19 |  #6

i think it's more motion blur than lack of focus, panning skills can take some time to perfect.
Next time up the ISO a bit and open up the lens a little to go quicker on the shutter speed and you should see better results.


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Oct 21, 2012 09:19 |  #7

Looking at the image it looks as if the front end of the car has some vertical movement upwards, with the car rotating about a point somewhere near the back axle. This causes some motion blur to be visible at the extremities but but the closer you get to the axis of rotation the less movement the is so the less it "shows" in the image.

You can see a similar effect on this Red Bull Sbach 300. The aircraft can roll at 540 degrees a second, so even though the shutter speed was 1/160 you can see quite a bit of rotation blur in the wing tips. In the second one you can see that the axis of rotation is not always where you might expect it to be, as in this one the axis is actually about 4 feet out on the port wing, and not parallel to the fuselage. This second shot was take at only 1/100th.

Alan


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ebiggs
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Oct 21, 2012 09:25 as a reply to  @ BigAl007's post |  #8

i think it's more motion blur than lack of focus, panning skills can take some time to perfect.

I agree:) with this assesment.


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DC ­ Fan
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Oct 21, 2012 10:06 |  #9

Arte Automobilistica wrote in post #15149237 (external link)
Hi

After reading far too much on the 7D's AF "problems" (yes I did read the manual etc etc...) I took it for a test yesterday using, among others, 50mm (w/o filter) and centre single AF point expansion. In the photo the AF point (centre) was just above the green sticker and the f/stop was 5.6.

Why is only specifically the interior of the car in focus?

Thanks

Because that's what happens sometimes when a slow shutter speed is used to generate blur that gives the illusion of motion in a still image of a moving subject.

From the vantage of a fixed point, the different parts of a moving object will appear to move at a different speed. That will mean different degrees of blur for each portion of the passing object. The parallax effect (external link) that defines the panning illusion is unpredictable when a human is operating the camera. There is nothing wrong or unusual in the sample image, and there is nothing wrong with the camera or the lens.




  
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Arte ­ Automobilistica
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Oct 21, 2012 10:49 |  #10

SMP_Homer wrote in post #15149394 (external link)
if the F point was above the green sticker - that's just a white area - AF sensor may have had issue locking to something for focus
helps to focus on something the sensor can use... like the stripe that runs front to back on the side, or the '37' on the door

Well that was where I was aiming :) but the car moving in 3 dimensions made it rather difficult.

TheBigDog wrote in post #15149408 (external link)
i think it's more motion blur than lack of focus, panning skills can take some time to perfect. Next time up the ISO a bit and open up the lens a little to go quicker on the shutter speed and you should see better results.

I tried to keep ISO as low as possible to limit noise. I used 5.6 as I wanted to use the cameras "enhanced" AF points and wanted to see motion in the wheels hence the lowish shutter speed

DC Fan wrote in post #15149710 (external link)
There is nothing wrong or unusual in the sample image, and there is nothing wrong with the camera or the lens.

Would a higher shutterspeed put more of the car in focus or would it rely totally on a different position?


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DC ­ Fan
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Oct 21, 2012 11:29 |  #11

Arte Automobilistica wrote in post #15149835 (external link)
Would a higher shutterspeed put more of the car in focus or would it rely totally on a different position?

Focus and motion blur are entirely different factors. A higher shutter speed would minimize motion blur, but it would not change focus. In this case, everything depends on what you really want from the picture. If you want every portion of the car perfectly sharp, the best way to accomplish that is to use a fast shutter speed. However, using a slow shutter speed means portions of the image will be blurred,and the blur creates the illusion of motion. The slower the shutter speed, the greater the motion blur and the less predictable that blur will be. A high shutter speed means a sharp subject that looks as if it's standing still. Example:


IMAGE NOT FOUND
IMAGE IS A REDIRECT OR MISSING!
Byte size: ZERO | Content warning: NOT AN IMAGE


Focal Length: 300.0mm
Aperture: f/5.6
Exposure Time: 0.0016 s (1/640)
ISO equiv: 320
Exposure Bias: none
Metering Mode: Matrix
Exposure: shutter priority (semi-auto)
White Balance: Auto
Flash Fired: No (enforced)
Orientation: Normal
Color Space: sRGB

It takes experience and skill to consistently generate images that balance motion blur and a sharp subject. Example:

IMAGE NOT FOUND
IMAGE IS A REDIRECT OR MISSING!
Byte size: ZERO | Content warning: NOT AN IMAGE


Focal Length: 183.0mm
Aperture: f/29.0
Exposure Time: 0.020 s (1/50)
ISO equiv: 100
Exposure Bias: none
Metering Mode: Partial
Exposure: shutter priority (semi-auto)
White Balance: Auto
Flash Fired: No (enforced)
Orientation: Normal
Color Space: sRGB



  
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FrayAdjacent
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Oct 21, 2012 11:48 |  #12

Looks like it caught focus on the rear driver's side roll cage, which is a bit far from the front passenger fender.


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amfoto1
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Oct 21, 2012 12:34 |  #13

It is not a focus issue... It's a motion blur issue... probably mostly subject movement, but might also be some camera shake blur happening .

50mm lens... you must have been pretty close. 1/160 was slow enough to produce a lot of subject motion blur. That's fine, if it's what you want. Panning rapidly with a 50mm lens, 1/160 might not be enough to fully deal with camera movement blur either.

Another thing... The shutter moves vertically, two curtains with a space or "slot" between that, along with the actual speed the shutter moves determine the "shutter speed". The higher the speed, the narrower the "slot", as well as the faster the movement of the shutter itself. But this means that different parts of an image are exposed at slightly different times. This was more apparent with older style shutters that moved horizontally. Motion blur would be different when the subject was moving left to right than it was moving right to left. But, even with modern, electronically controlled, vertically travelling shutters (based upon the venerable Copal Square invented in the late 1950s), there's still some effect with really fast moving subjects.

Add to that another variable... yourself. As you pan you probably don't do so exactly the same plane as the subject, nor at the exact same speed or even precisely the same direction. You probably "wobble" a little during the pan. You can minimize, but not eliminate this entirely with a monopod or a tripod (such as with a gimbal head and the panning axis left loose).

Whenever you "drag the shutter" (i.e. use a slow shutter speed deliberately to produce subject motion blur), there are a lot of variables and there are bound to be more shots lost to too much or too little blur. As best you can, experiment with different speeds and apertures and techniques. But differences in subject speeds, focal length (distance from the subject and the subject's distance from the background) are always going to happen. You never really "perfect" panning, per se, though you certainly can get better at it with practice and by refining your techniques. But even then there will always be some chance and luck involved. So take lots and lots of shots and simply plan to throw away more than you usually do, don't expect a high keeper rate.

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For the above, the car wasn't going as fast as in your shot (autocross, so probably no more than 40 to 50 mph)... but also I was using a smaller aperture (f16), a slower shutter speed (1/60) and a longer focal length (200mm) so was standing considerably farther away. The lens I used is stabilized, too (IS was left on, but in mode 2 which only corrects for vertical movement).

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Daship
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Oct 21, 2012 12:58 |  #14

If it is a nifty 50 it is a slow to focus lens also. I would say lens, and skill is what mucked the shot not the 7D.




  
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apersson850
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Oct 21, 2012 16:50 |  #15

amfoto1 wrote in post #15150163 (external link)
...as well as the faster the movement of the shutter itself.

Excellent post, but the shutter blinds move with the same speed at all shutter speeds. Only the time between the first and second blind changes.


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