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Thread started 17 Jul 2010 (Saturday) 13:57
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7D and 24L II focus issues, maybe?

 
Ross_Curtis
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Jul 17, 2010 13:57 |  #1

I seems to be getting inconsistent focus with this combo.

Say I've focused on a flat object (IE brick wall, shed etc) from about 10m away using centre spot. If I watch the focus markings on the lens almost every time I half press the shutter the focus shifts by a mm or two back and forth.
Is this normal?

The reason this came about was when I was simply focusing with the viewfinder then switching to live view and zooming into 10x to check it was spot-on, in almost every example I had to readjust.
I played around with microadjustment and although this did improve things when set to +2, it was still totally inconsistent, yet in EVERY situation I used auto focus in live view (set to 10x) it was perfect.

I know the 7D is notorious for user error, but I really can't see I'm doing anything wrong.....?


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Ziffle
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Jul 17, 2010 14:06 |  #2

are you in one shot or ai servo focus mode?

plus use a target with contrast. A plain wall is the worst thing to focus on.
you can use spot focus - standard setting and use a f stop of 4 or bigger.

also have a decent shutter speed so you are not inducing camera shake that looks like blurry focus. say 1/160 or faster.


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billybookcase
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Jul 17, 2010 14:11 |  #3

being spot on in gauging distance (which in itself could be a mm or two off at different spots) 10 meters away would be pretty hard for any type of machine, even a laser range finder.

at that ratio, your margin of error is around 0.2% which is pretty good for something you can hold in your hand.


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Ross_Curtis
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Jul 17, 2010 14:18 |  #4

This was all done with the lens set to 1.4, centre spot single focus with tripod. as most of my photos are taken wide open I wanted it be be realistic (are you saying lenses are less accurate when wide open?), the shutter was around 500th/sec.

I was aware of the subject having good contrast, which is why I tried a few different subjects.

Does micro adjustment affect auto focus in live view?


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billybookcase
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Jul 17, 2010 14:23 |  #5

I'm not saying it's the lens, I'm saying that it is a little unrealistic to assume that the AF sensor on a camera can be accurate to 1 mm at a distance of 10 m (1000 mm).

In addition, the only affect of opening the lens is that focus errors will be more pronounced. However, what is the likelyhood that your photography is concentrated on only flat walls where focusing error will be inherently present.


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Ross_Curtis
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Jul 17, 2010 14:26 |  #6

No, I mean the focus marking on the lens are moving back and forth a couple of mm's with every focus when the conditions haven't changed. Of course to expect the focus to be accurate to 1mm is totally unrealistic.


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jwcdds
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Jul 17, 2010 14:40 |  #7

Are you giving the AF sensor something VERY contrasty and distinct to focus on? Because if there is some variation on where the sensor might determine the focus, then once it locks onto something, it might be happy telling you it's in focus (although it might not be the exact thing you want to focus on).

Tripod or not, the AF sensor can't read minds. If you want to test for consistency, then I would suggest shooting a high contrast pole with nothing around it, using a tripod setup and see if the camera + lens can lock on the pole consistently. If it does, then well... it's doing fine, IMO.


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Ross_Curtis
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Jul 17, 2010 14:48 |  #8

I did also use a brown wheely bin with white letters on (this is about as contrasty as it gets), granted I was still at around 10m so the object was fairly small in the viewfinder.

What I don't get is why autofocus in live view is always perfect, yet when used in one shot, centre spot, the focus is only right 50% of the time.


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jwcdds
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Jul 17, 2010 15:27 |  #9

Different type of focus. Phase detection vs. strictly contrast based.


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Ross_Curtis
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Jul 17, 2010 15:47 |  #10

jwcdds wrote in post #10554498 (external link)
Different type of focus. Phase detection vs. strictly contrast based.


OK that makes sense, thanks.


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Ziffle
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Jul 17, 2010 15:58 |  #11

are you in ai servo or one shot?


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Ross_Curtis
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Jul 17, 2010 16:06 |  #12

One shot.


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Ross_Curtis
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Jul 18, 2010 03:54 |  #13

I had a little play around this morning using a test chart from about 1m away. Same setup. Tripod, single shot, remote release, and making sure the shutter was over 100sec.

Same inconsistent results.

I tried turning AI servo on in the same situation and watching the focus markings on the lens was interesting (kind of), it would hold for 5 secs, then move about for a bit, stop, then move again. This movement seemed consisten with the amount of focus shift I'm getting in single shot.

On another note, my 24L II requires +11 micro adjustment at 1m and +2 at 3m
!!!!

I'll post some examples later.


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Ross_Curtis
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Jul 18, 2010 03:59 as a reply to  @ Ross_Curtis's post |  #14

Here is an example of two consecutive shots, which clearly shows the focus shift I am getting. Taken at f1.4 from 1m, 100% crops. Straight from camera, no sharpening or PP.


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jwcdds
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Jul 18, 2010 04:24 |  #15

Hm... what about upclose?

I just tested mine out again, MFD = +10. about 1m also = +10 on mine. I'm not sure how to test 3m reliably since everything seems to be in focus by that distance. :D


Julian
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7D and 24L II focus issues, maybe?
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