7D AF query

Roy C

Roy C

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Forget the IQ as I was just experimenting taking shots through a double glazed window when I noticed two AF points were showing red in the viewfinder - I was just wondering how/why the AF can pick up these two points -all 19 focus point enabled (No, I do not usually use all 19 point - just prating around) . I was shooting from around 5 metres to the bird, The green oof hedge is a good 2 metres behind the bird so how come both are highlighted as being active points. I was shooting in one shot at the time with a aperture of f4.

I have had my 7D for 18 months and absolutely delighted with its AF performance which is usually with a Manual selected single point or point expansion for birds in flight so I am fairly certain there is no problem with my particular Camera but was just curious as to how and why the two points could both be highlighted (I have my own theory on this but was wondering what others thought).

AFtest1b.jpg
 
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Hi Rob

I had problems with my first 7D which sometimes completely lost the plot with chosing what to lock on to. For example if I took a picture of a bird against the plain sky it would lock ont to the sky with two or three squares and completely ignore the bird in the middle of the frame. It also had other quirkes and in the end I sent it back (thanks to the nice people at SRS Microsystems who sent me a replacement before making arangements to pick up the original). The second one was much better but having used it a bit more I find it better to use Manual select with or without AF point expansion.
 
Oggy said:
Hi Rob

I had problems with my first 7D which sometimes completely lost the plot with chosing what to lock on to. For example if I took a picture of a bird against the plain sky it would lock ont to the sky with two or three squares and completely ignore the bird in the middle of the frame. It also had other quirkes and in the end I sent it back (thanks to the nice people at SRS Microsystems who sent me a replacement before making arangements to pick up the original). The second one was much better but having used it a bit more I find it better to use Manual select with or without AF point expansion and AF is brilliant.
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Sorry Roger but I should have explained a bit more. I have had the 7D for 18 months, love it to bits and would not change it for the world. I have no problems with normal AF and the Camera is even pin sharp with a 2x tc on my 300/2.8 (see my photostream below and 2x samples all taken with the 7D). I normally only use single point AF with point expansion for flyers.

I have not got any reason ever to use all 19 points but do like playing around at times - just curious as to how this could happen (I have my own theories about all 19 points and zone AF but was just wondering what others thought)
 
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Was this in single shot mode or machine gun? Just wonder if you have AF as secondary priority and the camera is more interested in getting a shot off than AF in 19 point setting as nothing looks to be in focus
 
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Did you focus & recompose ?

I ask as if this were the case, those two particular points were used to set the focus, but then you moved the camera, leaving those points "active" but focus was locked.

Steve
 
Jelster said:
Did you focus & recompose ?

I ask as if this were the case, those two particular points were used to set the focus, but then you moved the camera, leaving those points "active" but focus was locked.

Steve
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That's interesting Steve - I always use the back button (AF-ON) to focus and normally I am in AI servo mode so I am in the habit of 'bumping' the focus - it could have been that I achieved focus on those two points and then released the AF-ON (focus) button, but I have just checked this and when you do it then there are no active focus points showing (which is what you would expect).
 
zooxanthellae said:
Was this in single shot mode or machine gun? Just wonder if you have AF as secondary priority and the camera is more interested in getting a shot off than AF in 19 point setting as nothing looks to be in focus
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This was in high burst mode which is where I think part of the problem could lie. I think it is a problem with the AF points keeping up, when you have all 19 points active (which I never do in practice !) and the camera or target is moving around then all the points are talking to each other and when you fire off a high burst the AF is just not quick enough to cope. There is nothing in focus as the bird was swinging around and I was not even in AI servo mode.
I have AF as first priority but I think the setting only effect AI servo mode anyway doesn't it?
Like I say, this is no problem for me as I never use all 19 points anyway and I almost always shoot in AI servo mode - I just had nothing better to do and was curious:bonk:
 
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Could it be the glass between you and the subject that's skewing things?

I know there's nothing obvious in the image that shows up on the glass but seeing as the subject isn't really in focus.

Seeing as AF will pick up on the closest target, my guess would be that the AF might have picked up on a slight reflection on the glass.
 
Russ77 said:
Could it be the glass between you and the subject that's skewing things?

I know there's nothing obvious in the image that shows up on the glass but seeing as the subject isn't really in focus.

Seeing as AF will pick up on the closest target, my guess would be that the AF might have picked up on a slight reflection on the glass.
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Could be Russ but the minimum focus of the lens is 2.5 metres and the glass was only a foot or so away so I do not think it was that.
 
Roy C said:
That's interesting Steve - I always use the back button (AF-ON) to focus and normally I am in AI servo mode so I am in the habit of 'bumping' the focus - it could have been that I achieved focus on those two points and then released the AF-ON (focus) button, but I have just checked this and when you do it then there are no active focus points showing (which is what you would expect).
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It was just a thought, it's the only feasible reason I could see for it happening :shrug:

Steve
 
If you indeed didn't hold the AF-ON button pressed when actually taking the picture, you wouldn't get any used AF point registered at all. Focusing must be active at the time of exposure for that information to be inserted into the Exif data.

It's much more likely that when locking focus, the leftmost point was close enough to the bird to see that. Remember that the points are quite a bit bigger than their representations in the viewfinder and the associated software.
 
It will be the double glazing which confuses things. I've had a (really bad) CPL which had the effect of making the AF think it was focused when it was not. Each pane of glass will have a reflection which will be visible to the camera regardless of the MFD of the lens.

For example, you can shoot the reflection in a mirror while standing a few inches away from the mirror.
 
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