auto focus accracy

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derrycity

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phil
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Auto-Focus Accuracy
I recently bought a new Sigma 24-70 F2.8 to pair up with my EOS 350D. Had it out there at the weekend and it seems pretty good, definately a good step up from the kit lens I had been using for the last year in terms of image quality and wider aperture available.

One thing I did notice however was the auto-focus accuracy doesn't appear to be as good as the kit lens. Is AF controlled by the lens, or by the camera? I was shooting a lot of photos with people in them outdoors and I could often see the AF point in the viewfinder lighting up on insignificant elements in the shot. I found I had to do a fair bit of manually selecting focus points or full time manual focusing to get solid results. Is this just a downside to buying lenses from a company other than the camera manufacturer? Is there anything I can be doing to help improve accuracy such as changing metering? Is the fact I only have 7 AF points on the 350D compared to more on newer models going to make much of a difference?

Would be interested to hear peoples comments on this, I know people have mixed views on lenses such as Sigma but unfortunately that is the budget I am on. Thanks
 
I wouldnt ever rely on having all the focus points active, allowing the camera to select where it should be in focus just doesnt work (in my experience anyway).

I would suggest you stick to a focus point which suits your medium, I normally use the central focus point for all shots.
 
The camera identifies focus by finding areas of contrast, if anything the lens is a big improvement, allowing the af points to pick up on a lot more than the kit lens was. I suggest playing around with the AF settings to see what suits your style of shooting.
 
Remember as well if you arew shooting af F2.8 you have less depth of field so you will notice any focus inaccuracies more.
 
RichardtheSane said:
Remember as well if you arew shooting af F2.8 you have less depth of field so you will notice any focus inaccuracies more.
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This is a bad thing?:thinking:

Surely, buying such a lens and using it at f/2.8 presupposes the intention of using it for that very purpose, i.e. narrow D.O.F. and blurred background.

With this comes improved technique?:shrug:
 
derrycity said:
Auto-Focus Accuracy
I recently bought a new Sigma 24-70 F2.8 to pair up with my EOS 350D. Had it out there at the weekend and it seems pretty good, definately a good step up from the kit lens I had been using for the last year in terms of image quality and wider aperture available.

One thing I did notice however was the auto-focus accuracy doesn't appear to be as good as the kit lens. Is AF controlled by the lens, or by the camera? I was shooting a lot of photos with people in them outdoors and I could often see the AF point in the viewfinder lighting up on insignificant elements in the shot. I found I had to do a fair bit of manually selecting focus points or full time manual focusing to get solid results. Is this just a downside to buying lenses from a company other than the camera manufacturer? Is there anything I can be doing to help improve accuracy such as changing metering? Is the fact I only have 7 AF points on the 350D compared to more on newer models going to make much of a difference?

Would be interested to hear peoples comments on this, I know people have mixed views on lenses such as Sigma but unfortunately that is the budget I am on. Thanks
Click to expand...

Now personally, I would never buy a Sigma (or any other 3rd party) lens but that is my choice and opinion...

I would go with previous advice from someone in this post and recommend you sticking with the central AF point. It has been a while since I had my 350 but IIRC only the central AF point is cross sensitive...i.e. sensitive to both horizontal and vertical lines.

The better the f/rating of a lens the better the AF should perform but do remember that the specific type of AF in the camera body will play a role...so, to answer your question, both lens and body should influence the AF performance.

Have you done some comparative tests...your Sigma lens on another 350 or a similar Sigma on your 350?

Metering should not affect AF in any way. Newer bodies with more AF points might be better but purely because there might be more cross-type AF points or improved AF technology...or both.

HTH
 
antonroland said:
This is a bad thing?:thinking:

Surely, buying such a lens and using it at f/2.8 presupposes the intention of using it for that very purpose, i.e. narrow D.O.F. and blurred background.

With this comes improved technique?:shrug:
Click to expand...

Never said it was a bad thing... not sure why you think I did.
 
I suppose I ASSUMED that someone who bought a f/2.8 lens would already know the benefits and qualities associated with it...

Only applied your comment on the O/P's question later...

No offence meant



:coat:
 
thanks for all your comments i'll be taking a load of test shot's from your comments
thanks philip
 
To answer the initial question.....

AF is controlled by the body in that the body decides the destination position of the lens' AF mechanism. If the lens is slower than f/2.8 (as per your kit lens) then the target postion will be within the depth of focus (not field). If the lens is f/2.8 or better then the target will be within 1/3 of the depth of focus.

The body's responsibility ends there. If the lens doesn't go to the commanded position then you're snookered. The AF system is open loop and there isn't any post-move checks done by the body other than to register the lens' response that a move has occurred.

Bob
 
Canon Bob said:
To answer the initial question.....

AF is controlled by the body in that the body decides the destination position of the lens' AF mechanism. If the lens is slower than f/2.8 (as per your kit lens) then the target postion will be within the depth of focus (not field). If the lens is f/2.8 or better then the target will be within 1/3 of the depth of focus.

The body's responsibility ends there. If the lens doesn't go to the commanded position then you're snookered. The AF system is open loop and there isn't any post-move checks done by the body other than to register the lens' response that a move has occurred.

Bob
Click to expand...

And for those of us who speak ENGLISH??:lol:
 
antonroland said:
And for those of us who speak ENGLISH??:lol:
Click to expand...

canon bob saying that the camera decides where the lens should focus based on the information - the contrast image got from the lens. and lens moves accordingly. but weather the lens focuses to the right focus point or not is not checked by the camera or there isn't any process to check it.

I think third party lenses, particularly sigma is more prone to AF issues, and the manufacturer lenses are not as bad, cos after all they make the damn lenses and the should know the exact characteristics of them when they build the cameras and thus the camera should be in perfect sync with the lens
 
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