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Archive 2013 · Possibly a stupid AF question.
  
 
Ed Swift
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p.1 #1 · Possibly a stupid AF question.


This is possibly a stupid question, but I can't find the answer to it on line (so far)

If I have a Canon 1000d with a cross type f5.6 af point in the center and a Canon 40d with a cross type f2.8 af point but the majority of my lenses are f4 or slower is the focus of the 40d actually going to be any better than the 1000d? Also, continuing that train of thought other than having more af points to choose will e.g. the 7d or 1d3 be better?

Does it all come down to the software for af in the camera?



Jul 19, 2013 at 08:21 PM
jcolwell
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p.1 #2 · Possibly a stupid AF question.


I can't answer your specific question, but it depends on every link in the chain; lens light transmissibility, type of lens AF motor, type and sensitivity of AF sensor(s), and processing speed, accuracy and algorithms in the camera processor(s), including (often) firmware version.


Jul 19, 2013 at 08:27 PM
Ed Swift
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p.1 #3 · Possibly a stupid AF question.


I was just trying to get my head round how things work when you are not using the 2.8 sensitivity to it's potential.

When i was at the British Superbikes last year I definitely had more keepers with the 40d, but have no idea why given they were both used center point only.

I was also thinking of your suggestion in my camera advice thread that maybe a 70-200/2.8 might be a good choice instead of a new body since it would use the AF point to the max as well ass being faster.

In my example, given the same lenses, same sensor in both body I guess it must be down to the algorithm in the camera.



Jul 19, 2013 at 08:34 PM
rscheffler
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p.1 #4 · Possibly a stupid AF question.


Not being super familiar with some those, it will depend on what the f/2.8 sensor in the 40D reverts to with slower lenses. Does it remain a cross sensor, but without the extra precision of the f/2.8 feature, or does it revert to a non-cross type sensor? According to DP Review, it seems to revert to an f/5.6 cross-type sensor, inline with its other AF points. The 1000D has only a single, central AF cross-type sensor. The others are non-cross, meaning they'll be less reliable with your slower lenses with lower contrast subject matter than the 40D. Considering both are 10MP cameras, you're likely to get a better user experience from the 40D. And likewise, if you compare the 1DIII vs. the 40D, it's going to be a much faster feeling camera, though AF with slower lenses might not be a noticeable difference.

But in general, I don't think you can look at only the type of AF points available. The software in each camera is going to be a factor, as well as the processor(s). Some of the higher-end cameras have more than one processor, where one might be specifically dedicated to AF, or camera functions, while the other deals with image processing, etc. Generally the more expensive the camera, the better the AF and overall performance will be. However, if the camera has many AF points that lose cross-sensor sensitivity with slower lenses, you might actually get a more positive AF experience from a camera that maintains f/5.6 cross-sensor sensitivity. Cameras like the 1D series are squarely aimed at high-end users who demand the highest performance and have the lenses to match. it's really a matter of deciding which features are most important to you.



Jul 19, 2013 at 08:46 PM
 

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scottam10
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p.1 #5 · Possibly a stupid AF question.


I agree that there probably wouldn't be much difference with the centre point for stationary subjects.
However the 40D has 9 cross-type AF points so you could use the outer AF points and expect better results than the 1000D

I expect that the 40D may track moving subjects better

Also, the 40D has a bigger viewfinder which makes it easier to visually confirm that the camera has focussed on what you want it to (and also helps you for manual focus, but modern cameras don't have split prism focus screens which makes them more difficult to use on manual focus; they're really designed for AF)

Having more AF points (eg 7D) is mostly useful for tracking moving subjects as the camera can follow the subject across the viewfinder. For stationary subjects I normally use single point AF anyway, as I want to control where the camera focuses - if I use the 19-point automatic AF mode it can focus on the wrong thing (eg catching something in the foreground)

With the 1000D you are pretty much limited to the 'focus lock and recompose' technique using the centre AF point. While this is fine, this can introduce a focus error if you are using a fast prime lens wide open (shallow DOF) and you are placing your subject on the edge of the frame, especially for wide-angle lenses. More AF points (eg 7D with 19 cross-type points) allows you to choose the AF point that is over your subject

The 40D is pretty cheap 2nd hand, I don't think it would cost too much to upgrade and I think the user experience and performance is better

Edited on Jul 19, 2013 at 09:05 PM · View previous versions



Jul 19, 2013 at 08:58 PM
Ed Swift
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p.1 #6 · Possibly a stupid AF question.


Thanks, I get the outer points on the 1000d aren't as good but couldn't understand how the central one works and why it is/isn't better.

Sounds like it's down to non "hardware", but it might be worth trying out better lenses with my current system and comparing it to my lenses with a better camera and seeing which should come first.



Jul 19, 2013 at 08:58 PM
Ed Swift
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p.1 #7 · Possibly a stupid AF question.


scottam10 wrote:
The 40D is pretty cheap 2nd hand, I don't think it would cost too much to upgrade and I think the user experience and performance is better

Hi Scott, I've already got both but have been thinking about upgrading to something like a 7d but i realized yesterday that I didn't know why center af was better with the 40d than the 1000d as it wasn't using the point at 2.8 and didn't know if/why AF would improve over my current kit with an upgrade.



Jul 19, 2013 at 09:04 PM





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