What is the difference between "good" AF and "bad" AF?

[mcintoshi]

This probably seems like a strange question to begin with, but...can anyone tell me what the difference is between "good" autofocus and "bad" autofocus?

While waiting for the 5D3, I've done lots of reading and poring over potential specs etc, and one thing that always crops up is talk of improved AF over the supposedly bad AF in the 5D2, and comparisons with the supposedly good AF in the 7D. Never having shot with either, I can only speculate based on my experiences with my lowly 1000D.

In using the 1000D, like many people I set the centre AF point and then when shooting I focus, then recompose, then release the shutter. Most of the time it seems to work fine, so I'm wondering what specifically it was about the 5D2's AF that was "bad". Surely it was better than that of the 1000D - so what factors exactly are we talking about when comparing the AF of these high-level models?

cheers,
Ian

 

[dirtcastle]

I always wondered why so many of my 5D2 shots were out of focus.

Turns out, my tendency to use peripheral AF points, rather than the center point, seems to have been the culprit. No surprise, I learned about this issue here in the CR forums. Needless to say, now I exclusively use the center point. And a lot more shots are in focus.

Even if the 5D2 peripheral points were more accurate, it still limits you to a small diamond shape. I find that limiting. Since when is everything I want to focus in the shape of a diamond? I would have preferred a big rectangle because usually I'm just guessing where I'll be focusing and then slightly recomposing for the crop.

I'm curious to hear if others have had similar issues with peripheral focus points being unreliable.

 

[wickidwombat]

I always wondered why so many of my 5D2 shots were out of focus.

Turns out, my tendency to use peripheral AF points, rather than the center point, seems to have been the culprit. No surprise, I learned about this issue here in the CR forums. Needless to say, now I exclusively use the center point. And a lot more shots are in focus.

Even if the 5D2 peripheral points were more accurate, it still limits you to a small diamond shape. I find that limiting. Since when is everything I want to focus in the shape of a diamond? I would have preferred a big rectangle because usually I'm just guessing where I'll be focusing and then slightly recomposing for the crop.

I'm curious to hear if others have had similar issues with peripheral focus points being unreliable.

Absooolutelty!
But i went from nikon to shooting with a 1D then got the 5D2 so you can imagine my horror when I first experienced the AF of the 5D but i am mostly over the shock now just some light scaring.
but the IQ of the 5D2 is soooo nice i have to keep forgiving it for its one legged AF

 

[briansquibb]

I always wondered why so many of my 5D2 shots were out of focus.

Turns out, my tendency to use peripheral AF points, rather than the center point, seems to have been the culprit. No surprise, I learned about this issue here in the CR forums. Needless to say, now I exclusively use the center point. And a lot more shots are in focus.

Even if the 5D2 peripheral points were more accurate, it still limits you to a small diamond shape. I find that limiting. Since when is everything I want to focus in the shape of a diamond? I would have preferred a big rectangle because usually I'm just guessing where I'll be focusing and then slightly recomposing for the crop.

I'm curious to hear if others have had similar issues with peripheral focus points being unreliable.

Absooolutelty!
But i went from nikon to shooting with a 1D then got the 5D2 so you can imagine my horror when I first experienced the AF of the 5D but i am mostly over the shock now just some light scaring.
but the IQ of the 5D2 is soooo nice i have to keep forgiving it for its one legged AF

+1 - sometimes you have to work at it rather than having the DSLR as a point and shoot

 

[Hillsilly]

I'd say a good AF system: -

1. Works reliably in subdued lighting (this is the EV thing you see mentioned - the lower the better)
2. Has autofocus points across most of the image (not just the central portion)
3. Has more "cross type" autofocus points (these are more accurate)

From there, a good AF system would have a number of options relating to tracking objects better and choosing groups of sensors to use. If you have a chance, download a 7D manual and you'll see the different options available and what's possible.

 

[briansquibb]

I'd say a good AF system: -

1. Works reliably in subdued lighting (this is the EV thing you see mentioned - the lower the better)
2. Has autofocus points across most of the image (not just the central portion)
3. Has more "cross type" autofocus points (these are more accurate)

From there, a good AF system would have a number of options relating to tracking objects better and choosing groups of sensors to use. If you have a chance, download a 7D manual and you'll see the different options available and what's possible.

Or download a 1D4 manual to see the extra functions and capabilities

 

[Drizzt321]

This probably seems like a strange question to begin with, but...can anyone tell me what the difference is between "good" autofocus and "bad" autofocus?

While waiting for the 5D3, I've done lots of reading and poring over potential specs etc, and one thing that always crops up is talk of improved AF over the supposedly bad AF in the 5D2, and comparisons with the supposedly good AF in the 7D. Never having shot with either, I can only speculate based on my experiences with my lowly 1000D.

In using the 1000D, like many people I set the centre AF point and then when shooting I focus, then recompose, then release the shutter. Most of the time it seems to work fine, so I'm wondering what specifically it was about the 5D2's AF that was "bad". Surely it was better than that of the 1000D - so what factors exactly are we talking about when comparing the AF of these high-level models?

cheers,
Ian

Well, it's really that it's 'bad' in comparison to the vast majority of it's contemporary bodies at similar target audience. For example, the AF compared to 10+ years ago, it's probably quite a bit better. Compared to the 7D/D700/etc it's generally considered to be not nearly as good. So, it's all your perspective and what you're comparing it to. Plus, most people think, for the price, it should have had something better that what it has. And right now, the AF is, by comparison to the new bodies coming out, about ready to let out one last breathe and drop into an open grave.

 

[briansquibb]

This probably seems like a strange question to begin with, but...can anyone tell me what the difference is between "good" autofocus and "bad" autofocus?

While waiting for the 5D3, I've done lots of reading and poring over potential specs etc, and one thing that always crops up is talk of improved AF over the supposedly bad AF in the 5D2, and comparisons with the supposedly good AF in the 7D. Never having shot with either, I can only speculate based on my experiences with my lowly 1000D.

In using the 1000D, like many people I set the centre AF point and then when shooting I focus, then recompose, then release the shutter. Most of the time it seems to work fine, so I'm wondering what specifically it was about the 5D2's AF that was "bad". Surely it was better than that of the 1000D - so what factors exactly are we talking about when comparing the AF of these high-level models?

cheers,
Ian

Well, it's really that it's 'bad' in comparison to the vast majority of it's contemporary bodies at similar target audience. For example, the AF compared to 10+ years ago, it's probably quite a bit better. Compared to the 7D/D700/etc it's generally considered to be not nearly as good. So, it's all your perspective and what you're comparing it to. Plus, most people think, for the price, it should have had something better that what it has. And right now, the AF is, by comparison to the new bodies coming out, about ready to let out one last breathe and drop into an open grave.

Back to 5D2 AF bad mouthing time again

 

[Drizzt321]

This probably seems like a strange question to begin with, but...can anyone tell me what the difference is between "good" autofocus and "bad" autofocus?

While waiting for the 5D3, I've done lots of reading and poring over potential specs etc, and one thing that always crops up is talk of improved AF over the supposedly bad AF in the 5D2, and comparisons with the supposedly good AF in the 7D. Never having shot with either, I can only speculate based on my experiences with my lowly 1000D.

In using the 1000D, like many people I set the centre AF point and then when shooting I focus, then recompose, then release the shutter. Most of the time it seems to work fine, so I'm wondering what specifically it was about the 5D2's AF that was "bad". Surely it was better than that of the 1000D - so what factors exactly are we talking about when comparing the AF of these high-level models?

cheers,
Ian

Well, it's really that it's 'bad' in comparison to the vast majority of it's contemporary bodies at similar target audience. For example, the AF compared to 10+ years ago, it's probably quite a bit better. Compared to the 7D/D700/etc it's generally considered to be not nearly as good. So, it's all your perspective and what you're comparing it to. Plus, most people think, for the price, it should have had something better that what it has. And right now, the AF is, by comparison to the new bodies coming out, about ready to let out one last breathe and drop into an open grave.

Back to 5D2 AF bad mouthing time again

Hey, I'm not bad mouthing it. I have one, and it works fine for the most part. I'm just saying that compared to other bodies of similar age/price, it's not generally considered to be as good.

 

[briansquibb]

And right now, the AF is, by comparison to the new bodies coming out, about ready to let out one last breathe and drop into an open grave.

That doesn't read as anything else but bad mouthing .....

No one claims it is a pro AF but with a f/2.8 lens that centre point is still the best in low light

 

[Loswr]

No one claims it is a pro AF but with a f/2.8 lens that centre point is still the best in low light

Perhaps...but I was in an aquarium last weekend, and while ISO 3200 - 6400 were sufficient for the shots, the AF (center point) was struggling to achieve lock much of the time (and that was with a 35mm f/1.4L).

...like many people I set the centre AF point and then when shooting I focus, then recompose, then release the shutter.

Most of the time that does work fine, with a slow lens. But if you're shooting a fast lens (f/2 and wider) at wide aperture, focus-recompose will result in out of focus shots, simply due to geometry (the sensor is flat, the lens field is flat, ideally at any rate, and you're moving the camera in an arc when you recompose).