I just realized something about 5DII autofocus system...

Alfred Juras said:

Doesn't the cross-type AF points only kick in with an f/2.8 lens?

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These are two separate concepts: a cross sensor means that is has two different sensor lines, one vertical and one horizontal like this (+). A sensor can only see detail that is perpendicular to the sensor line. (If you've ever used a split image focus aid in a MF SLR, it's similar in principle.)

A high-precision line is one that has a longer baseline and needs a faster lens to see all of the light over the entire length. The higher precision comes from this longer baseline.

You will find the following arrangements in Canon DLRs:

All canon DSLRs have a center AF sensor that is a cross sensor. Both lines of this cross will work with f/5.6 or better (f/4 or better for the 1D series.)

The Canon 20D/400D/30D/5D/450D have an extra horizontal line in the center cross that is higher-precision with f/2.8 lenses. The center point is still a cross, but it will only detect vertical detail with the higher precision. If you point it at something that has only horizontal detail, only the f/5.6 regular precision sensor will see anything to focus on.

On the 40D/50D, the center sensor has a high-precision X instead of a +. It can use the higher-precision f/2.8 focusing for both horizontal and vertical detail.

On all cameras except the 40D/50D/1-series, all of the outer sensors are standard-precision f/5.6 one direction only sensors. The orientation of the sensor is indicated by the shape of the rectangle in the viewfinder. On the 40D/50D, the outer sensors are still standard f/5.6 sensors, but are cross (+).

On the 1D3 AF, most of the outer ring of points have high-precision segments as well as the inner ring just outside the spot circle. These segments are horizontal only. All other points are vertical only.

On 1DII and earlier AF, only the center column(s) of sensors have high-precision segments. (see below)

1DIII vs. 1D2:



40D (from whitepaper)



20D/30D/400D/450D:



5D2:



--
Erik

No text

Thanks for this great post. I've been somewhat dissatisfied with the AF on my 350D when using the peripheral AF points. There are times when I swear they have enough contrast to operate on, but I just can't get AF even after repeatedly trying for 10 seconds.

I never knew the shape of the rectangles told you what sensor type you had. Using this information, I just tested it out in a dark corner with my 24/1.4. I was able to focus on some amazingly dark subjects just by looking for detail which was perpendicular to the AF point in question. Of course, the 24/1.4 helps AF, but this allowed me to utilize it even better. I'm still getting a 5DII though.

Actually, this makes me feel better regarding that purchase as well. All the talk of its mediocre AF system was getting me down...but now I think that as long as I use the AF points intelligently, knowing the capabilities of each point, I'll be totally satified with it, even in very low light.

So thanks again, for breathing some new life into an older camera as well as increasing confidence in my purchase of a new one!

OK, any conclusions...

Which one (AF in 5D2 or 50D with lenses 2.8) is better-faster-precise in your opinion.
Thanks.

ginn said:

OK, any conclusions...

Which one (AF in 5D2 or 50D with lenses 2.8) is better-faster-precise
in your opinion.
Thanks.

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50D, slightly better, because it's got that diagonal high precision cross. 5D is high precision sensitive only to vertical contrasts. On the other hand, I'm not sure that the 45degree rotated cross of the 50D would work very well with the horizontal and vertical contrasts. It works well with the contrasts that are oriented 45degrees from vertical/horizontal.

They should put 1D2 AF in all but perhaps the lowest-end bodies and get rid of gazillion AF units for ours and theirs sake.

--
Cheers,
Martin

These are the three components one would like to optimize in a perfect AF system.

Each component has some technological counterpart:

1/ Detectivity is the ability to find something to focus on. It is helped by cross focus points (which are sensitive to horizontal and vertical lines) and a contrasty lens (which is often not the case with wide primes).

2/ Precision is the ability to find the correct distance from your subject within a given accuracy (usually given relatively to a fraction of the DoF). It is helped by sensors having a long basis (hence the "f2.8" sensors) but cannot work on lenses having a smaller max aperture than the AF sensor basis (hence an f4 lens cannot use an f2.8 sensor to focus because the sensor will get an uneven illumination).

3/ Speed comes from the amount of computational work required to detect the subject and use the AF sensor to estimate the distance. It is helped by optimized AF algorithms and faster hardware.

All this comes together to form an AF system which idealy would have excellent detectivity (ie be able to find it's way in low light/low contrast situations), be precise (long AF base) and fast (lots of computing power + smart algorithms to get within the required precision in one try).

Of course, these things are often playing against each other (like cross points being "slower" than line points because they require more work to focus, or having 51 points being slower than having 9 points) but it also goes to show that the exact same "AF sensor" can lead to completely different results when it comes to speed for instance.

I used ye ole Pentax *istDs for 3 years (11 AF points with 9 cross: not bad for a 2004 camera!) before going to the K20 briefly (same 11 AF points with 9 cross) and all I can say is those have little in common in practice: despite having the "same AF sensor", the K20 trumps the Ds on speed and tracking by a factor of 2 easily when the same lens is attached.

So there you have it:

1/ People complaining that the 5DII have the same AF system as the 5D are wrong. Same layout, different algorithm, different hardware ==> different performances.

2/People complaining that detectivity has not been improved are likely to be right though.

3/ People compaining that precision hasn't been improved might be right or wrong depending of the software/hardware tweaks introduced on the 5DII

4/ People complaining that speed and tracking has not been improved are likely to be wrong.

Canon says it is the same.
if it wasn't, then they would absolutely say so and describe the improvements

lol101 said:

1/ People complaining that the 5DII have the same AF system as the 5D
are wrong. Same layout, different algorithm, different hardware ==>
different performances.

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Camera Dog said:

Canon says it is the same.
if it wasn't, then they would absolutely say so and describe the
improvements

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They said it was the same layout and AF point definition, hence detectivity and precision might be the same, but they also have a new processor and probably different algorithms, hence speed is likely to be improved and quite possibly precision too (a sizeable part of the AF's precision comes from algorithms).

But then again, it might be 100% the same as the 5D's one, making the 5DII the world's first DSLR not to improve on its predecessor in the AF department, time will tell....

Interesting.

I went to a camera show over the weekend and asked a Canon representative two very specific questions. I also played with the 5DII for a few minutes.

1.) Is the AF on 5D different from 5DII?
Yes. Algorithm and others.

2.) Which camera has a better AF, 50D or 5DII?
Answer: 50D.

I am well aware Canon representatives may not always have the correct info, if you heard otherwise, please let us know and site the information source.

Tim

Camera Dog said:

lol101 said:

1/ People complaining that the 5DII have the same AF system as the 5D
are wrong. Same layout, different algorithm, different hardware ==>
different performances.

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Click to expand...

Alfred Juras said:

All the bitching about the 5DII autofocus (sick of it, you make me
sick).

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Fair enough... but this is what makes me sick:

Alfred Juras said:

Tired of only the Nikons having hi-ISO and low noise.

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Umm do you realise that what you said is actually much worse than "bitching about" the AF? At least it's a fact that AF hasn't really been updated... but to imply that Canon doesn't have hi-ISO and low noise is pure ignorance!?!?!

GTW
--
http://www.flickr.com/genotypewriter

I don't get it. True, sometimes you want DOF, but decent F1.4 lens lets in 8 time more light than a F4, so ISO 3200 on that F4 is ISO 26,000 on a F1.4.

Or a 40d can be shot at ISO 800 w/F1.4 when a 5d would need ISO 6400!

I guess overall you saved money - buying one good camera body is better than buying 4 fast lenses - but lenses are forever, cameras are for a year (or for the 5d, 3 years!)

lol101 said:

But then again, it might be 100% the same as the 5D's one, making the
5DII the world's first DSLR not to improve on its predecessor in the
AF department, time will tell....

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I don't think there was much change between 20d and 30d either...
--
Riku

Chuck Westfall said that 50D focus system was not used because the 5D system was deemed to be more accurate for AI Servo.
--
http://www.pbase.com/victorengel/

Alfred Juras said:

All the bitching about the 5DII autofocus (sick of it, you make me
sick).

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So people who have a different opinion than you make you sick. Are you aware of the fact that this is actually a discussion forum? People are supposed to express different opinions here. If you can't cope with that I suggest that you stop visiting.

--
Riku

You talk like its so bad its unusable. Coming from the 10-40D series and having used a 5D many times, I think you are being ridiculous! True, and upgrade would have been nice but by no means are the auto focus systems in these bodies poor.

Ive considered moving into the Nikon realm a few times and auto focus was definitely one of their main strengths but whether I had 9, 15, or a 100 focus points I would be using the center one regardless. Ask most Nikon users and most of them do exactly the same. Take that away and the EOS system does fine. You just need to know what to lock onto. The micro lens adjust is going to really help out. I can't wait to get my5DmrkII

Well, everything is relative. And the 5D AF is simply not good enough. It obviously depends on what you shoot and no, I am not even talking about sports. Even for simple portraits at shallow DOF the 5D AF is inadequate (due to the lack of accurate outer AF sensors). However, if f/11 landscapes is all you do, you will be very happy with the AF of the 5D. As for the 5D MkII, I believe I have given some good reasons why I believe that the AF will be even worse.

The only hope I have for the 5D MkII is the Live View AF mode - perhaps that will help where the normal AF fails. We will see...

Barnett

Cliffee said:

You talk like its so bad its unusable. Coming from the 10-40D series
and having used a 5D many times, I think you are being ridiculous!
True, and upgrade would have been nice but by no means are the auto
focus systems in these bodies poor.
Ive considered moving into the Nikon realm a few times and auto focus
was definitely one of their main strengths but whether I had 9, 15,
or a 100 focus points I would be using the center one regardless. Ask
most Nikon users and most of them do exactly the same. Take that away
and the EOS system does fine. You just need to know what to lock
onto. The micro lens adjust is going to really help out. I can't wait
to get my5DmrkII

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