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09-13-2010, 11:40 AM   #1
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Predictive Focusing-what it is?

After the recent (false) rumour of K5 having 18 focus points vice Pentax's usual 11, there were comments to the effect that one only needed the center focus point and some putdown remarks for Nikon's 51 focal points. I don't consider i fully understand the issue, but here's a Nikon discussion paper on the subject of focusing that helped clear up some of my confusion:

Nikon | Technology | Predictive Focus Tracking System

If one does landscaping or portraits, there's nothing at all wrong with the number of focal points found in Pentax and Canon (usually 9 in 50D and 60D) for focusing on mostly static subjects. If one uses DOF expeditiously relative to the area that the subject(s) will be in, existing Pentax and Canon systems are more than adequate.

Nikon's breakthrough is installing 51 focus sensors in the DXXX series and trying to detect where the focused object is going to be after the lag time of pressing the shutter button and waiting for the camera to expose the picture. As the short article says, this works pretty well in their judgement for items coming towards the photographer, but there are obviously situations where such a system does not fair as well. They mention it does not work for erratic moving objects. I can guess that there are other problematic situations as well, e.g. if you tried to focus on a flock of birds, how would the camera know which bird you wanted to focus on, e.g. the one moving to the right or the one moving to the left or the one moving vertically?

I don't think Canon has anything equivalent to Nikon in their mid tier models, not sure in the upper level, but Canon has a super sensivitive focus sensor for f2.8 lenses to help focus in low light.

I would like Pentax to go with a similar approach that Canon uses with a f2.8 sensitive center sensor for those low light occasions that give our camera fits. For them to go with the Nikon approach is probably not as doable and still achieve a lower cost camera body.

Pentax changing algorithms is fine, but i think one reaches a hardware limitiation at some point. If anyone has better information about AF limitations and issues, please speak up.

09-13-2010, 06:28 PM   #2
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The powerfull thing about Nikon's autofocus system, is that you can customize how it works.

You can set the camera to 11 point if you would rather not use all 51. You can set the points so they function in groups, or to catch the nearest thin in the frame, or you can joint points into "wide" points, or you can just use the centerpoint if you like. There are setting to change how long the focus point will "hold" before if refocuses (handy to follow a bird flying behind tree branches for example). Or, you can set the hold delay to a shorter count, if you need to focus to snap fast from on object to another.

The System in the D300 will also lock onto colour, so if the system is set to use all 51 points, and you are traking a soccer player in a red jersey, the camera will try to keep the red object in focus as it passes it from one point to the next.

When people talk about the Nikon system being better, it is more then just focus speed. It is also teh ability to customize the system to your needs.

I'd like to hear comments from any of the Canon users, as I've nver used a modern Canon autofocus system.
09-14-2010, 07:46 AM   #3
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QuoteOriginally posted by philbaum Quote
If one does landscaping or portraits, there's nothing at all wrong with the number of focal points found in Pentax and Canon (usually 9 in 50D and 60D) for focusing on mostly static subjects. ...

Nikon's breakthrough is installing 51 focus sensors in the DXXX series and trying to detect where the focused object is going to be after the lag time of pressing the shutter button and waiting for the camera to expose the picture. ... They mention it does not work for erratic moving objects. I can guess that there are other problematic situations as well, e.g. if you tried to focus on a flock of birds, how would the camera know which bird you wanted to focus on, e.g. the one moving to the right or the one moving to the left or the one moving vertically?
Thanks for the link to the article. I myself have wanted to know more about predictive focusing.

But I have been learning about the new focusing system in the Sony A33 and A55 DSLTs. Because they don't have a reflex mirror, they can actually keep their "eye" on a moving subject—even an erratically moving one—and keep it in focus, while shooting 7-10 fps. And apparently it actually works pretty well. In fact the whole focusing system in the A33/A55 seems pretty exciting. Certainly has me looking....

Will
09-14-2010, 07:55 AM   #4
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QuoteOriginally posted by KungPOW Quote
The System in the D300 will also lock onto colour, so if the system is set to use all 51 points, and you are traking a soccer player in a red jersey, the camera will try to keep the red object in focus as it passes it from one point to the next.
According to Thom Hogan, the Nikon Multi-CAM3500 system also uses information from the metering system (1005-segment RGB color) to supplement the AF sensors, and can reportedly "follow" some subjects moving in and out of the AF sensors coverage area.

09-14-2010, 08:29 AM   #5
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Actually the only issue there was, is that some idiot claimed that 51 points is a fu#@ing joke and is totally useless because he (the idiot in question) is only using the center point.

Some people tried to explain that despite the fact that most people only use the center point 99% of the time, there are cases and there are types of photography where having a lot more than 11 focusing points is actually useful... USEFUL, not "MUST HAVE".... Nobody ever said that it's impossible to do photography with less than 51 points... But, as usual, it went south....

As far as predictive AF is concerned, there's another side of the story, the way how Nikon's AF works. Basically Nikon's AF system is measuring the exact distance to the subject and it send the lens there right away without any fallowing re-confirmation or re-adjusting like all current Pentax bodies do... Fundamentally that's how Nikon's AF system was done. It puts some very strict tolerances on the lens's AF mechanism, but you get the speed. And if i'm not mistaken, Canon uses the same principle.

Last edited by alexeyga; 09-14-2010 at 08:51 AM.
09-14-2010, 07:34 PM   #6
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One time, in Band Camp, I took a photo by prefocusing the lens to its hyperfocal length...
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