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auto focus mechanism
A lots of questions. bothering me for a while..

According to wikipedia. ps camera uses active autofocus that emit beam to calculate the distance. but I have you guys talked about this phase or contrast focus thing..

Do dSLR don't use active focus but phase or contrast instead? which of these 3 are more advance or accurate. And what are the pros and cons of them?.

And also the live view and the focus problem. cameras can't focus when doing live view? I don't understand the mirror flipping up and down to do the focus thing.

Http://www.dpreview.com/news/0801/08013003sonyalpha1st.asp..

Comments (8)

New_type wrote:.

According to wikipedia. ps camera uses active autofocus that emitbeam to calculate the distance. but I have you guys talked about thisphase or contrast focus thing..

Digital cameras don't use active autofocus (there may be a rare exception)..

Digicams generally use contrast detection. DSLRs generally use phase detection..

Do dSLR don't use active focus but phase or contrast instead? whichof these 3 are more advance or accurate. And what are the pros andcons of them?.

Contrast detection requires that the main sensor be "live" during autofocus. That rules it out for most DSLRs, where the sensor is behind the mirrors and shutter..

Contrast detection is generally slower than phase detection. It can be more accurate, though..

And also the live view and the focus problem. cameras can't focuswhen doing live view? I don't understand the mirror flipping up anddown to do the focus thing.

Phase detection relies on the image being reflected off of a mirror to an autofocus unit. If the mirrors are up, the autofocus unit is blind..

The new Canon Digital Rebel XSi/450D can autofocus with the mirrors up in live view, but it uses contrast detection in that case. It normally uses phase detection. The same is true of the Nikon D300 and D3..

The new Sony A300 and A350 can autofocus in live view, using phase detection, because the live view isn't using the main sensor and the mirrors are still down, reflecting the image to the autofocus unit...

Comment #1

Thanks..

As far as I know, most ps cameras shoot a orange auto assist beam in dark to help focus. isn't that infraed beam?.

And I don't understand what's phrase focus. contrast is to detect the contrast of the object. what's phrase?..

Comment #2

Http://www.dpreview.com/news/0801/08013003sonyalpha1st.asp.

According to the sony a 350, it can only do live view or optical view finder at one time?.

And also when exposing, there will not be any image in the live view and optical view finder coz the mirror is up?..

Comment #3

New_type wrote:.

Thanks.as far as I know, most ps cameras shoot a orange auto assist beam indark to help focus. isn't that infraed beam?.

Infra-red would be invisible to the eye. In any case, the beam is normally used simply to illuminate the subject to give the camera sensor some chance of detecting the details in the subject, for the contrast-detection method to work..

I seem to remember at least one camera having a combined active beam to measure the subject distance, together with contrast detection, which worked faster than contrast detection alone.Regards,Peter..

Comment #4

Correct, the Sony's can do one or the other, not both at the same time..

There is an entire mirror assembly in the view finder which shifts to send the image to either the second sensor that does the LV, or to the Optical VF. When in LV mode, a current covers the view finder too..

On the plus side is that they both get full up phase AF..

Further what would be the point in having both at once. It's not really physically possible for a human to look through the OVF and look at the rear screen at the same time. So no compromise there. And of course it's not like the others can do LV and OVF at the same time either..

While the setups that Canon, Nikon, and newer Oly's (verses the 330) may be simpler from a parts standpoint. Their performance isn't that great because of having to flap the mirror up and down for some focusing, or the slow contrast AF..

I'm not a LV fan. But the Sony's look to be one of the better systems. Still far from going EVIL...

Comment #5

And I don't understand what's phrase focus. contrast is to detect thecontrast of the object. what's phrase?.

There are explanations of phase-detection autofocus here... both clearer (to me) than the Wikipedia version:.

Http://www.nikon.co.jp/...lio/about/technology/nikon_technology/caf/index.htmhttp://photonotes.org/cgi-bin/entry.pl?id=Phasedetection.

Best wishesMike..

Comment #6

Doug Pardee wrote:.

Contrast detection is generally slower than phase detection. It canbe more accurate, though..

I don't necessarily agree about the accuracy statement. Contrast detection is limited to the main sensor resolution to do it's work. Phase AF sensors are generally have higher resolutions than the main sensor and thus can do a more accurate focus estimate..

My 1DMKII sensor has an MTF of about 1/2 at 56 lp/mm. The AF high precision spec is equivalent to about 72 lp/mm for the same MTF. A properly calibrated phase AF system with a good focus generally can perform well above this spec.Leonhttp://homepage.mac.com/leonwittwer/landscapes.htm..

Comment #7

For Phase detection try.

Http://www.sciam.com/...D=9&articleID=000E3DD0-F20E-1C73-9B81809EC588EF21.

Orhttp://www.markerink.org/WJM/HTML/af_expla.htm.

It's useful to have a basic understanding of how both contrast and phase work (especially contrast detection) then you can work around their limitations them rather than be disappointed when the results aren't what you would like!Sandy..

Comment #8

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