Headline - DSLRs have imprecise focusing due to their mirrors

Started Apr 5, 2014 | Discussions thread
Lee Jay Forum Pro • Posts: 52,235
Re: Your faulty logic
1

peevee1 wrote:

Lee Jay wrote:

peevee1 wrote:

Lee Jay wrote:

peevee1 wrote:

Lee Jay wrote:

Greg A A wrote:

Lab D wrote:

Lee Jay wrote:

conceptually, the optical paths cab be different lengths leading to consistently missed focus, and single-shot focus can be open loop.

In practice, I use closed-loop AI-servo and all of my cameras and lenses are calibrated right out of the box meaning I get 99% or so in-focus shots in the most difficult conditions. My contrast-detection cameras can't come close to matching that even with much more depth-of-field and much easier conditions.
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Lee Jay

http://www.lensrentals.com/blog/2012/08/autofocus-reality-part-3b-canon-cameras

1. Phase-detection autofocus (even using still targets and center-point only) wasn’t nearly as accurate as contrast detection.

2. The contrast-detection autofocus was about as accurate as the most careful manual focusing.

There have been many tests by unbiased sources showing Contrast Detect is usually more accurate.

For those who missed it here is a test that contradicts the test results you posted.

"The contrast detect results on the 5Dmk2 and 7D show a big focus error with a reasonable amount of variability between the shots too. This just translates to out of focus shots – plain and simple" - from:http://www.reikan.co.uk/focalweb/index.php/2012/12/af-consistency-comparison-nikon-canon-phase-detect-contrast-detect/

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f8 and be there

So 2 PDAF cameras do not perform well in Live view/CDAF mode? 2 cameras designed touse PDAF and have CDAF as an afterthought. Do you see your faulty logic? The other 2 cameras designed to use PDAF actually performed well in CDAF mode (although all 4 of them are very slow and do not have the best logic).

Sorry, but your link does NOT contradict the test results. It only show that when camera makers add CDAF as an afterthought, it may not work very well. The CDAF designed cameras are still as accurate as you can get.

On large, contrasty targets. On low contrast targets where precision is required, they often fail.

Exactly like PDAF, and for the same reason.

In my experience, dedicated PDAF is at least one, if not two, orders of magnitude better on such targets.

Not my experience. Why would that be? The PDAF sensors in DSLRs lose light to OVF, unlike the main sensor in mirrorless cameras.

Because phase is a dramatically more sensitive approach than contrast. You need almost no scene contrast to create a phase difference measurable by dedicated phase sensors.

Do you have proof of that?

Proof?  It's that way on every camera I've tried, and that's how it should be physically.  Why did film cameras have split-prism focusing screens?  Because detecting contrast - even with your eyes - is pretty hit-and-miss.  It's hard to tell if you're exactly, precisely, on the dot because a little either way changes the contrast almost not at all.  In other words, it's a pretty flat top of the curve.  On the other hand, with phase, you can easily see a break in a line across the split.

In my book, photons measured are exactly the same. But I am not talking about PDAF vs CDAF, all mirrorless systems have OSPDAF now - I am talking about on-sensor focusing, which is in principle always properly aligned, with light-loosing and ALWAYS improperly aligned off-sensor focusing employed by the old film-era tech (only the amount of misalignment varies, even shot to shot).

On-sensor PDAF is certainly an improvement over CDAF, but there's still a catch.  The micro lenses don't have anywhere near the resolving/splitting power of the optics on dedicated AF sensors because they are so small.  That, and the main imaging sensor isn't optimized for AF like the dedicated AF sensors are.  So, Canon is going for the brute-force approach.  Since each AF sensor on the main imaging sensor is so lousy, they're using tons and tons of them (every pixel that can see the subject, in some cases) and using all that information to try to come up with what just a few dedicated AF pixels can do at a far higher speed.

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Lee Jay

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