Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
10-15-2012, 01:15 PM   #16
Loyal Site Supporter
bwDraco's Avatar

Join Date: Jul 2010
Location: New York
Photos: Albums
Posts: 1,015
Original Poster
QuoteOriginally posted by soalle Quote
A curiosity... do you know if the k30 has any f/2.8 precision point?
Not that I'm aware.

--DragonLord

10-15-2012, 01:56 PM   #17
Veteran Member
froeschle's Avatar

Join Date: Sep 2007
Location: Germany
Photos: Gallery
Posts: 552
QuoteOriginally posted by Class A Quote
One needs to bear in mind, though, that the f/2.8 sensor should still work for f/4 lenses. There aren't that many lenses that are slower than this.

Also, given the relatively large size of the sensor areas, in practice they should be able to pick up some structure with contrast. Artificial patterns, as used on some focus charts, could be problematic but most real world targets should not.

Anyhow, a lot of speculation (as you also acknowledge) based on a marketing diagram which may or may not reflect the actual design. The AF areas are a lot larger than they are marked in the marketing illustration, for example.
Hopefully, the central f2.8 sensor will still work (at least to some extent) with f4 lenses. However, lenses with f5.6 (18-55, 18-135, etc.) could probably perform worse. At least, I am not so convinced anymore that the AF of the K-5 II will perform better than that of the K-5. The opposite might even be true.
10-15-2012, 02:41 PM   #18
Loyal Site Supporter




Join Date: Apr 2010
Location: Tennessee
Posts: 5,795
I would hope that Pentax chooses to overlay the F/2.8 sensor over the F/5.6 so that the center AF point would work with both F/2.8 and F/5.6 lenses. It is not an either or situation as Canon and Nikon have shown.

If Pentax does it right you wont lose any functionality of the center AF point with F/5.6 glass, but you gain AF accuracy if you are using faster glass.
10-15-2012, 06:57 PM   #19
Pentaxian




Join Date: Jun 2009
Location: GMT +10
Photos: Albums
Posts: 10,834
QuoteOriginally posted by Winder Quote
you wont lose any functionality of the center AF point with F/5.6 glass, but you gain AF accuracy if you are using faster glass.
I'm pretty sure (fingers crossed!) that is the whole idea.

10-16-2012, 02:37 AM   #20
Veteran Member
froeschle's Avatar

Join Date: Sep 2007
Location: Germany
Photos: Gallery
Posts: 552
QuoteQuote:
I would hope that Pentax chooses to overlay the F/2.8 sensor over the F/5.6 so that the center AF point would work with both F/2.8 and F/5.6 lenses. It is not an either or situation as Canon and Nikon have shown.
What you suggest would be a dual cross-type AF point, which probably would be the best solution.
In the case of the K-5 II it seems that at the center there is only one cross-type AF point.
Such AF points have been used before e.g. by Canon.
However, they then additionally improved center focus accuracy.
QuoteQuote:
The central AF point strikes a compromise, working as a full cross-type sensor at f/4, and as a horizontal-only sensor down to f/8. Canon added one more interesting twist with the central array, though: While it doesn't cover as wide an area as the "f/2.8" arrays, it does have twice as many sensor elements per unit of length. This gives it accuracy equivalent to the f/2.8 arrays, but at a smaller aperture of f/4.
See e.g.
Canon EOS-1D Mark III

Canon White Paper
What are the aperture limits for autofocus on Canon DSLR cameras?
QuoteQuote:
  • A horizontal-sensitive point or vertical-sensitive point is an autofocus point that can detect horizontal or vertical lines, respectively. These points cannot detect both at the same time.
  • A high-precision sensor is an autofocus sensor that is capable of focusing within 1/3 of the depth of field of the lens, instead of simply within the depth of field. These sensors require faster maximum apertures - typically at least f/2.8 - in order for them to work.
  • A cross-type point is an autofocus point that can detect both horizontal and vertical lines simultaneously. Depending on the camera model, the maximum aperture may need to be at least f/2.8 or f/4 for a point to be cross-type, because the vertical-sensitive part of the AF point is a high-precision sensor.
  • A dual cross-type point is an autofocus point that can detect diagonal lines as well as horizontal and vertical lines. The diagonal sensors require at least f/2.8 in order to function because they are high-precision sensors.
10-16-2012, 04:27 AM   #21
Pentaxian
Class A's Avatar

Join Date: Aug 2008
Location: Wellington, New Zealand
Posts: 9,193
QuoteOriginally posted by froeschle Quote
Such AF points have been used before e.g. by Canon.
Thanks for all the info.

The one passage you quote
While it doesn't cover as wide an area as the "f/2.8" arrays, it does have twice as many sensor elements per unit of length. This gives it accuracy equivalent to the f/2.8 arrays, but at a smaller aperture of f/4
does not make sense (EDIT: When interpreting "twice as many sensor elements" as "twice as many AF areas"; reading it as "twice as many AF area sensels" makes sense indeed).

I believe Image Resource misunderstood something. I'd be surprised to find this claim anywhere else. Accuracy can only be improved by larger baselines, not by having smaller AF areas. Perhaps they meant "targeting precision" rather than "AF precision".

Last edited by Class A; 10-17-2012 at 03:38 AM.
10-16-2012, 12:03 PM   #22
Loyal Site Supporter
bwDraco's Avatar

Join Date: Jul 2010
Location: New York
Photos: Albums
Posts: 1,015
Original Poster
QuoteOriginally posted by froeschle Quote
What you suggest would be a dual cross-type AF point, which probably would be the best solution.
In the case of the K-5 II it seems that at the center there is only one cross-type AF point.
Such AF points have been used before e.g. by Canon.
However, they then additionally improved center focus accuracy.


See e.g.
Canon EOS-1D Mark III

Canon White Paper
What are the aperture limits for autofocus on Canon DSLR cameras?
If you're wondering, I was the one who wrote this answer on Stack Exchange Photography!

I just updated the answer to add details on the Canon EOS 6D. Canon says that the center point is high-precision at f/2.8, but does retain cross-type functionality to f/5.6.

--DragonLord

Last edited by bwDraco; 10-16-2012 at 12:11 PM.
10-16-2012, 08:47 PM   #23
Veteran Member




Join Date: Jan 2007
Location: Northern California
Posts: 475
QuoteOriginally posted by Class A Quote
Thanks for all the info.

The one passage you quote
While it doesn't cover as wide an area as the "f/2.8" arrays, it does have twice as many sensor elements per unit of length. This gives it accuracy equivalent to the f/2.8 arrays, but at a smaller aperture of f/4
does not make sense.

I believe Image Resource misunderstood something. I'd be surprised to find this claim anywhere else. Accuracy can only be improved by larger baselines, not by having smaller AF areas. Perhaps they meant "targeting precision" rather than "AF precision".
We had this discussion already, but as I recall, you got wrapped around flying teapots and clearly did not understand.

The AF sensors are just pixel arrays. The baseline between the pixel array pairs is just one part of the equation. The other part is the pixel pitch of the particular sensor array pairs. An array with more pixels in the same unit length has more resolution than one with fewer pixels and therefore can make a more accurate determination of the phase difference, even if the baseline is smaller.

Canon is saying that you can increase the pixel resolution 2x, or increase the baseline 2x and the AF calculation will be equally as accurate.

The higher resolution arrays will have smaller pixels, or course, so they probably have a disadvantage in light sensitivity, and are also working with less light to begin with given the smaller aperture, but that is a different problem.

Ray

10-17-2012, 03:31 AM   #24
Pentaxian
Class A's Avatar

Join Date: Aug 2008
Location: Wellington, New Zealand
Posts: 9,193
QuoteOriginally posted by Ray Pulley Quote
We had this discussion already, but as I recall, you got wrapped around flying teapots and clearly did not understand.
You recalled that incorrectly.
  1. It wasn't "this" discussion. It was a discussion about your claim that longer baselines would prolong AF calculations.
  2. I did not get wrapped around flying teapots, but used Russel's famous example to illustrate that the argumentum ad ignorantiam technique you used is flawed.
  3. I understood the discussion.

QuoteOriginally posted by Ray Pulley Quote
The other part is the pixel pitch of the particular sensor array pairs.
I did not interpret "twice as many sensor elements" this way (I interpreted it as meaning "twice as many AF areas"), but your interpretation makes more sense and you are probably right.

QuoteOriginally posted by Ray Pulley Quote
An array with more pixels in the same unit length has more resolution than one with fewer pixels and therefore can make a more accurate determination of the phase difference, even if the baseline is smaller.
This is only true if the AF area sensels are too big to resolve a phase difference that could be detected by twice the number of AF area sensels.

I'm not sure whether the above case actually occurs because the AF area sensels see an out of focus (i.e., blurred image). If the phase difference has become too small to allow further AF adjustments then there is not point in knowing this with twice the resolution.

I do not know whether doubling the accuracy of the AF system is as easy as doubling the number of AF area sensels. My gut feeling says "no" but I cannot further support this gut feeling beyond what I wrote above.
10-18-2012, 05:32 PM   #25
Veteran Member




Join Date: Jan 2007
Location: Northern California
Posts: 475
QuoteQuote:
I did not interpret "twice as many sensor elements" this way (I interpreted it as meaning "twice as many AF areas"), but your interpretation makes more sense and you are probably right.


This is only true if the AF area sensels are too big to resolve a phase difference that could be detected by twice the number of AF area sensels.

I'm not sure whether the above case actually occurs because the AF area sensels see an out of focus (i.e., blurred image). If the phase difference has become too small to allow further AF adjustments then there is not point in knowing this with twice the resolution.

I do not know whether doubling the accuracy of the AF system is as easy as doubling the number of AF area sensels. My gut feeling says "no" but I cannot further support this gut feeling beyond what I wrote above.
I would also suspect that the equation might not be perfectly linear, or true in all circumstances. For example:

If you double the accuracy/resolution of the AF array making the reading but do not have enough resolution in the lens AF motor/gear train/motor encoder to make a small enough adjustment to hit the more accurately calculated "perfect spot", then your accuracy might not change at all.

Most such claims or tests/specifications are made under conditions that are not always clearly spelled out, but that are most likely optimal, like using one specific lens in daylight at a certain light level.

I do not understand what you are trying to say with the blurry comment? The whole process is maing the image sharper and sharper as you move towards being in-phase.

Ray

Last edited by Ray Pulley; 10-18-2012 at 07:15 PM.
10-18-2012, 07:14 PM   #26
Veteran Member




Join Date: Jan 2007
Location: Northern California
Posts: 475
BTW

QuoteOriginally posted by Class A Quote
You recalled that incorrectly.
I understood the discussion.
On both points, I beg to differ.

My comment from the earlier discussion you "participated" in:

"The reason seems to be that the f2.8 sensor areas have more resolution or more pixels in the same area (which also means they are smaller). This makes sense as the baseline is not the only thing that affects AF accuracy, the resolution of the pixels measuring the split light beams is also part of the equation. If you only increased the baseline for a more accurate calculation, you might not gain the full accuracy advantage if your array resolution stayed the same. After all, the camera looks at the two split rays on a the pixels of the arrays and moves the lens to hit the right spot. The resolution of these arrays determines how accurately you can judge the difference between the split rays and which direction to move and also how far to move."

So, we have had this discussion before, and you clearly did not understand.

Ray
10-18-2012, 08:11 PM   #27
Pentaxian
Class A's Avatar

Join Date: Aug 2008
Location: Wellington, New Zealand
Posts: 9,193
QuoteOriginally posted by Ray Pulley Quote
On both points, I beg to differ.

My comment from the earlier discussion you "participated" in:

"The reason seems to be that the f2.8 sensor areas have more resolution or more pixels in the same area (which also means they are smaller). This makes sense as the baseline is not the only thing that affects AF accuracy, the resolution of the pixels measuring the split light beams is also part of the equation. If you only increased the baseline for a more accurate calculation, you might not gain the full accuracy advantage if your array resolution stayed the same. After all, the camera looks at the two split rays on a the pixels of the arrays and moves the lens to hit the right spot. The resolution of these arrays determines how accurately you can judge the difference between the split rays and which direction to move and also how far to move."

So, we have had this discussion before, and you clearly did not understand.
In general, I'm happy to have discussions but I lose interest when the other party has other goals than improving their own understanding of matters. I could point out multiple aspects in which I disagree with your latest statement but I find all that too tedious. Let's terminate this discussion.
10-18-2012, 11:21 PM   #28
Veteran Member




Join Date: Dec 2010
Location: Manila
Photos: Gallery | Albums
Posts: 2,190
I hope some Pentax reps are reading this so they know how much frustration their being silent about their products and features have on the technical side, breaking heads and the sanity of their customers...
10-19-2012, 05:22 AM   #29
Veteran Member




Join Date: Aug 2011
Location: Niagara
Photos: Gallery | Albums
Posts: 792
QuoteOriginally posted by Alizarine Quote
I hope some Pentax reps are reading this so they know how much frustration their being silent about their products and features have on the technical side, breaking heads and the sanity of their customers...
Amen to that, let's get some user feedback!
10-19-2012, 07:14 PM   #30
Loyal Site Supporter
bwDraco's Avatar

Join Date: Jul 2010
Location: New York
Photos: Albums
Posts: 1,015
Original Poster
The SAFOX X system does includes the diffractive optical element from the SAFOX IXi+ system. From K-5II / K-5IIs?PENTAX RICOH IMAGING, via Google Translate (emphasis added):

QuoteQuote:
Newly developed (the difference between Fox Ten) AF module new generation, SAFOX X. New AF sensor with high sensitivity strong on the darkness. In addition, the diffractive lens optical system is adopted. Effectively canceling the chromatic aberration in the module, which significantly improve the detection accuracy. Also use sensors to detect light, and is suppressing the defocusing especially under artificial light sources.
With regard to the f/2.8 high-precision sensor, I believe that the new sensor is in addition to the existing standard-precision sensor, rather than replacing it outright. Consider the AF sensor design for the Canon EOS 40D, 50D, 60D, and Rebel T4i:

Link to image

... and for the EOS-1D Mark IV:

Link to image

... and for the EOS-1D X:

Link to image

From these images, and from this one, I would guess that the K-5 and K-5 II have AF sensor layouts like this (I'm probably not exactly right):



As such, the addition of the high-precision sensor should not require the removal of the the existing standard-precision sensor.

--DragonLord

Last edited by bwDraco; 10-19-2012 at 08:51 PM.
Reply

Bookmarks
  • Submit Thread to Facebook Facebook
  • Submit Thread to Twitter Twitter
  • Submit Thread to Digg Digg
Tags - Make this thread easier to find by adding keywords to it!
af, camera, dslr, f/2.8, high-precision, ii, k-5, photography, sensor, sensors
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
For Sale - Sold: K-5 with precision matte focusing screen and 18-55 WR montman Sold Items 7 03-14-2012 09:43 AM
Kino Precision - Kiron 28mm f/2.8 Iocus Pentax SLR Lens Discussion 7 09-25-2011 05:30 PM
For Sale - Sold: Kiron (Kino Precision) 28mm/2.0 (US) systemA Sold Items 3 12-22-2010 06:47 PM
High School Senior Pictures Question lovemypentax Photographic Technique 3 07-30-2009 04:42 PM
K20D High Speed Sync Question thomps6s Pentax DSLR Discussion 15 12-03-2008 09:48 PM



All times are GMT -7. The time now is 02:31 AM. | See also: NikonForums.com, CanonForums.com part of our network of photo forums!
  • Red (Default)
  • Green
  • Gray
  • Dark
  • Dark Yellow
  • Dark Blue
  • Old Red
  • Old Green
  • Old Gray
  • Dial-Up Style
Hello! It's great to see you back on the forum! Have you considered joining the community?
register
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]
Top