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04-10-2016, 08:03 PM   #1
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Select AF point(s) vs. single (spot) point ... any difference in accuracy?

I have been using my K3 (new - recent) with the single (spot) AF point .
So, I am wondering if the "select" point (single) makes any difference in accuracy.
My gear is the K3 with the DA*300/4.
Doing mostly birding.

Any comments/suggestions/ advice welcomed!

JP

04-10-2016, 08:07 PM   #2
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If I understand your question correctly.
Accuracy should be no different, but in multipoint mode the camera may choose the wrong subject.
04-10-2016, 08:30 PM   #3
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Also I believe certain AF Points (IIRC the 3 in the center: The main one in the middle, the one above it and the one below it) are more sensitive than the rest. For example in low light situations the center 3 will be more sensitive and therefore better at locking focus.

I found this on the Ricoh Japan site that gives a clearer explanation:



High-precision AF with F2.8 luminance flux linear sensors

The center sensor and two sensors just above and below it are designed to detect the luminance flux of an F2.8 lens for high-precision autofocusing. When using a very fast lens with a shallow depth of field, these sensors greatly improve the focusing accuracy.

Dependable AF at super-low minimum luminance of –3 EV

The K-3’s high-sensitivity AF sensor assures dependable autofocus operation down to a minimum luminance of –3 EV (when using 25 middle sensors), preventing the AF mechanism from slowing down or being confused by high-contrast areas in back-lit situations. It also captures low-contrast subjects with great accuracy.

Last edited by a5m; 04-10-2016 at 08:44 PM.
04-10-2016, 08:52 PM   #4
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Per the illustration above, the center point and the points directly above and below it may be more sensitive with certain lenses, but I think in practice the other cross-type points are already pretty accurate.


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04-10-2016, 09:47 PM   #5
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QuoteOriginally posted by jpzk Quote
I have been using my K3 (new - recent) with the single (spot) AF point .
So, I am wondering if the "select" point (single) makes any difference in accuracy.
As noted above, the three points in the center are tuned to provide higher precision when paired with lenses with f/2.8 and wider maximum aperture (focus sensitivity* of f/2.8). The other 24 focus points are tuned to a focus sensitivity of f/5.6, the de facto industry standard for PDAF. As I understand the Pentax literature, any of the 25 cross-point AF points will work well with your DA 300/4. Whether the f/2.8 points will offer any advantage is not clear. In general, f/2.8 points are a hybrid component having both f/2.8 and f/5.6 sensors. The f/2.8 "blacks out" somewhere between f/2.8 and f/5.6 with its task being taken up by its f/5.6 partner when used with slower lenses.

* Focus sensitivity refers to the ability to detect an out-of-focus condition. For an f/5.6 sensor that means that the point's ability to detect OOF (equivalent to precision) is never better than f/5.6 maximum with characteristic DOF for that aperture. With a fast lens, the lack of precision results in a higher incidence of missed focus regardless of calibration accuracy. With slower lenses the relative low precision is covered to a great extent by DOF. these limits to focus sensitivity are determined by aperture and are true regardless of the amount of light present or the minimum light required for the system to work.


Steve

Last edited by stevebrot; 04-10-2016 at 09:54 PM.
04-10-2016, 11:14 PM   #6
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QuoteOriginally posted by stevebrot Quote
With slower lenses the relative low precision is covered to a great extent by DOF. these limits to focus sensitivity are determined by aperture and are true regardless of the amount of light present or the minimum light required for the system to work.
Ok, so, practically, that means, stopping down the lens lessen the effect of focus error.

QuoteOriginally posted by bertwert Quote
Accuracy should be no different, but in multipoint mode the camera may choose the wrong subject.
That's true for AFC auto, but that's not the case for AF select when the user selects the primary point of focus that is used by the camera to acquire focus.

QuoteOriginally posted by jpzk Quote
Select AF point(s) vs. single (spot) point ... any difference in accuracy?
Are you using AFA, AFS or AFC? If you are using AFS, the answers are above.
If you are using AFC, beyond the limits of accuracy you'd get in AFS mode, I think what makes the most difference on focus errors is if the camera AF servo has acquired target and how long it has been tracking the target before the shot. AFC acquire focus with a loose accuracy and once the target is in range , it tries to reduce the focus error by prediction of where the target is going to be. There are two reasons for miss focused shots with the K-3: 1) the user starts firing shutter before the AF servo enters tracking mode ; 2) the target is moving too fast in the Z direction relative to the speed of actuation.
04-11-2016, 12:41 AM - 1 Like   #7
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QuoteOriginally posted by biz-engineer Quote
Ok, so, practically, that means, stopping down the lens lessen the effect of focus error.
Not what I said, but close enough. The statement applied to focusing wide open. Even at f/4 for the DA 300, DOF will usually cover for missed focus from an f/5.6 sensor.* Note that f/5.6 sensors are the industry standard and that f/2.8 are the exception and found only on higher end models. The relatively poor precision has been the bane of PDAF systems since pretty much forever. Were it not for focus speed and the ability to predict direction of focus, PDAF would probably not have become the standard it is today.


Steve

* As always, regardless of DOF there is only one plane of focus and missed focus is always evident if you have sufficient magnification.

Last edited by stevebrot; 04-11-2016 at 12:50 AM.
04-11-2016, 04:54 AM - 2 Likes   #8
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QuoteOriginally posted by stevebrot Quote
and missed focus is always evident if you have sufficient magnification.
Definition of "acceptable sharpness" = "How many angels can dance on the head of a pin?"

04-11-2016, 05:22 AM   #9
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is this post applicable here? i came across it the other day and bookmarked it for more intensive review when i had more time to devote to it. its not the way i shoot--i also use single point center--but a cursory read made me think perhaps that method does not take full advantage of what these cameras have to offer.

https://www.pentaxforums.com/forums/38-photographic-technique/236970-perfect-...everytime.html
04-11-2016, 07:05 AM   #10
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QuoteOriginally posted by rbelyell Quote
is this post applicable here? i came across it the other day and bookmarked it for more intensive review when i had more time to devote to it. its not the way i shoot--i also use single point center--but a cursory read made me think perhaps that method does not take full advantage of what these cameras have to offer.

https://www.pentaxforums.com/forums/38-photographic-technique/236970-perfect-...everytime.html
Probably a little off topic, but good thoughts all the same. There are very few, if any, features on our cameras that are not useful at some point and worth knowing how to use.

BTW...Behavior of your Pentax camera may be is different than in the linked article. Some of the described features are "Easter Eggs" (undocumented behavior) and have not been carried over in new models.


Steve

Last edited by stevebrot; 04-11-2016 at 07:26 AM.
04-11-2016, 09:32 AM   #11
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Thanks for all this information, folks !

So, if I just always use spot AF (single point) and usually between f4 -5.6, I should be OK ?
The way I "interpret" the information, there wouldn't be much difference between that and using a "selected" (SEL) point?

Cheers!
04-11-2016, 02:02 PM - 1 Like   #12
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I only use the center, then recompose. It is fast and easy......am I missing something?

Regards!
04-12-2016, 04:11 AM - 2 Likes   #13
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If you are not using Expanded Area AF and AF Hold in AF-C mode, you are completely subverting the camera's ability to track. The K-3 and K-3 II have sophisticated tracking algorithms based on an "86,000-pixel RGB light-metering sensor for extra-accurate detection of the subject’s shape and color". Without these features you may as well be using a K-5 or K-7.

Here are my current recommendations for BIF with a K-3:
TAv mode:
- Set aperture for sharpest MTF - f/5.6 with the DA*300, f/8 if you're also using the TC. Shoot at f/8 with 55-300mm, do not use a TC. If you're shooting in dim conditions, the DA*300 is acceptable at f4. The 55-300 is unacceptably soft wide open.
- Shutter speed according to conditions. I find 1/1000s works most of the time. Never go below 1/500s. Don't be afraid to push ISO. Noise is correctable, motion blur is not
AF-C, Hi-Speed continuous burst:
Center AF point, with 25 point expansion
Center-weighted metering, add 0.5EV compensation, fine tune in p-p
Shake reduction off

Custom Menu Parameter Settings:
16. 1st frame action in AF-C - Focus priority (make sure focus is on the target before shooting, or you'll have a string of misses)
17. Action in AF-C Continuous - Focus Priority
18. Hold AF status - Medium, bump up to High if required. IME, AF Low allows focus to drift to the background too readily.

Read more at: https://www.pentaxforums.com/forums/38-photographic-technique/317498-birds-fl...#ixzz45bs8GE68

Last edited by audiobomber; 04-14-2016 at 06:40 PM.
04-12-2016, 04:20 AM   #14
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I often wonder with longer lenses if a little bit of field curvature is an issue on bird shots when one uses the centre point to focus on an eye and then reframes significantly away from the eye to take the shot..... especially near wide open apertures at closer shooting distances.
04-12-2016, 05:39 AM   #15
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QuoteOriginally posted by audiobomber Quote
If you are not using Expanded Area AF and AF Hold in AF-C mode, you are completely subverting the camera's ability to track.
I see now....you guys are tracking moving objects. I never track moving objects, so recomposing is idea for me but not for you. Makes perfect sense, if your tracking is accurate and fast.

Regards!
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