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04-26-2014, 11:57 AM - 1 Like   #1
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K3 autofocus accuracy

I am curious to know if anyone has been able to compare a K3 with a K30 or similar type Pentax DSLR when shooting with a f1.8 or f1.4 aperture (or even similar cameras from canikon used at large apertures). Does the K3 show significant improvement in accuracy with its new f2.8 optimized af sensors? If so please do tell or share any links confirming improvement in accuracy (not necessarily speed of focus). Thanks

04-26-2014, 12:07 PM   #2
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mm I have a K-5IIs and the center point (that is f/2.8 sensitive) is deadly accurate. So in the K-3 with 2 more of this points it should be as good or better...
04-26-2014, 01:21 PM   #3
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I didn't realize the K-5IIs had an f2.8 center point. Good to know! What fast lenses have you used on your K-5IIs?
04-26-2014, 01:26 PM   #4
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The main advantage of the K-3 and K-5 II in terms of accuracy is in low light. They are also slightly faster. On paper the center AF points can take advantage of F2.8, but IMO in practice the difference isn't too significant as the AF in the K-30 is pretty good already.


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04-26-2014, 02:46 PM   #5
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The benefit of the f/2.8 AF sensors is not so much a matter of accuracy as it is precision. Without them, the camera will do as good a job finding the same focus point twice in a row with a f/2.8 lens as with a f/5.6 lens. Precision takes precedence over accuracy since you can't determine the latter if the former is not present.


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04-26-2014, 03:44 PM   #6
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QuoteOriginally posted by stevebrot Quote
The benefit of the f/2.8 AF sensors is not so much a matter of accuracy as it is precision. Without them, the camera will do as good a job finding the same focus point twice in a row with a f/2.8 lens as with a f/5.6 lens. Precision takes precedence over accuracy since you can't determine the latter if the former is not present.


Steve
Perhaps you could define your terms, since in everyday use they are interchangeable.
04-26-2014, 05:39 PM   #7
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QuoteOriginally posted by DSims Quote
Perhaps you could define your terms, since in everyday use they are interchangeable.
The way it was described in my textbooks:
Precision = consistency, repeatability of results, low variance
Accuracy = actually being correct

Which seems consistent with stevebrot's meaning.
04-26-2014, 07:46 PM   #8
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QuoteOriginally posted by Scintilla Quote
Which seems consistent with stevebrot's meaning.
Which is consistent with my intent. The two terms are often misused, but they both have very specific meanings. Focus acquisition (green hexagon) at f/1.4 with one of the f/5.6 sensors is less likely to be accurate (correct) than at f/1.4 with one of the f/2.8 sensors.


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Last edited by stevebrot; 04-26-2014 at 08:40 PM.
04-26-2014, 08:50 PM   #9
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QuoteOriginally posted by stevebrot Quote
Which is consistent with my intent. The two terms are often misused, but they both have very specific meanings. Focus acquisition (green hexagon) at f/1.4 with one of the f/5.6 sensors is less likely to be accurate (correct) than at f/1.4 with one of the f/2.8 sensors.
Thanks for the clarification.
04-28-2014, 03:36 AM   #10
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QuoteOriginally posted by stevebrot Quote
Which is consistent with my intent. The two terms are often misused, but they both have very specific meanings. Focus acquisition (green hexagon) at f/1.4 with one of the f/5.6 sensors is less likely to be accurate (correct) than at f/1.4 with one of the f/2.8 sensors.


Steve
My favourite (mis)-use of these two words is when the military talks about precision-guided weapons. Great, but how accurate are they?
04-28-2014, 07:12 AM   #11
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can one be accurate with 900 pounds of high explosive?
04-29-2014, 10:36 PM   #12
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QuoteOriginally posted by Scintilla Quote
The way it was described in my textbooks:
Precision = consistency, repeatability of results, low variance
Accuracy = actually being correct

Which seems consistent with stevebrot's meaning.
After buying K-3, I was not able to get as many sharp photos as I was with K-5 II. So I did some testing and found K-3 to be a lot worser than K-5 II regarding precision on screw driven AF lenses with long focal lengths. (That is 200mm +++). With SDM lenses the precision is very good on K-3, but unfortunately older lenses sucks big time.

I am not sure at this time if this is a problem with only my own K-3 or if it is a common problem. I will do some more testing with another K-3, but it would also be interesting to hear from others who have seens similar behaviour.
04-30-2014, 06:16 AM   #13
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I have not had time to look into this but it does seem to me that I get sharper pictures when I use the SMC TC 1.7 on the K-01 than when it is on the K3. this is based on very few pictures so I could be completely wrong about it. I had intended to check some more this weekend. This post makes me think there may be some truth to the idea. However, others seem to have had good results with K-3 and screw focus.
05-09-2014, 06:57 AM   #14
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I have written a little about the focusing system of the K-3 against the K-5 mk1 in my review of the K-3, mostly concerning portrait photography
05-09-2014, 07:32 AM   #15
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The K-3 has smaller focus points than previous 11-point Pentax DSLR's and I find the difference easily noticeable. I can pick out the bird from the branches with pinpoint accuracy on the K-3. I expect the same would be true if I were shooting portraits and wanted focus on the eye instead of the brow.
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