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11-12-2022, 10:43 AM   #1
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AF on an approaching subject

A very popular AF test in reviews is to test continuous AF on a subject that's approaching the camera (e.g. a car/child/dog). In pretty much every review I have seen (not necessarily of Pentax cameras) the camera would produce a decent percentage of sharp images. The reviewer would usually be upset that some images are out of focus. Well, whenever I attempt to replicate this with children approaching me, and camera in AF-C mode, and central AF sensor on the subject, I always reliably get exactly 0% of usable images. I never had a single usable image of anything approaching me produced with a KP. I do not see how my technique (whatever it might be) might contribute to this. This seems to be a 100% autofocus function. What has your experience been? Any suggestion what I might be doing wrong?


Last edited by Pentaxxoid; 11-12-2022 at 10:54 AM.
11-12-2022, 12:03 PM   #2
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QuoteOriginally posted by Pentaxxoid Quote
A very popular AF test in reviews is to test continuous AF on a subject that's approaching the camera (e.g. a car/child/dog). In pretty much every review I have seen (not necessarily of Pentax cameras) the camera would produce a decent percentage of sharp images. The reviewer would usually be upset that some images are out of focus. Well, whenever I attempt to replicate this with children approaching me, and camera in AF-C mode, and central AF sensor on the subject, I always reliably get exactly 0% of usable images. I never had a single usable image of anything approaching me produced with a KP. I do not see how my technique (whatever it might be) might contribute to this. This seems to be a 100% autofocus function. What has your experience been? Any suggestion what I might be doing wrong?
First question to ask you is what lens are you using ?



Have you experimented with the AF-C settings in the KP menu ? Focus-priority or Release-priority can make a difference, as can The AF Hold function.

If your subject is of a uniform colour, you may find the centre point AF will struggle to get a lock. Try using the Expanded Area AF (S) instead, this works well for me.

perhaps uploading some images which show the problem would help too.

I use the K-1 II and with a modern lens like the DFA 150-450 have no problem with AF-C. I would expect the K3 III to be just as good.

Last edited by pschlute; 11-12-2022 at 12:13 PM.
11-12-2022, 12:21 PM   #3
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QuoteOriginally posted by pschlute Quote
First question to ask you is what lens are you using ?
DA16-85, DA55-300, FA50 macro.

QuoteOriginally posted by pschlute Quote
Have you experimented with the AF-C settings in the KP menu ? Focus-priority or Release-priority can make a difference, as can The AF Hold function.
No, I only use Focus-priority.

QuoteOriginally posted by pschlute Quote
If your subject is of a uniform colour, you may find the centre point AF will struggle to get a lock. Try using the Expanded Area AF (S) instead, this works well for me.
I tried using that, but the result is even worse.
11-12-2022, 12:26 PM   #4
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QuoteOriginally posted by Pentaxxoid Quote
I tried using that, but the result is even worse
Presumably the camera focusses perfectly on a still subject ?

What Continuous Shooting mode are you using .... H/M/L ?

11-12-2022, 01:21 PM   #5
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QuoteOriginally posted by Pentaxxoid Quote
A very popular AF test in reviews is to test continuous AF on a subject that's approaching the camera (e.g. a car/child/dog). In pretty much every review I have seen (not necessarily of Pentax cameras) the camera would produce a decent percentage of sharp images. The reviewer would usually be upset that some images are out of focus. Well, whenever I attempt to replicate this with children approaching me, and camera in AF-C mode, and central AF sensor on the subject, I always reliably get exactly 0% of usable images. I never had a single usable image of anything approaching me produced with a KP. I do not see how my technique (whatever it might be) might contribute to this. This seems to be a 100% autofocus function. What has your experience been? Any suggestion what I might be doing wrong?
I photograph moving trains - often 50mph or so.
I have found that - to my frustration - my KP takes a second or so to ‘lock onto’ the subject - my aiming point is normally a place with lot’s of ‘detail’ - but once it has it, the photograph is sharp. I use “single point” to speed up the processing.

I used to use a DA 18-135mm lens
now I use a DA 20-40mm Ltd lens

I’m afraid that I don’t know of any secret.

Last edited by reh321; 11-12-2022 at 02:34 PM.
11-12-2022, 02:04 PM   #6
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The FA50 and DA 55-300 don't have very fast autofocus mechanisms. The 16-85 is the most likely to succeed here.
11-12-2022, 02:07 PM - 1 Like   #7
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I can barely do it either. I have a k10D and a K5II and I hardly notice any difference.
I tested it with my dog that I run up to me and found that the best results were with the old “SMC PENTAX-FA 1:3.5-5.6 28-80” . But the main problem I think is the reaction of the camera itself.
I found out when you use catz in focus setting with a manual lens the camera does not respond or just too late. You should actually experiment with behind or in front of focus and the “SR” slows the system down in my opinion as well.
Did quite a few dog-agility photos with “Sigma 70-200 HSM II” and that is quite a challenge.
My experience is if you can focus on the grass just before the dog's route that you have the most success.
https://www.agilityhans.nl/picture.php?/216/category/45 All photos taken with k10D and Sigma 70-200 HSMII.

My English is not that good so I used Google Translate.

Greetings Hans

11-12-2022, 02:09 PM   #8
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I hardly ever use AFC, however by often reading various negative statements about it I did some testing to see if I could use it when needed. I must say that I have not had any particular problems. I set AFC, focus priority, center point. K70 and 55 300. Fast burst, jpeg only. press the shutter button halfway and wait for the focus lock sound (beep). Then start the burst.

The dog ran towards me. Not very fast but more or less like a child can do. Try to get some exercise while always keeping the point on the subject. Many think that cameras can do it all by themselves, in fact we can still help them a little. we just have to know them.
Here the link of the sequence.

test by Mirko Fambrini | JuzaPhoto

Last edited by nonpigliounoshoot; 11-12-2022 at 02:16 PM.
11-12-2022, 02:46 PM   #9
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Just make sure your speed is fast enough (shooting TAv or M) for *moving targets*.
11-12-2022, 03:40 PM   #10
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My KP can keep up with a bird flying towards me using a Sigma 150-500mm, provided I can keep the single focussing point on the subject … this also assumes that focus lock has been achieved in the first place … check out the little green hexagon in the viewfinder to confirm focus lock.
If there's no green, there's no lock … and little point in pressing the shutter button and hoping
I'll admit to not ever using the "beep" myself, but can see that it might help in an environment where it wouldn't be considered intrusive.
Remember, the red square that lights up in the viewfinder just indicates which a/f sensor is in use, not that focus is achieved!
It may also be of interest that the a/f system in the KP did seem to get a noticeable upgrade with the latest (last?) firmware release. From experience, if your camera hasn't been updated I'd suggest that it was well worth while.
Good luck
11-12-2022, 04:08 PM   #11
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QuoteOriginally posted by mhoule418 Quote
Just make sure your speed is fast enough (shooting TAv or M) for *moving targets*.
This does bring up the question of motion blur or out-of-focus blur.
11-12-2022, 11:12 PM   #12
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AFC and tracking objects and being successful in this is dependant on many factors. One of these is the skill of the photographer. Few people can pick up a camera and be successful in this on their first go. Fine motor skills are required and often need practice to refine. It is interesting to have someone make a video of you while attempting to do this so that you can see how your body is moving and how you move the camera. I used to wonder why I had trouble getting sharp photos with a p&s while looking at the rear screen when it was pointed out to me that I moved my whole arm down when pressing the shutter button. An involuntary movement that I was unaware of. After this being pointed out my keeper rate skyrocketed.
11-12-2022, 11:51 PM   #13
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QuoteOriginally posted by Pentaxxoid Quote
I do not see how my technique (whatever it might be) might contribute to this. This seems to be a 100% autofocus function
The secret Pentaxian technique is called "luck factor" or using an old film technique known as "zone focusing".

First, let me tell you about the "luck factor": camera AF tracking use what's know as a servo loop. The servo loop has a bandwidth or speed or operation within which is is effective. So when your subject move at a relative speed (relative to camera+lens FL) that fit within AFC servo operation, the camera AF servo is able to tack the subject and you'll get sharp photos. Problem is, in real life, relative subject speed can vary a lot from situation to situation. Pentax KP AF servo bandwidth is narrow and not particularly fast. That's why most of the time, Pentax KP (or K1 etc..) servo isn't able to focus track subjects reliably. Sometimes, when the subject speed is right for Pentax, you'll hear the lens AF zzziiippp continuously, that's when the subject speed matches Pentax AF servo operating bandwidth, that's when the Pentax servo is able to predicts and track, but that doesn't happen very often.

Second, let me tell you about "zone focusing": with zone focusing, you pre-focus the lens manually at a distance where you can predict the subject will be as it's moving (e.g towards you), you disable camera AF, set burst mode L,M,H depending on buffer depth and subject speed. When the subject approaches you, you press the shutter button, fires until the buffer is full, and you'll get a couple of frame in focus. Alternatively, you can zone focus (pre-focus the lens) in front of where the subject will be, enable AF.C with Release Priority (and not Focus Priority), fire a continuous burst, you'll get frames in focus when the AF move and subject move counter direction will cross. That's the secret Pentax technique.

Good luck!

---------- Post added 13-11-22 at 08:16 ----------

QuoteOriginally posted by pschlute Quote
I use the K-1 II and with a modern lens like the DFA 150-450 have no problem with AF-C.
AF speed is not absolute, it's relative. An airplane flying at 300 mph, 5 miles away, or a race car running at 100 mph at 500 meters, are much easier to track than a human running at 8 mph at 8 meters, due to depth-of-field / subject-depth and lens focusing travel near to far gear ratio not being constant.
11-14-2022, 07:37 PM   #14
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Make sure "Hold AF Status" page 59 in the manual is turned off. It allows the camera to hold AF if something momentarily gets between you and your focus point. It will wreak havoc with tracking a moving object.
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