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07-09-2018, 05:01 AM   #1
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K1 Autofocus Behaviour : Autofocus lock

Hello all,

I just had my Pentax K1 and Sigma 35mm lens calibrated by CR.Kennedy and am still waiting for the kit to arrive. During the conversation with the service personnel today, I was advised on the following behaviour of Pentax K1 autofocus system which I could not understand completely.

"when I refocus from infinity to the near object, say about 5 meters, the autofocus lock will slightly be back focussed but the subject will still be within focus range. Similarly when I refocus from near object to far, it will slightly front focus. This is default behaviour of the Pentax K1 autofocus system"

I tried searching for any article to confirm this behaviour but could not find any. Hope our fellow pentaxians can shed some light on this.

07-09-2018, 05:13 AM   #2
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Large aperture prime lenses around 35-50 mm have more inconsistent front and back focus patterns depending on the distance (see excerpt and link below). By lock I think they mean the final focus position acquired by the camera.

The below is pasted from the lens rentals article

"When it comes to 35mm and 50mm primes, you’ll often find that making an adjustment at shorter focus distances may cause longer distances to be off"
Lens Rentals | Blog
07-09-2018, 06:29 AM   #3
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I saw your earlier post and wondered the same question as you. I think the answer lies in consideration of PDAF vs CDAF.

CDAF I understand hunts around either side of sharp focus and comes back eventually to the point where contrast was found to be highest and locks at that point. The camera sensor is used for this.

PDAF uses a different method using a dedicated AF sensor to direct the focus motor to rotate just enough to align two split images of the same scene (simplistically speaking) and stops when the two split images 'overlap'. It avoids hunting if it can to give fast autofocus (which we all keep demanding). It sounds as though it stops once it detects the overlap is tolerably close. So if coming from earlier near focus towards a now more distance subject I can see why the technician says it slightly front focusses. That is, to avoid the need to hunt, it pulls up and locks just before perfect focus to avoid overshoot. And visa versa for focussing on a near subject after previously being more distantly focussed.

I kind of make use of this focussing my M lenses when using the focus confirm indicator without knowing what the technician said. I get better results focussing upwards from being too closed focussed initially. The focus indicator probably lights just before perfect focus and by the time I react and stop moving the focus ring, I probably end up about right for focus. I've rationalised it to date on the grounds that there is more depth of field behind a lens' point of focus than in front. So if I'm a bit out coming up from a too closely focussed start point then that extra dof behind where I stop may well include my intended focus point and I'll get a keeper. Coming down from infinity means I have less in-focus latitude in front of where I stop.

If I extrapolate that to your Sigma woes, did you ever observe if it is more problematic focussing in from infinity?

Anyway, that's my lunchtime AF technical opining. Happy to defer to anyone more knowledgeable (are you out there Digitalis).

Last edited by southlander; 07-09-2018 at 06:41 AM.
07-09-2018, 11:08 AM   #4
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QuoteOriginally posted by matroxication Quote
This is default behaviour of the Pentax K1 autofocus system
Well, if that isn't an interesting statement. It will be interesting to see if anyone on this site is able to confirm.

FWIW...It is always good to do focus evaluation with equal number of trials from both directions, not so much because the Pentax system is more prone to front/back depending on direction, but because PDAF sensitivity to OOF is really not so good at times and the range of plane-of-focus that returns acceptable results can be fairly broad. This is true for ALL brands. It is easiest to demonstrate by using a manual focus lens and the focus confirmation feature. Slowly turn the focus ring and note how much further one may turn it after the green hexagon appears until it disappears. Technically, both points may be front/back focused if pixel-peeped.


Steve

07-10-2018, 07:37 PM   #5
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Thanks everyone for the information provided. Am now able to understand what the service personnel was referring to
07-11-2018, 07:15 AM   #6
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QuoteOriginally posted by matroxication Quote
Thanks everyone for the information provided. Am now able to understand what the service personnel was referring to
That is good to hear. You can usually expect someone here to help.
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