Pentax 15 mm f4 limited missing focus points on K3 mark iii

Libor · 10-11-2021, 11:14 AM

Hello, I have tried Pentax limited lens 15 mm f4 on K3 iii. It works, however, I found limited range of focus points. Comparing to a.g. 55*SDM, there is less focus points. Did not experience this with any other lenses such as sigma 17-50, Pentax 55-300 plan or 50 -200.
My question is if anyone experience issue with limited range of focus points with 15 mm f4 limited on K3III. I attach picture of back display with focus points 15 mm on K3III. Please advice if this is ok.
Thank you Libor.

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licht96 · 10-11-2021, 12:41 PM - **1 Like**

As I know it's mentioned in the manual for a lot of lenses. There are certain lenses wher you don't get the full amount of AF points

Paul the Sunman · 10-11-2021, 01:54 PM - **1 Like**

I think it has something to do with field curvature, for which the DA 15 Limited is well-known. The edges are not a strong point of the lens, so it makes sense not to choose a focus point there.

The DA 15, 21 and 560 are "Type B" according to the manual (p 117), which is the most restricted class. Even the DA 10-17 fisheye is only Type A, which has an extra column of points on either side. This indicates just how dodgy are the DA 15 edges. Great lens though.

house · 10-11-2021, 02:08 PM

It would be interesting to know the actual reason. Is it mentioned by Pentax anywhere?

I don't understand why field curvature would prevent use of focus points. It should on the contrary be very useful. General softness and smearing would probable freak out the phase detect.

Is there a list of affected lenses?

Paul the Sunman · 10-11-2021, 02:18 PM

Originally posted by house

Is there a list of affected lenses?

As I said, p 117 of the manual.

**luftfluss** · 10-11-2021, 03:21 PM

Can this "feature" be disabled? There have been times when shooting the 15/4 that I move the focus point so that it is halfway between the center and edge, resulting in a more evenly in-focus image.

Libor · 10-12-2021, 10:53 PM

Originally posted by licht96

As I know it's mentioned in the manual for a lot of lenses. There are certain lenses wher you don't get the full amount of AF points

Thank you very much for this note. Lens work fine except those focus points. I have asked Pentax in Czech via the service and haven't obtained any relevant answer.
Answers below make a good point as well.
Thanks again. Libor

**StigVidar** · 10-15-2021, 12:35 AM

Originally posted by house

It would be interesting to know the actual reason.

It depends on the lens diameter for the lens nearest the mirror box (or the backside of the lens). When this lens is small, light will not hit the AF sensors which is made for looking out of the lens on either side. The AF sensors use light from each side of the lens to do some triangulation and this require a large enough opening in the lens. For the center AF sensor it works for lenses with f/5.6 to f/8 opening. For AF sensors to either side an additional requirement is how wide the light beam is inside the mirror box.

**Larrymc** · 10-15-2021, 06:28 AM

Originally posted by luftfluss

Can this "feature" be disabled?

Nope!!

10-11-2021, 01:54 PM - 1 Like	#3
Paul the Sunman Pentaxian Join Date: Aug 2011 Location: Melbourne Photos: Gallery Posts: 4,846	I think it has something to do with field curvature, for which the DA 15 Limited is well-known. The edges are not a strong point of the lens, so it makes sense not to choose a focus point there. The DA 15, 21 and 560 are "Type B" according to the manual (p 117), which is the most restricted class. Even the DA 10-17 fisheye is only Type A, which has an extra column of points on either side. This indicates just how dodgy are the DA 15 edges. Great lens though. Last edited by Paul the Sunman; 10-11-2021 at 02:02 PM.

10-11-2021, 02:18 PM	#5
Paul the Sunman Pentaxian Join Date: Aug 2011 Location: Melbourne Photos: Gallery Posts: 4,846	Originally posted by house Is there a list of affected lenses? As I said, p 117 of the manual. Last edited by Paul the Sunman; 12-16-2021 at 10:49 PM.

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10-11-2021, 12:41 PM - 1 Like	#2
licht96 Forum Member Join Date: Jan 2021 Location: Braunschweig Posts: 92	As I know it's mentioned in the manual for a lot of lenses. There are certain lenses wher you don't get the full amount of AF points

10-11-2021, 02:08 PM	#4
house Pentaxian Join Date: May 2015 Photos: Albums Posts: 3,306	It would be interesting to know the actual reason. Is it mentioned by Pentax anywhere? I don't understand why field curvature would prevent use of focus points. It should on the contrary be very useful. General softness and smearing would probable freak out the phase detect. Is there a list of affected lenses?

10-11-2021, 03:21 PM	#6
luftfluss Site Supporter Join Date: Jun 2011 Location: NJ Photos: Gallery \| Albums Posts: 11,626	Can this "feature" be disabled? There have been times when shooting the 15/4 that I move the focus point so that it is halfway between the center and edge, resulting in a more evenly in-focus image.

10-12-2021, 10:53 PM	#7
Libor Forum Member Join Date: Sep 2014 Location: Brno Posts: 59 Original Poster	Originally posted by licht96 As I know it's mentioned in the manual for a lot of lenses. There are certain lenses wher you don't get the full amount of AF points Thank you very much for this note. Lens work fine except those focus points. I have asked Pentax in Czech via the service and haven't obtained any relevant answer. Answers below make a good point as well. Thanks again. Libor

10-15-2021, 12:35 AM	#8
StigVidar Site Supporter Join Date: Nov 2006 Location: Norway Photos: Gallery \| Albums Posts: 705	Originally posted by house It would be interesting to know the actual reason. It depends on the lens diameter for the lens nearest the mirror box (or the backside of the lens). When this lens is small, light will not hit the AF sensors which is made for looking out of the lens on either side. The AF sensors use light from each side of the lens to do some triangulation and this require a large enough opening in the lens. For the center AF sensor it works for lenses with f/5.6 to f/8 opening. For AF sensors to either side an additional requirement is how wide the light beam is inside the mirror box.

10-15-2021, 06:28 AM	#9
Larrymc Site Supporter Join Date: Jun 2014 Location: Mississippi, USA Posts: 5,252	Originally posted by luftfluss Can this "feature" be disabled? Nope!!