I have fixed my front focus problem - how to

[wlachan]

My only doubt was (still is) do I adjust the 3 screws (I did, it
seemed logical - I hope I was right) or just the bottom one? On the
2 links I posted, you get 2 different pieces of advice.

If you wish to adjust the AF, you don't just adjust the centre sensor, but all 11 sensors at the same time. Altering 1 screw will change all of them. All 3 screws are critical for adjustment. My opinion is that attempting to adjust the AF w/o any proper instrument is just guess work at best (same as adjusting focus screen). You can adjust the center AF sensor easily, but you will never be able to get all 11 sensors aligned properly.
--
Alan Chan
http://www.pbase.com/wlachan

 

[wlachan]

do you have more information on what that 3 screws is used for ?
I am a little bit scare of doing something without knowing what is
the truth behind this..

are there use to allign the sensor? horizontal,vertical and
front-back alignment ? if so, that what you need to adjust is JUST
one screw then, the front-back screws..but which one is this ?

See my post above.
--
Alan Chan
http://www.pbase.com/wlachan

 

[wlachan]

adjusting the camera AF for one lens might result in BF/FF with
other AF lenses.

for Nikon and Pentax systems, the focusing motor is in the camera.
So, this issue should not surface. But it's safer to check, of
course.

What I mean is, Pentax seem to know and expect some BF/FF with certain camera/lens combinations for some technical limitation. That's why the very same camera will AF perfectly with some lenses, and BF/FF with others. Since there is nothing to be adjusted inside Pentax AF lenses, altering the default AF alignment of the camera might solve nothing at all. What the end user merely did was to shift AF accuracy from lens A to lens B.
--
Alan Chan
http://www.pbase.com/wlachan

 

[ahilgaz]

So can someone pls make it clear? Do we adjust only one screw or all the 3 screws?
--
Ahmet
*ist-anbul +2GMT

 

[wlachan]

So can someone pls make it clear? Do we adjust only one screw or
all the 3 screws?

3
--
Alan Chan
http://www.pbase.com/wlachan

 

[ozdean]

Hi Allan, I can't see how the focus can change from lens to lens, once the sensor is adjusted spot on , using spot focus and a shallow DOF at say 1.4, then it has to read and adjust all lenses correctly. The auto focus is sensing and adjusting within the camera so once the sensor is accurate I don't see how different lenses can change this setting. The camera will put the lens where it needs to be to give the sharpest readout by the sensor.
Someone please correct me if I am wrong.
--
Capturing Creation

 

[Sweets]

That is really interesting, thanks. My *istD focuses well with
every lens I have except the FA35/2 which is a little spotty, but
it is very interesting to know there are possibilities for
correction.

I might be wrong, but from earlier threads on this I believe this
cannot be done on the *istD, only the DS and later.

--
Brett
http://www.pbase.com/shreder



The Journey is the Thing

Brett, time ago there was a threat on this concerning the *istD, with this picure.



--
rgrds. Henk (*istD&DBG-1).

 

[wlachan]

Hi Allan, I can't see how the focus can change from lens to lens,
once the sensor is adjusted spot on , using spot focus and a
shallow DOF at say 1.4, then it has to read and adjust all lenses
correctly. The auto focus is sensing and adjusting within the
camera so once the sensor is accurate I don't see how different
lenses can change this setting. The camera will put the lens where
it needs to be to give the sharpest readout by the sensor.
Someone please correct me if I am wrong.

Wish I could answer that. But to give you an idea. My newly purchased DA16-45/4 AF precisely at 45mm with my DS, but BF at 16mm (at all distances). If what you said was true, it should AF equally precise at any focal length, but it doesn't. Gladly the focus stays when zooming from 45 to 16mm so I can AF at the long end and just zoom out afterward. A twist to this BF issue is that this lens AF precisely with my pair of Z-1p. Perhaps the DS series were budget models so the AF system was designed with certain limitations to keep the cost down?
--
Alan Chan
http://www.pbase.com/wlachan

 

[ozdean]

Allan it would be interesting for you to do the test and see if the tuning is out. I think the screw that adjusts the front/back focus is the one towards the front of the camera as in the above shot.
Regards Dean
--
Capturing Creation

 

[thw]

My newly purchased DA16-45/4 AF precisely at 45mm with my DS, but
BF at 16mm (at all distances).

In this case, it means you have a firmware issue in the camera whereby the camera motor does not correctly. I'm afraid for such a situation, only Pentax engineers can fix the problem. You'll probably need to take ALL of one's lenses with the camera to Pentax for calibration.

-------------------------------------------
See the colors of my world in:
thw.smugmug.com

 

[barneycg]

Wish I could answer that. But to give you an idea. My newly
purchased DA16-45/4 AF precisely at 45mm with my DS, but BF at 16mm
(at all distances).

I would have thought that the barrel distortion caused by a very wide angle lens could throw off the AF system slightly, which is why it is seen at 16mm and not any other focal length.

Barney

 

[wlachan]

Allan it would be interesting for you to do the test and see if the
tuning is out. I think the screw that adjusts the front/back focus
is the one towards the front of the camera as in the above shot.
Regards Dean

I am afraid I have no intention to mess the factory setting of my DS. I used to do a little repairs on some used cameras and it was not as easy or simple as it seems believe me. By altering these 3 screws, you are actually changing the phyical plane of the AF sensors which are embedded on the same fixed module. It is no different from the way the focus screen was calibrated. Imagine if the focus screen is adjusted through 3 screws (like K1000 or Super A etc), altering any one screw will throw everything out of balance. If you are altering 1 screw on the bottom of the camera, you may bring the centre AF sensor to focus properly with one lens, but at least 5 other AF sensors will be thrown out of alignment.
--
Alan Chan
http://www.pbase.com/wlachan

 

[wlachan]

I would have thought that the barrel distortion caused by a very
wide angle lens could throw off the AF system slightly, which is
why it is seen at 16mm and not any other focal length.

No idea, but I would imagine the centre AF sensor would not be affected at least. I always use spot AF anyway.
--
Alan Chan
http://www.pbase.com/wlachan

 

[wlachan]

In this case, it means you have a firmware issue in the camera
whereby the camera motor does not correctly. I'm afraid for such a
situation, only Pentax engineers can fix the problem. You'll
probably need to take ALL of one's lenses with the camera to Pentax
for calibration.

Unlike Canon AF system, I don't think Pentax AF can be fixed this way. A friend of mine is having BF/FF issue with his 80-200/2.8 with D70 & D200. The technican spent considerable amount of time of them but explained that it was the limitation of AF and perfect AF with all lenses was not possible. He did adjust the AF of the cameras but only to make them AF better at certain focal length and lenses. Before the calibration, it AF fine at the long end; after the calibration, it AF fine at the short end (the reason was explained in my other post).
--
Alan Chan
http://www.pbase.com/wlachan

 

[thw]

If you are altering 1 screw on the bottom of the camera,
you may bring the centre AF sensor to focus properly with one lens,
but at least 5 other AF sensors will be thrown out of alignment.

That is correct.

-------------------------------------------
See the colors of my world in:
thw.smugmug.com

 

[thw]

Unlike Canon AF system, I don't think Pentax AF can be fixed this
way. A friend of mine is having BF/FF issue with his 80-200/2.8
with D70 & D200. The technican spent considerable amount of time of
them but explained that it was the limitation of AF and perfect AF
with all lenses was not possible. He did adjust the AF of the
cameras but only to make them AF better at certain focal length and
lenses. Before the calibration, it AF fine at the long end; after
the calibration, it AF fine at the short end (the reason was
explained in my other post).

OK.

Incidentally, I used to have the 16-45 and DS too, but did not encounter any focus issues (apart from slow focusing in lowlight). But I switched to Canon more than a year ago and have been rather happy with Canon USM system.

-------------------------------------------
See the colors of my world in:
thw.smugmug.com

 

[Digital C]

from the way I see it look like the 3 screw one is to adjust the front/rear focus. one to adjust the focus sensor to the L/R and one to adjust it up/down. I may be wrong.

So can someone pls make it clear? Do we adjust only one screw or
all the 3 screws?
--
Ahmet
*ist-anbul +2GMT

--
SH
Singapore

 

[64mini]

All of this is supposition, I have not opened up my DS or have any info about how the AF sensor works other than what people have posted but as I see it-

The AF sensors are CCD based? and all 11 sensors are mounted as a single unit in the configuration as displayed when looking through the viewfinder?

The hardware and software in the camera are used in selecting which point is used.

The AF unit is not solid mounted to the body as tolerances in production are too wide, hence the screw adjustment to fine tune AF sensor alignment.

If the below diagram gives an idea of how the 3 screws affect the AF sensor alignment in 3 axis-



Then if the 3 screws are adjusted by the same amount, the sensor is only moved in 1 plane, say closer or further away, perpendicular to the mirror and affecting focus forward or backward.

If you were to adjust only one screw, then this should change the plane of the focus sensor points from the perpendicular with respect to the mirror in the vertical or horizontal plane (assuming it is set right in the first place) so that-

The left sensor point might front focus and the right sensor point back focus or vice versa,

or the top sensor point would front focus and the bottom sensor point back focus or vice versa, when using individual sensor points.

This might have interesting effects when using automatic focus point selection.

OK, I know there is more to it (zoom lenses/light type etc) but does this make sense in a purely mechanical way to anyone else or an I talking gibberish?

pick away

cheers

Kevin

--

When Gandhi was in England in 1930, he was asked by a journalist, 'Mr Gandhi, what do you think of civilised society?' Gandhi answered, 'That would be a good idea'

 

[ozdean]

Kevin you have put a lot of work into your theory which is appriciated.

I may be on the wrong track but I was thinking the screws were adjusting the signal from the sensor electronically rather than mecanically adjusting the sensor itself. It would sure be nice to know. We need a hot line to Pentax.
--
Capturing Creation

 

[64mini]

Hey Ozdean,

thanks ) but not too much thought!

I hadnt thought in terms of ajusting the signal, I would have imagined using pots or software/firmware to ajust signal electrically rather than the allen screws described which is what got me thinking along the mechanical line..

There was mention of pentax techs using software to calibrate the AF as well, A tech repair manual would be really handy )

cheers

Kevin
--

When Gandhi was in England in 1930, he was asked by a journalist, 'Mr Gandhi, what do you think of civilised society?' Gandhi answered, 'That would be a good idea'