All the fuss about AF point size

**MSL** · 03-06-2012, 08:48 AM

Originally posted by westmill

Dont get the wrong idea ! The big sensor does not make it any slower or less sensative. Neither does it affect low light performance. There is nothing you can do about the size, other than be aware and use that circle rather than the little square for you focus point.

Ok, let me rephrase this question. Knowing that the center spot may focus on the wrong place - might it help to use one of the side spots and recompose, or does it help to focus away and focus back, or does one need to treat the lens a bit like a manual focus under these conditions and occasionally adjust the focus.

jmg257 · 03-06-2012, 08:56 AM

The sensors are all pretty much the same size, so I don't think using another would make much of a difference; although the two extremes are linear verticle only - might help on horizontal edges. I can't think why focus away/back would help, other then to get new readings (but on what?).

Maybe just be aware that you might not be able to pick and choose so precisely what you want to focus on, and select a greater DOF so more of the subject(s) are in focus?

westmill · 03-06-2012, 09:08 AM

Originally posted by MSL

Ok, let me rephrase this question. Knowing that the center spot may focus on the wrong place - might it help to use one of the side spots and recompose, or does it help to focus away and focus back, or does one need to treat the lens a bit like a manual focus under these conditions and occasionally adjust the focus.

Since they are all roughly the same size I doubt it would make much differance. At least with the centre you have an accurate marker with the circle. What i have now done, is simply be less critical. Instead of focussing on an eye, i place the circle over the face. I now know that at worst it may focus on the end of a nose instead of the eye. It may not be spot on... but close enough.
Obviously, if you can stop down a bit, this will hopefully cover the gap with the exrea DOF. If your subject is static and critical focus is important then manual focus would prob be best but its not always poss.
A lot of people like to shoot really fast lenses. Those that do will have to concentrate far more on this.
Focus is always critical here with such a shallow DOF.

Philoslothical · 03-06-2012, 09:20 AM

If Pentax AF systems follow in the trend of other brands, the difference between those two outlying points and the 9 cross type points is that cross type points are good to f./8, whereas the two side points are only good to f./5.6. For us, that usually won't have much effect, unless using mirror lenses, m42 lenses, or anything where you stop down the aperture (and see the viewfinder darken) before trying to focus. If the aperture is too small, you won't get the AF confirmation. The other big issue with this is the use of teleconverters, which eat light and increase the effective aperture number. This is why with some slower lenses, even an AF capable TC won't work properly.

In any case, there's no reason to use the two side (non-cross type) AF points. They're less sensitive. It's better to focus normally and recompose.

Winder · 03-06-2012, 10:14 AM

Originally posted by westmill

Since they are all roughly the same size I doubt it would make much differance. At least with the centre you have an accurate marker with the circle. What i have now done, is simply be less critical. Instead of focussing on an eye, i place the circle over the face. I now know that at worst it may focus on the end of a nose instead of the eye. It may not be spot on... but close enough.
Obviously, if you can stop down a bit, this will hopefully cover the gap with the exrea DOF. If your subject is static and critical focus is important then manual focus would prob be best but its not always poss.
A lot of people like to shoot really fast lenses. Those that do will have to concentrate far more on this.
Focus is always critical here with such a shallow DOF.

Yes. It is frustrating. I think this is one of the big reasons why Pentax has not developed a DA* 85mm F/1.4 or 135mm F/1.8. They know AF is not up to the challenge.

**MSL** · 03-06-2012, 10:40 AM

Thanks. This does help. What it tells me is that if I'm using a 50/1.7 for a low light situation, I really can't depend on going down to 1.7 because the focusing issue means I need to preserve more DOF. But if I'm already pushing ISO or shutter speed limits (for hand held), I need to be extra careful on the focus.

As far as my idea of focusing away and back again, yes - with luck if you are taking a series of shots you might get some of them right, whereas a burst without refocusing with either be all right or all wrong...

RonHendriks1966 · 03-06-2012, 10:48 AM

Originally posted by Winder

Yes. It is frustrating. I think this is one of the big reasons why Pentax has not developed a DA* 85mm F/1.4 or 135mm F/1.8. They know AF is not up to the challenge.

Well they better hurry, since I want that better AF and DA*135mm/f2.0

jmg257 · 03-06-2012, 11:33 AM

Quote:

As far as my idea of focusing away and back again, yes - with luck if you are taking a series of shots you might get some of them right, whereas a burst without refocusing with either be all right or all wrong...

Ahh - gotcha! Thought you meant for each/one shot.

I found this interesting...pretty cool to chek out the actual AF Sensor for the Canon 1DX, and compare it to the AF Points shown in the ViewFinder...

The 5 blue AF points are for the high-precision diagonal cross types. The pinks for the regular cross types.

And this patent sketch of a potential Pentax grid.

This is an official Pentax representation of the SAFOX sensors from the *st film booklet I found on this forum.

lightbulb · 03-06-2012, 06:23 PM

Wow, like a few steps back. Hopefully we will have 69 af points by k3.

pscl57 · 03-10-2012, 04:41 AM

Originally posted by westmill

Yes ! i cant tell you how grateful i am to you too ! I did a shoot today and took 512 model pics without a single missfocus !
Its an eye opener for sure if people actually start to realise the implcations.
All these front and rear focussing problems ! I wonder..... rear focus.... on the ear maybe ? front focus.... nose maybe !
Its taken me two months to crack it. But judging from today, I do beleive its cracked.
I thought too that my lens was front and back focussing inconsistantly. That doesnt make sense though.
Ive no doubt in my mind that this is the main cause of the vast majority of complaints.
It explains almost everything.... it even explains why some people have zero problems too, since its down to subject matter.

Thank you for your very clear and useful explanations.

westmill · 03-10-2012, 05:55 AM

I thought I would show an example here of real world pics !
The first pic.... the model is full frontal...

and you can see the focus has latched onto
the nose. The second... because its closer and the

model has her head turned to one side
has allowed uninterupted focus on the eye !
The first pic

is an example of what i call close enough. Not spot on but not an oomph either.

westmill · 03-10-2012, 05:58 AM

the pics havnt come up in order lol The 100% pic of the pic showing is the red box directly above the main pic.

**Michaelina2** · 03-10-2012, 06:12 AM

Originally posted by westmill

Since they are all roughly the same size I doubt it would make much differance. At least with the centre you have an accurate marker with the circle. What i have now done, is simply be less critical. Instead of focussing on an eye, i place the circle over the face. I now know that at worst it may focus on the end of a nose instead of the eye. It may not be spot on... but close enough.
Obviously, if you can stop down a bit, this will hopefully cover the gap with the exrea DOF. If your subject is static and critical focus is important then manual focus would prob be best but its not always poss.
A lot of people like to shoot really fast lenses. Those that do will have to concentrate far more on this.
Focus is always critical here with such a shallow DOF.

In addition, within outdoor walkaround settings... I'm confident of getting 'high probability' acceptable K-5 results using AF,C with Focus Priority (C-16, 1) while locked on to the Center Spot focus point and two /f stops less (or lower) than the lens' max /f stop in TAv mode (1/300 sec, or faster, ISO Auto: 80 - 25,600, SR=OFF, AF Adjustment=OFF (C-26, 1)).

Cheers...

westmill · 03-10-2012, 09:04 AM

This pic from the same shoot.... Because the models face is now so small in the frame,
the circle was placed over the face. If I had tried to set focus on the eye, it is likely or
poss the sensor would have latched on the background. That said.... since the background
is not that contrasty, it may have worked, but at that distance.... it makes sense to simply
place the circle over the models face.

Philoslothical · 03-10-2012, 09:29 AM

I can't see any pics in your last post, and only one in the post up above (that indicates there are two). Not sure what's up with that, but thought you'd want to know.

03-06-2012, 11:33 AM	#68
jmg257 Inactive Account Join Date: Jan 2012 Posts: 202	Quote: As far as my idea of focusing away and back again, yes - with luck if you are taking a series of shots you might get some of them right, whereas a burst without refocusing with either be all right or all wrong... Ahh - gotcha! Thought you meant for each/one shot. I found this interesting...pretty cool to chek out the actual AF Sensor for the Canon 1DX, and compare it to the AF Points shown in the ViewFinder... The 5 blue AF points are for the high-precision diagonal cross types. The pinks for the regular cross types. And this patent sketch of a potential Pentax grid. This is an official Pentax representation of the SAFOX sensors from the st film booklet I found on this forum. Last edited by jmg257; 06-04-2012 at 09:40 AM.*

03-10-2012, 06:12 AM	#73
Michaelina2 Site Supporter Join Date: Jan 2008 Location: SE Michigan USA Photos: Gallery Posts: 1,300	Originally posted by westmill Since they are all roughly the same size I doubt it would make much differance. At least with the centre you have an accurate marker with the circle. What i have now done, is simply be less critical. Instead of focussing on an eye, i place the circle over the face. I now know that at worst it may focus on the end of a nose instead of the eye. It may not be spot on... but close enough. Obviously, if you can stop down a bit, this will hopefully cover the gap with the exrea DOF. If your subject is static and critical focus is important then manual focus would prob be best but its not always poss. A lot of people like to shoot really fast lenses. Those that do will have to concentrate far more on this. Focus is always critical here with such a shallow DOF. In addition, within outdoor walkaround settings... I'm confident of getting 'high probability' acceptable K-5 results using AF,C with Focus Priority (C-16, 1) while locked on to the Center Spot focus point and two /f stops less (or lower) than the lens' max /f stop in TAv mode (1/300 sec, or faster, ISO Auto: 80 - 25,600, SR=OFF, AF Adjustment=OFF (C-26, 1)). Cheers... Last edited by Michaelina2; 03-10-2012 at 07:20 AM.

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03-06-2012, 08:48 AM	#61
MSL Site Supporter Join Date: Feb 2012 Location: Greater Toronto Area Photos: Gallery \| Albums Posts: 4,749	Originally posted by westmill Dont get the wrong idea ! The big sensor does not make it any slower or less sensative. Neither does it affect low light performance. There is nothing you can do about the size, other than be aware and use that circle rather than the little square for you focus point. Ok, let me rephrase this question. Knowing that the center spot may focus on the wrong place - might it help to use one of the side spots and recompose, or does it help to focus away and focus back, or does one need to treat the lens a bit like a manual focus under these conditions and occasionally adjust the focus.

03-06-2012, 08:56 AM	#62
jmg257 Inactive Account Join Date: Jan 2012 Posts: 202	The sensors are all pretty much the same size, so I don't think using another would make much of a difference; although the two extremes are linear verticle only - might help on horizontal edges. I can't think why focus away/back would help, other then to get new readings (but on what?). Maybe just be aware that you might not be able to pick and choose so precisely what you want to focus on, and select a greater DOF so more of the subject(s) are in focus?

03-06-2012, 09:08 AM	#63
westmill Veteran Member Join Date: Jan 2012 Location: Stoke on Trent Photos: Gallery \| Albums Posts: 1,146	Originally posted by MSL Ok, let me rephrase this question. Knowing that the center spot may focus on the wrong place - might it help to use one of the side spots and recompose, or does it help to focus away and focus back, or does one need to treat the lens a bit like a manual focus under these conditions and occasionally adjust the focus. Since they are all roughly the same size I doubt it would make much differance. At least with the centre you have an accurate marker with the circle. What i have now done, is simply be less critical. Instead of focussing on an eye, i place the circle over the face. I now know that at worst it may focus on the end of a nose instead of the eye. It may not be spot on... but close enough. Obviously, if you can stop down a bit, this will hopefully cover the gap with the exrea DOF. If your subject is static and critical focus is important then manual focus would prob be best but its not always poss. A lot of people like to shoot really fast lenses. Those that do will have to concentrate far more on this. Focus is always critical here with such a shallow DOF.

03-06-2012, 09:20 AM	#64
Philoslothical Veteran Member Join Date: May 2011 Posts: 1,889	If Pentax AF systems follow in the trend of other brands, the difference between those two outlying points and the 9 cross type points is that cross type points are good to f./8, whereas the two side points are only good to f./5.6. For us, that usually won't have much effect, unless using mirror lenses, m42 lenses, or anything where you stop down the aperture (and see the viewfinder darken) before trying to focus. If the aperture is too small, you won't get the AF confirmation. The other big issue with this is the use of teleconverters, which eat light and increase the effective aperture number. This is why with some slower lenses, even an AF capable TC won't work properly. In any case, there's no reason to use the two side (non-cross type) AF points. They're less sensitive. It's better to focus normally and recompose.

03-06-2012, 10:14 AM	#65
Winder Veteran Member Join Date: Apr 2010 Location: Tennessee Posts: 6,617	Originally posted by westmill Since they are all roughly the same size I doubt it would make much differance. At least with the centre you have an accurate marker with the circle. What i have now done, is simply be less critical. Instead of focussing on an eye, i place the circle over the face. I now know that at worst it may focus on the end of a nose instead of the eye. It may not be spot on... but close enough. Obviously, if you can stop down a bit, this will hopefully cover the gap with the exrea DOF. If your subject is static and critical focus is important then manual focus would prob be best but its not always poss. A lot of people like to shoot really fast lenses. Those that do will have to concentrate far more on this. Focus is always critical here with such a shallow DOF. Yes. It is frustrating. I think this is one of the big reasons why Pentax has not developed a DA* 85mm F/1.4 or 135mm F/1.8. They know AF is not up to the challenge.

03-06-2012, 10:40 AM	#66
MSL Site Supporter Join Date: Feb 2012 Location: Greater Toronto Area Photos: Gallery \| Albums Posts: 4,749	Thanks. This does help. What it tells me is that if I'm using a 50/1.7 for a low light situation, I really can't depend on going down to 1.7 because the focusing issue means I need to preserve more DOF. But if I'm already pushing ISO or shutter speed limits (for hand held), I need to be extra careful on the focus. As far as my idea of focusing away and back again, yes - with luck if you are taking a series of shots you might get some of them right, whereas a burst without refocusing with either be all right or all wrong...

03-06-2012, 10:48 AM	#67
RonHendriks1966 Banned Join Date: Jan 2009 Photos: Gallery Posts: 9,675	Originally posted by Winder Yes. It is frustrating. I think this is one of the big reasons why Pentax has not developed a DA* 85mm F/1.4 or 135mm F/1.8. They know AF is not up to the challenge. Well they better hurry, since I want that better AF and DA*135mm/f2.0

03-06-2012, 06:23 PM	#69
lightbulb Veteran Member Join Date: Jan 2012 Posts: 691	Wow, like a few steps back. Hopefully we will have 69 af points by k3.

03-10-2012, 04:41 AM	#70
pscl57 Inactive Account Join Date: Oct 2009 Location: Lille, France Photos: Gallery Posts: 27	Originally posted by westmill Yes ! i cant tell you how grateful i am to you too ! I did a shoot today and took 512 model pics without a single missfocus ! Its an eye opener for sure if people actually start to realise the implcations. All these front and rear focussing problems ! I wonder..... rear focus.... on the ear maybe ? front focus.... nose maybe ! Its taken me two months to crack it. But judging from today, I do beleive its cracked. I thought too that my lens was front and back focussing inconsistantly. That doesnt make sense though. Ive no doubt in my mind that this is the main cause of the vast majority of complaints. It explains almost everything.... it even explains why some people have zero problems too, since its down to subject matter. Thank you for your very clear and useful explanations.

03-10-2012, 05:55 AM	#71
westmill Veteran Member Join Date: Jan 2012 Location: Stoke on Trent Photos: Gallery \| Albums Posts: 1,146	I thought I would show an example here of real world pics ! The first pic.... the model is full frontal... and you can see the focus has latched onto the nose. The second... because its closer and the model has her head turned to one side has allowed uninterupted focus on the eye ! The first pic is an example of what i call close enough. Not spot on but not an oomph either.

03-10-2012, 05:58 AM	#72
westmill Veteran Member Join Date: Jan 2012 Location: Stoke on Trent Photos: Gallery \| Albums Posts: 1,146	the pics havnt come up in order lol The 100% pic of the pic showing is the red box directly above the main pic.

03-10-2012, 09:04 AM	#74
westmill Veteran Member Join Date: Jan 2012 Location: Stoke on Trent Photos: Gallery \| Albums Posts: 1,146	This pic from the same shoot.... Because the models face is now so small in the frame, the circle was placed over the face. If I had tried to set focus on the eye, it is likely or poss the sensor would have latched on the background. That said.... since the background is not that contrasty, it may have worked, but at that distance.... it makes sense to simply place the circle over the models face.

03-10-2012, 09:29 AM	#75
Philoslothical Veteran Member Join Date: May 2011 Posts: 1,889	I can't see any pics in your last post, and only one in the post up above (that indicates there are two). Not sure what's up with that, but thought you'd want to know.