[jmg257]

Just jumping in here to ask a question -

Don't think you are wrong...

The 9 cross-type sensor areas - each one made of a longitudinal and lateral sensor - are part of a continuous grid with the sensor areas end to end and overlapping a bit. The center area extends the furthest overall in up/down left/right directions, so that the center sensor beams extend as wide and tall as that Spot Meter circle (the outside beams of the 8 sensors along the edges & in corners are limited by those edges - they are sized more like T & L then +). The problem is there is no real way to know what contrasty item the (center) sensor may have focused on, when more then one focusable item appears within that circle (along the sensor cross actually). Did the longitudinal part lock on? Did the lateral part? Did the cross (indicated by the AF point)?

I.E. if you are trying to adjust Micro AF, and there is a nice dark line across the middle of your focus chart, but the lighter lines on either side of it are contrasty enough, the part of the sensor overlapping these lighter lines may focus on one of those lines, and not the intended dark one in the middle.

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Last edited by jmg257; 03-05-2012 at 08:21 PM.

[JinDesu]

It would be easier for you to read the first page that explains it quite well.
rather than me or anyone going through it all over again lol. Certainly not better and prob worth reading matey

From the first page - one person mentions putting the subject of AF on the edge of the circle.

You also mention how using a wire hanger to test will register the moment it hits the edge of the circle.

However, the inner portion of the circle will read focus as well, correct?

[westmill]

Don't think you are wrong...

The 9 cross-type sensors - each one made up of a longitudinal and lateral sensor - are part of a continious grid with the sensors end to end, and the center extends the furthest overall in up/down left/right directions, so that the center sensor legs extend as wide and tall as that Spot Meter circle (the outside 'legs' of the 8 sensors along the edges are limited by those edges - they are sized more like t then +). The problem is there is no real way to know what contrasty item the (center) sensor may have focused on, when more then one focusable item appears within that circle (the sensor cross actually). Did the longitudinal part lock on? Did the lateral part? Did the cross (indicated by the AF point)?

I.E. if you are trying to adjust Micro AF, and there is a nice dark line across the middle of your focus chart, but the lighter lines on either side of it are contrasty enough, the part of the sensor overlapping these lighter lines may focus on one of those lines, and not the intended dark one in the middle.

Yup.... you got your thinking cap on now Its a bit of an eye opener isnt it.

[westmill]

From the first page - one person mentions putting the subject of AF on the edge of the circle.

You also mention how using a wire hanger to test will register the moment it hits the edge of the circle.

However, the inner portion of the circle will read focus as well, correct?

Sorry, you need to read a bit further lol. Im not being awkward, honnestly, Its just a lot to repeat and explain.
but your miles off the mark here.

[jmg257]

...However, the inner portion of the circle will read focus as well, correct?

Just to clarify...
The sensor area isn't really a circle, its a cross. If any part of the cross finds focus, the AF point will light red and focus will be acheived. This can happen anywhere along the + (with extra importance given to the lateral sensor, and much emphasis on 'closer is better' and 'smaller lens movement is better'. Moving the (center) sensor area across a blank wall into a specifc line such as a coat hanger (or door jam) will show just how far those sensors extend out from the AF point....focus is acheived at about the radius of the ( ), way before the AF point is over the focus target.

You are right that it might not really help us focus on say an eye instead of a nose or ear, but being aware that such a thing may happen - AND WHY(!) - could help in some circumstances, and is always good to know! I.E. I tend to think alot of so-called 'BF/FF issues' are actually the AF locking on other items then the user intended.

[rawr]

I tend to think alot of so-called 'BF/FF issues' are actually the AF locking on other items then the user intended.

Agreed. I often think people believe cameras can read their minds about what the camera should have focussed on, rather than understanding properly how AF works.

[westmill]

Just to clarify...
The sensor isn't really a circle its a cross. If any part of the cross finds focus, the AF point will light red and focus will be acheived. This can happen anywhere along the +. Using a specifc line such as a coat hanger (or door jam) will show just how far those sensors extend out from the AF point.

You are right that it might not really help us focus on say an eye instead of a nose or ear, but being aware that such a thing may happen - AND WHY(!) - could help on in circumstance, and is always good to know! I.E. I tend to think alot of so-called 'BF/FF issues' are actually the AF locking on other items then the user intended.

Just so no more confusion..... the cross fills the semicircle and can be relied on as an accurate guide !
The point I was trying to make with the nose and ear thing is that the focus area is so large you wouldnt know where it focussed.
and that, if it happned to focus on the ear when they think they focussed on the eye.... they just think the lens is back focussing.
To put things into perspective.... when I look at the sensor in the nikon and then look at the size of the area of the K5 its obviously
a guestimate here but I would say that I would fit at least 6 of the nikon squares in there.
And YUP ! If you understand how it works you can understand its limatations.
The best workaround or advice I can come up with at the moment is just try to focus on something that fits inside the circle and that there
is nothing between you and your objective. IE... Microphone in front of singer etc.
I just feel its something everyone should be aware of. Like you, i too, Totaly 100% convinced these BF and FF issues are mostly related
to this.

[simico]

All of this was already discovered and discussed in 2009

[Philoslothical]

All of this was already discovered and discussed in 2009

Nice find! That post by wlachan with the images is really good for helping to visualize it.

[westmill]

Yes ! exactly the same thing..... the more people know about it the better I think.
Then next time someone is asking about focussing probs someone is bound to
mention this ! I noticed wlachan camera wasnt even centraly aligned too.
Its one of those things that anyone using the K5 should just know about.
I would imagine obviously same goes for K7 etc etc.
Sumone mentioned the older cameras are even bigger !
Anyone at all having focussing issues.... this has to be the first stop !

[JinDesu]

That picture with the cross type sensors really helps a lot - I had no idea they looked like that!

[westmill]

One other sumwat related thing springs to mind........ You have a good advantage here
if you use pentax lenses. I dont know about all of them, but most of them it seems
allows you to fine tune focus while shooting in single AF. The only way i can focus my
Tamron is switch to full manual focus. Sumit tells me Pentax may have designed this into
the lenses as a bit of a way of compensating !

[jmg257]

All of this was already discovered and discussed in 2009

Yep - but in 2009 I had a Canon - didn't have to worry about such things!

[MSL]

Ok, so I come back to my previous question: What tricks can you use to get around this particularly when shooting it tough conditions - like low light and moving people?

[westmill]

Ok, so I come back to my previous question: What tricks can you use to get around this particularly when shooting it tough conditions - like low light and moving people?

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.