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09-11-2012, 02:16 PM   #1
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Too big focus points

I often hear that Pentax cameras have focus points that are too large, causing inaccuracies. I don't quite understand what this means and what issue this causes. Anyone willing to educate me on this?

Thanks

09-11-2012, 02:26 PM   #2
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https://www.pentaxforums.com/forums/pentax-dslr-discussion/181885-analysis-p...provement.html

See above thread for more info:

Basically the cross type AF sensors are literally crosses. They look for points of contrast in the scene and determine the AF distance. They will try to focus on a point of most contrast that lands anywhere on the cross. A larger cross means more of the scene is being reviewed by the AF system. This can lead to the system focusing on the wrong subject.

A smaller cross (and more of them) allows for more pinpoint control of what the AF system is choosing for focusing. The downside is that it taxes the AF processor more (more information, etc).
09-11-2012, 02:37 PM   #3
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This leads to the camera focusing on something else on the focus point. For example, you want to take a photo of a person in front of you, but the camera focuses to the person behind them, even though the red focus confirm light in the viewfinder makes it appear as if it focused on the person you wanted. And on the tiny screen you often cannot tell fine misfocuses. And often you don't expect them at all.
You can work around this by.. well, by really figuring out how the camera works and making sure you focus on what you want.
09-11-2012, 05:02 PM   #4
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I see...given the size of the focus sensors, is there some approach to take to mitigate the effect? And, does it impact manual focus?

09-11-2012, 06:32 PM   #5
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QuoteOriginally posted by Na Horuk Quote
This leads to the camera focusing on something else on the focus point. For example, you want to take a photo of a person in front of you, but the camera focuses to the person behind them, even though the red focus confirm light in the viewfinder makes it appear as if it focused on the person you wanted. And on the tiny screen you often cannot tell fine misfocuses. And often you don't expect them at all.
You can work around this by.. well, by really figuring out how the camera works and making sure you focus on what you want.
I don't really understand why people have this problem. When I focus, I look through the viewfinder to ensure the target is in focus. The AF is just a substitute for me adjusting the focus manually, It does the work of turning the dial, but I am responsible to ensure it focussed on my intended subject. Don't people check where the focus is when they use AF?
09-11-2012, 06:37 PM   #6
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First, the stock screen is not going to help you with fast lenses to check focus is on eye, nose, eye brow or ear. Second, firing a burst in AF-C (say of a player in a sports team) doesn't allow you to fine tune focus @ 5.5 fps. I have so many in focus pictures of wire fences and trees in the background it isn't funny when I shoot my kids in sport, all down to a too big of a centre AF point.
09-11-2012, 06:39 PM   #7
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So, what appears to be focused on with the selected focus point may not be what is really being focused on?
09-11-2012, 06:42 PM - 1 Like   #8
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QuoteOriginally posted by twitch Quote
First, the stock screen is not going to help you with fast lenses to check focus is on eye, nose, eye brow or ear. Second, firing a burst in AF-C (say of a player in a sports team) doesn't allow you to fine tune focus @ 5.5 fps. I have so many in focus pictures of wire fences and trees in the background it isn't funny when I shoot my kids in sport, all down to a too big of a centre AF point.
I suspect that most people with AF complaints are using AF-C and/or letting the camera control focus. I use center point and recompose or MF only. AF-C is only good for spray and pray. It will miss, take lots of shots.

09-11-2012, 06:48 PM   #9
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QuoteOriginally posted by audiobomber Quote
I suspect that most people with AF complaints are using AF-C and/or letting the camera control focus. I use center point and recompose or MF only. AF-C is only good for spray and pray. It will miss, take lots of shots.
What you call spray and pray is sports/action photography, and recompose doesn't help you if the centre point is so big it grabbed the wrong thing.

I had my K-5 so confused last weekend in AF-C shooting sport that it just pumped focus back for and forth for seconds trying to decide whether to focus on player or on tree. Sure I get lots of hits too, but the misses are frustrating.
09-12-2012, 03:26 AM   #10
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Pentax should add a little joystick so we can help the camera whether to focus one something near or something far
09-12-2012, 04:04 AM   #11
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QuoteOriginally posted by Spodeworld Quote
I see...given the size of the focus sensors, is there some approach to take to mitigate the effect? And, does it impact manual focus?
I think it could potentially impact manual focus if you are relying on the camera's focus confirmation before taking a picture. With manual focus, the camera always uses the center focus point for focus confirmation purposes. However, if you have a good focusing screen, you should be able to focus pretty closely on exactly what you want. I've actually had a much higher keeper rate since I started using manual focus and manual focus lenses.
09-13-2012, 05:24 AM   #12
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this I posted in another similar thread, https://www.pentaxforums.com/forums/pentax-k-5/198105-please-solve-my-focus-issues.html

OK here is a test I have just carried out . I was going to post the pictures but if you are really that interested I suggest you go try it out your self.

I took a plain white sheet of A3 size paper as that in landscape is the same ratio as a K5 and therefore fills the frame. I very carefully measured the centre and drew with a thin black marker pen a vertical line 1/4 of a inch each side of the centre, and stuck the sheet to a door. target area is therefore 1/2 inch high by about 1/8 " wide

K5 on a tripod set to single focus centre point with 50mm f1.7 fitted. and placed so the sheet of paper just filled the viewfinder.

if the black line in on or in side the right hand ) in the centre of the viewfinder the focus lock green hex comes on.

if the black line is inside but NOT on the left hand ( in the centre of the view finder then focus lock is achieved.

if you imagine the ( ) creating a circle the focus lock comes on if the top or bottom edge of the black line touches the imaginary completed circle or any where inside that circle.

nothing new here we knew that any thing inside that ( ) will be found no matter what the dot is on.

but what is very interesting is to try this same exercise with any of the user selectable points.

you will find that in fact the distance either side of the black line where the red dot is placed is not at all equal. if you take the far left one you will see it is much larger and locks a bit left of the line but not at all to the right and also quite some distance above and below the line.

the same applies to the other smaller usable selected points, the distance that they lock on is is not equal either, they are not crosses so much as we have been told.

this is quite hard to explain and either needs images with the red dot added or you you need to test for your self.

what it all therefore indicates is that even if you have put the centre dot on a object say a face, if there is something more contrast in the ( ) area it can lock on that and not the face. So your chosen subject should fill the ( ) area.

What is more important is the distance away from the line that the user selectable chosen point will still lock up can lead to the lock being on completely the wrong thing.

whats worse is that in a small dslr viewfinder with no split screen focus can be very hard to spot.

if you go back to page one of this thread and look at the original image you can possible see why the edge of the crib is in focus and not the face.

it also make no difference what f stop you are using as far as I can see.

as the op said [see below]" I use my arrow keys to move the red dot over what I want to be in focus." if you do that you could easily get the crib edge in focus and not the face.
Try it out you will soon see what I mean.

https://www.pentaxforums.com/forums/pentax-k-5/198105-please-solve-my-focus-issues.html
09-13-2012, 05:28 AM   #13
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QuoteOriginally posted by audiobomber Quote
I don't really understand why people have this problem. When I focus, I look through the viewfinder to ensure the target is in focus. The AF is just a substitute for me adjusting the focus manually, It does the work of turning the dial, but I am responsible to ensure it focussed on my intended subject. Don't people check where the focus is when they use AF?
Dan I would agree in principle but given the size of the viewfinder and the lack of a split focusing screen it is often impossible to be 100% sure that what you want is actually in focus and not something else within the range of the focus point that is of higher contrast .
see this post it is a typical problem well illustarted in this thread in post #one .
https://www.pentaxforums.com/forums/pentax-k-5/198105-please-solve-my-focus-issues.html
09-13-2012, 09:17 AM   #14
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Is this problem more noticeable under certain conditions?
09-13-2012, 09:42 AM   #15
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QuoteOriginally posted by Spodeworld Quote
Is this problem more noticeable under certain conditions?
not in my experience but then I regularly see people posting about the poor focusing of Pentax in low light, not some thing I have ever had a issue with and at the end of the day I still count myself as lucky as I still have a hand that can operate a focus ring.

I am never convinced that Pentax is a any worse than any other brand, if you have quick search on any of the other brand forums there are plenty of posts complaining about missed focus and low light focus issues.

I think some posters expect a bit to much from the automation of electronic focus.
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