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10-05-2011, 03:27 AM   #1
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Samsung GX20 massive back focus problem

Hi all.

My Samsung GX20 has a serious back focus problem (about half a meter).
I tryed reducind the back focus in "AF fine adjustment" form Settings menu and even Debuging mode but with no luck.
In "AF fine adjustment" i couldn't even get the camera to remeber the lense, and in "Debuging mode" i had no luck finding the AF adjustment menu(for my debugging menu experience i used instructions for Pentax K20 but two menus aren't the same).

So if anyone had the same problem please share some tips even thow they weren't helpfull. Anything Could help.

PS: i tryed to post on the existing GX20 debug mode post but it was too old.

10-05-2011, 07:31 AM   #2
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The debug mode details are all here. The file needs to be named modset.454 for the GX20.

On the K20D you use 'menu', 'settings', then you'll find the item called 'AF Test' as the last-but-one item in the settings list. Go right into that and down one to give the focus to the adjustment slider, and experiment.

Add value to correct back-focus, subtract value to correct front-focus.

Sorry, I can't say what it looks like on the GX20 but I wouldn't expect it to be very different.
10-05-2011, 11:52 AM   #3
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QuoteOriginally posted by karma mechanic Quote
On the K20D you use 'menu', 'settings', then you'll find the item called 'AF Test' as the last-but-one item in the settings list. Go right into that and down one to give the focus to the adjustment slider, and experiment.
Well the problem is that in gx20's debug mode "AF Test" is missing. I only have "Cature/System Test" and "String Test".
I searched extensive for a solution but i only saw other people who have the same problem but no solution.
10-05-2011, 12:09 PM   #4
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Hmmmm... I see the problem. There must be a solution...

10-05-2011, 04:33 PM   #5
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QuoteOriginally posted by macgyverx86 Quote
My Samsung GX20 has a serious back focus problem (about half a meter).
Huh? What lens has the problem, and what is the distance from your focus target?

BF/FF issues (where they actually exist and are not imaginary) are not related to the body alone, but specific body/lens combinations.

BF/FF also needs to be tested in the right way. BF of 50cm against a focus target 1m away is a big issue, but no issue at all against a focus target 100m away. And BF of 50cm measured at an aperture of f11 isn't the same as BF of 50cm measured at an aperture of f1.4 at any distance. Etc.
10-05-2011, 10:20 PM   #6
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Hi rawr

Currently i have the 18-55 kit lens and a Tamron 18-200 lens capable of auto-focus. I've set both lenses at 50-55 mm and f8 and pointed them at an object 2 meters away. With "AF fine Adjustment" on maximum (+10 for One type) seems to be a bit smaller but still auto focus isn't usable. "AF fine Adjustment" isn't cumulative. it remembers the value set for a lens.

I'll research how to test this AF problem to post some exact values.

Thanks for your reply
10-06-2011, 05:12 AM   #7
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QuoteOriginally posted by macgyverx86 Quote
I've set both lenses at 50-55 mm and f8 and pointed them at an object 2 meters away
You couldn't be doing it more wrong, if that is how you are doing your testing.

Here, for example, is how Pentax suggest a focus test be performed, from the K100D service manual:



Other guides online will offer similar advice. Researching the issue more is a good idea before you start attempting any focus adjustments on your camera, otherwise you really risk making a total mess of your auto-focus.

You may also be interested in buying ($50) a product like this if you really want to be confident about doing the AF adjust reasonably well:
SpyderLensCal

I have both of the lenses you have BTW. Never had any problem with them.
.

Last edited by rawr; 10-06-2011 at 05:21 AM.
10-06-2011, 11:10 AM   #8
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Hi

So i made a little AF test. I printed a AF adjustment chart from here, and i pasted two scales together to see where the camera focuses.
The result is below.

And this is the exif info:
Code:
Filename - SG200011.JPG
Make - SAMSUNG DIGITAL IMA
Model - SAMSUNG GX20
Orientation - Top left
XResolution - 72
YResolution - 72
ResolutionUnit - Inch
Software - GX20 Ver 1.03
DateTime - 2011:10:06 19:49:51
YCbCrPositioning - Co-Sited
ExifOffset - 234
ExposureTime - 1/160 seconds
FNumber - 5.60
ExposureProgram - Not defined
ISOSpeedRatings - 1600
ExifVersion - 0221
DateTimeOriginal - 2011:10:06 19:49:51
DateTimeDigitized - 2011:10:06 19:49:51
ComponentsConfiguration - YCbCr
ExposureBiasValue - 0.00
MeteringMode - Multi-segment
Flash - Flash fired, Compulsory flash mode
FocalLength - 55.00 mm
FlashPixVersion - 0100
ColorSpace - sRGB
ExifImageWidth - 3008
ExifImageHeight - 2000
InteroperabilityOffset - 53920
SensingMethod - One-chip color area sensor
FileSource - Other
SceneType - Other
CustomRendered - Normal process
ExposureMode - Auto
White Balance - Auto
FocalLengthIn35mmFilm - 82 mm
SceneCaptureType - Standard
Contrast - Hard
Saturation - High
Sharpness - Hard
SubjectDistanceRange - Macro

Maker Note (Vendor): -

Thumbnail: -
Compression - 6 (JPG)
XResolution - 72
YResolution - 72
ResolutionUnit - Inch
JpegIFOffset - 54044
JpegIFByteCount - 7306
And here is a 100% crop photo: (AF point was in the red spot)


10-06-2011, 11:48 AM   #9
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I would say that you have a more fundamental problem than just the need for AF adjustment.

Is this a recent phenomenon, or has the camera always been way out?
Had it been dropped or otherwise upset?
Does the problem exist for all lenses?

Please try something else. Outdoors try manually-focussing (via the viewfinder) on a distant object with a lens wide-open.
In the resulting image is the focus where you'd expect it to be?
Was the lens focus ring near or at the infinity mark?
What happens if you switch to AF, defocus the lens by putting your hand in front of it, then allow the AF to focus on the distant object?
Using AF do you get a sharp image at infinity, does it lock focus?

What I'm trying to determine is whether the AF is in error or the sensor position has moved for some reason. Both are physically adjustable with varying degrees of complexity and risk.
10-06-2011, 12:08 PM   #10
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I have this problem for about a year and i think i got it while i was trying to calibrate the focus of a Tamron 28-200 lens that i dismantled.
I used this camera in 90% of the cases in manual focus mode and i got really some really good shots. And yes, this problem appears to all AF lenses.
I have another 50mm, f2.0 prime and when using that lenses at f2.0 i get a slight out of focus when looking through the viewfinder, but when i use live view and zoom in to focus all images are sharp and accurately focused.
10-06-2011, 12:28 PM   #11
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Focussing the Live View image will always work because it is based on what the sensor is seeing. You say that you got the problem when you were trying to calibrate the focus of a Tamron. What did you actually do? That's probably the crux of the matter - it either started on its own for unknown reasons, or you did something...
10-06-2011, 12:42 PM   #12
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By calibrating the focus of a lens i meant that i was playing with the distance between the first lens (the one in the front of the tens) and the other group of lenses by screwing and unscrewing the first lens and finding a position that would let me put the three screws in place .
I don't really know if someone could possibly understand what i was trying to say
10-06-2011, 11:35 PM   #13
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Hi karma mechanic.

This morning i did a few photos as you said, outside, wide open apperture, with manual focus through viewfinder, with autofocus and manual focus in Live View.
I did two sets of photos at 55mm, f5.6 with the kit lens. One set at a distant object at more then 2 Km away and at a relativley near object at 300 meeters.

For the fisrt set (2 Km away object) there was a really big difference between AF and MF and a slight difference between MF through viewfinder and Live View. AF was way out of focus, MF looked goog (there were details but not as clear as in Live View mode), and MF via Live view looked best.

For the second test (relatevely near objekt) i had the same AF problem but no differences between the two MF modes (Live View and Viewfinder), but here the focus ring was right in it's maximum position and images wheren't as focused as first set. When i took the first set the focus ring hat a little space left and when rotated to maximum the image was a bit blurry.
10-07-2011, 03:06 AM   #14
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That sounds like progress, although I am puzzled about one thing.

For the most distant objects the focus ring should have been closer to the infinity position (the 'end stop') than for nearer objects. I think your relatively near object wasn't really very near, something like a poster on a wall at 5 metres would be better for that one. Both your tests were effectively at infinity, and they do show that the AF is probably the factor that is completely wrong. There can often be a small difference between viewfinder and Live View, nothing to worry about compared to the huge difference in AF. GX20 Live View is very poor resolution compared to the K7/K5.

One more thing to examine. Go into the menu and select 'setup','sensor cleaning' and use it to flip the mirror up. With some good light have a look at where the AF module sits in the bottom of the lens throat. This is just to be sure that there isn't something like a big bit of fluff obstructing the view of the AF sensors, or maybe scratches or physical damage. A long shot.

Apart from that all I can suggest is that you try to get the camera serviced, but whether there is anywhere in Romania is a different matter... Or just use MF

(Edited to add: the position of the AF assembly can be adjusted with the bottom plate of the body removed. Since your AF is already broken there is probably little to lose in trying this, or getting somebody else to do it for you. I don't know whether the available adjustment would be enough, or whether it would work at all).

Last edited by karma mechanic; 10-07-2011 at 03:38 AM.
10-07-2011, 01:23 PM   #15
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This post is not any help at all, but that has to be the worst focus test chart ever made.
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