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07-10-2008, 04:18 PM   #16
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QuoteOriginally posted by ckanthon Quote
Sorry for uncleared explanation of my story. (OK no SDM there)

1) at wide angle 24mm and 18mm. F/3.5 and F/4. shot at "distant" objects far away... The AF will stop beyond the infinite mark (see attached picture with yellow arrow) and it was possible to hit the end of the focus ring. AF stopped there and AF just takes action and pictures taken.

2) when Aperature decreases to F/8, the AF stop at around the infinite mark. no above issue.

3) Yes, for my kit lens, I noticed that the focus depended on the f-stop. When F/3.5, the focus is longer than at F/8. That was why the focus ring gone beyound the infinite mark when shot at F/3.5 or F/4. (I dont know why...)
I'm having a hard time understanding the problem except it seems "at some times" the AF will accidently hit the hard stop. Prior to your servicing for a backfocus issue this was NOT a problem.
Also you are a bit bothered by the fact that sometimes the infinity mark is correct and sometimes not.
The infinity stop is adjustable I believe so the lens could be "fixed"
Lenses also change properties due to heat/cold expansion as well. One of the reasons infinity is not infinity and lenses are painted white.
Luminous Landscape Forum > Infinity focus

07-10-2008, 05:51 PM   #17
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QuoteOriginally posted by jeffkrol Quote
I'm having a hard time understanding the problem except it seems "at some times" the AF will accidently hit the hard stop. Prior to your servicing for a backfocus issue this was NOT a problem.
Also you are a bit bothered by the fact that sometimes the infinity mark is correct and sometimes not.

.......
Thank you very much for your inputs and information.

A) In fact, At wide angle settings, when shot at distant objects, Aperature wide open (F3.5, F4, F5.6 etc), the AF always hits the hard stop and stary there.
Thing only goes much normal when F-stop down to F8. And there is not problem at 35mm, 55mm lenght.

B) Before sending the K200d kit to Jebsan for focus fine-tune service, the K200d kit was Front Focused without hitting a hard stop during AF

Thanks again for caring this matter

Last edited by ckanthon; 07-10-2008 at 08:32 PM.
07-10-2008, 08:13 PM   #18
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That, and...

QuoteOriginally posted by MRRiley Quote
I believe the issue egordon99, that the OP is asking about is the fact that the lens appears to focus PAST infinity.

From what I understand this is a fairly common behaviour on modern AF lenses. I've heard it is intentional in order to insure that the AF motor does not run up against a hard stop when focusing to infinity.

I think the only people who notice this are folks who routinely focus manually. If you do so by feel and just 'turn it all the way to infinity" you will have over focussed and the pics will be a little soft. I've learned to focus to the stop then back off a tad with both my 16-45 and my Tamron 28-75. Seems the better the lens, the more likely you ae to see this "feature".
it is to allow for expansion and contraction in extremes of temperatures.

Every lens I have ever owned does that. Even old manual focus ones.

Cheers,
Cameron
07-11-2008, 04:33 AM   #19
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Its Physics, but also just an *inadequate* design.

Just because Pentax's AF design lack a "f/2.8" AF sensor. As you say, every lens, no matter how expensive it is, has spherical and aspherical errors and they are not perfect. As such, Pentax's AF system should have a f/2.8 sensor, at least for the more expensive models.

Here is the article which you will learn about why a f/2.8 sensor design will increase AF accuracy for faster lens:

http://doug.kerr.home.att.net/pumpkin/Split_Prism.pdf

And also why f/2.8 AF sensor cannot be used for slow lenses. So, they must have *both*.

Now that even the Canon 450D has a f/2.8 AF sensor, what Pentax have? (in none of their DSLRs, never)

QuoteOriginally posted by ckanthon Quote
Hi Gentlemen,

Thanks for your great advises as well as the linked information (which I quickly read thro').
Basically, I think what happened to Ben DA 21 also happened to my Kit Mk2. Except mine was Front Focus before the repair service
In fact, for very long time, I had decided to used DC "P&S" instead of using DSLR what's that I knew optics system(s) could be very tricky to deal with: Slightly mis-alignment with a little bit error could already ruin the system's performance because optics required accuracy in "one wave-lenght" levels. I learned this while I did laser projects in polyU time. Simple setup is better than system built with lens + mirror(s) etc....
I eventually switched to DSLR because I really started to feel not satisfact with my 7M pixels P&S whereas the latest P&S with higher M-pix seemed did not have real improvement on IQ (In fact, IQ of most of the Higher MegaPixels P&S really stepped backwards I realized). Obviously, now I get more after entering the "P" world but I still did not get what I supposed to....Ok....

Yes, I can confirm the "German guy's finding": focus (focal lenght of a piece of lens) depends on F-number. I can explain this phenomeon using my limited knowledge learnt in high school times.....It is caused by "lens imperfection". A real lens cannot be perfect in real life....In fact, when the Aperature is widely open, the focus at the centre of a piece of lens can NEVER be concidely overlapping with the focus point obtained at the "rim" of the same lens (not matter how high quality of the lens produced). This resulted that the "a real focus" of a bundle of parallel light from infinite, caught by a wide open lens, is in fact "an area" or in fact "a 3D oval region" rather than "a spacial spot". An "AL" or "ED" lens improved a little but not all.
But the "problem" here also caused by (or mixed complicately by) the characteristic of the AF sensor...how it responds to those images produced by non-ideal lenses.....

What I could say now is that I have a bit of BAD LUCK... that I already find a problem with my kit mk2.....not even before I buy a second piece of lens....In fact, the problem really not purely caused by the lens but by the "combined" error of the lens + the body when put them together..... List them: design error + illed tolerances + manufacturer QC + lack of final fine-tune + combined unceratinty when puttng them together + .. ..
I will cantact Jebsun again and decide whether I will buy more lenses to play with...!


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