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Old Aug 20, 2006, 7:52 PM   #1
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I have come to the conclusion that auto focus on the *ist DS is really useless. It spends all its time running to the end of travel then stalling or running right past the focus point.



This is with the Pentax kit lenses.



Does anyone have a good word that could change my opinion of this "feature"?
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Old Aug 20, 2006, 8:06 PM   #2
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Hi John

I also Have the DS and Do Not have that issue normally. I Occsaionally Do using my Sigma 70-300 while Shooting at an Airshow and i miss the plane. I cannot really Blame the camera as there is nothing to focus on in a clear sky.
My gut feeling is if this happens all the time and with Both lenses, you might want to Contact Pentax and see what they say

Phil

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Old Aug 20, 2006, 8:08 PM   #3
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I never have that problem either with my DS and the kit lens unless it is very dark out, so you may have an issue with your DS.

Tom
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Old Aug 20, 2006, 9:37 PM   #4
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Hi John,

Sometimes knowing how something works makes the difference. I have friends who have recently taken up digital photography and are sometimes frustrated by the technology. I found that once I discovered how the system worked, I had much more success with AF, and my friends have generally come to the same conclusion.

AF is a case in point. On your DS, in the menu, you can set the camera so it determines the focus point, or you set it manually, or so it just uses the center point. I would suggest the last setting so you can concentrate on the focusing point more easily.

The AF system looks for a line of contrast to determine if that point is in focus. The focus sensor is a "+" type so should be able to work with either vertical or horizontal contrast lines (most p&s cameras just use "I" shaped sensors, so look for vertical lines only). If there are no contrast lines, as in a cloudless sky or a blank wall, it will usually refuse to focus -- so manual focus is really your only choice. Specific situations to illustrate the point would be:

In the case of a head shot type portrait, get you focus on the eye, then reframe to center the face since the cheeks and nose usually don't supply the contrast necessary for reliable focus (and you really don't want to focus on the nose anyway).

In the case of moon shots, get the focus point to the edge of the moon instead of the center where there's no real contrast.

As light levels get lower, the AF system has a harder time "seeing" the contrast it needs to determine focus, so sometimes you have to help it by trying to center the focus sensor on an area where the contrast is more defined. In lower light you also have to hold the camera more steadily, especially when using longer focal length lenses, as the normal swaying of the body will allow the contrast area to move in and out of the focus sensor's effective area without giving it time to lock on something. In worst case situations, you might have to rotate the camera to get an off-angle contrast line aligned with the camera's vertical or horizontal, then once the focus is locked, rotate the camera back and take the shot.

In very low light, faster lenses will usually focus better as they allow more light through the lens, but sometimes the only alternative you have is to supply some light. I personally don't find the onboard flash "buzzing" all that great, and prefer the AF assist lights available on some external flashes, but if it's all you have, the onboard flash can help in this function. Another option is to get a small keyring LED flashlight and either hand hold it with the left hand, or temporarily attach it to the side or top of the camera (I use a small dot of the adhesive putty usually used to stick posters to walls).

That's about it for tips to make AF work for you. If you try all of this, and find your DS still doesn't AF, then I'd first suggest you try some other lenses, and if it still doesn't work, then the camera is probably at fault

Hope that might help.

Scott




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Old Aug 20, 2006, 9:59 PM   #5
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Thanks for the advice everyone, there are good points there I will need to study just a little.

Since opening this topic I went for a long walk beside the river (I left the camera at home though!) thinking about what could be the issues.

One of the things with my camera is that there are a few dust specks that show in the viewfinder, now I have been of a mind that I should leave well enough alone so I have never tried to find and clean these specks as they do not show in the captured images.

Potentially though they could be messing up the auto focus? Maybe? My little rubber bulb air blower did not dislodge them all, there is one little speck in particular right close to the middle of the view I would like to eliminate but I cant decide which surface it is on. Releasing the lens lock and gently turning the lens while looking in the view finder confirms this speck is in the camera but where? On the mirror? On the focus screen? If it is on the mirror can I clean that with anything other than air? What about the focus screen, is it OK to use lens cleaner swabs on that?



Thanks.


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Old Aug 20, 2006, 10:07 PM   #6
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John, did you walk by the Hutt River? Just a guess, but my wife is a Kiwi and we lived there for 13 years - loved it. You have an endless wonder of photographic opportunities there in Aotearoa.

I found this following thread extremely helpful, particularly the tips that Ira shared - don't know if you saw it, but it is well worth reviewing, because it has been a real help. The tips on alternative focusing are very good.

I was also impressed with Scott's information. Very well written, and if I can understand it, most anyone else can - thanks Scott.

http://www.stevesforums.com/forums/v...mp;forum_id=80

John Hill wrote:
Quote:
Thanks for the advice everyone, there are good points there I will need to study just a little.

Since opening this topic I went for a long walk beside the river (I left the camera at home though!) thinking about what could be the issues.

One of the things with my camera is that there are a few dust specks that show in the viewfinder, now I have been of a mind that I should leave well enough alone so I have never tried to find and clean these specks as they do not show in the captured images.

Potentially though they could be messing up the auto focus? Maybe? My little rubber bulb air blower did not dislodge them all, there is one little speck in particular right close to the middle of the view I would like to eliminate but I cant decide which surface it is on. Releasing the lens lock and gently turning the lens while looking in the view finder confirms this speck is in the camera but where? On the mirror? On the focus screen? If it is on the mirror can I clean that with anything other than air? What about the focus screen, is it OK to use lens cleaner swabs on that?



Thanks.

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Old Aug 20, 2006, 10:13 PM   #7
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I have had the same problem with my 55-200 Sigma Lens (very cheap lens) at full zoom.



I upgraded to a Sigma 70-200 f/2.8 APO and problem solved.



It was extremely frustrating, I missed out on a lot of shots (read $$$$) becuase of the focusing issue that particular lens had.
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Old Aug 20, 2006, 10:27 PM   #8
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Autofocus is also affected by the strength/charge of your batteries. When my DS begins to "hunt", I know it's time to put in a fresh set.
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Old Aug 20, 2006, 10:34 PM   #9
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Thank you, Scott, for your detailed post - it tracks with my experience. There have been several times when I've been trying to photograph sunsets and the sky doesn't have quite enough contrast. I've solved this by pointing the camera to the top of the mountain across the valley from me (a mile away) and have never had a problem getting it to focus.

The other thing I've found is that keeping it on center focus works better for me - my past experience was with a DS so I'm used to it. When I've used the matrix, it will sometimes focus on the wrong thing and I may or may not notice (I do tend to go into P&S mode for certain subjects).
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Old Aug 20, 2006, 11:07 PM   #10
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Quote:
John, did you walk by the Hutt River? Just a guess, but my wife is a Kiwi and we lived there for 13 years - loved it. You have an endless wonder of photographic opportunities there in Aotearoa.


Yes, our office is in Lower Hutt City, I can walk upstream which is a really nice area to walk through then cross over and come back via the shopping area. Or I can walk down stream. Upstream the only wildlife is sea gulls (about 3 varieties) and dull looking sparrows! Downstream however takes me through a golf course and an area where there are tribes of feral geese and a pair or two of paradise ducks, a few tuis in the trees too but they are very hard to capture always staying in the shadow and quite high in the trees, one day though I will get one! Unfortunately thats about a two hour round trip and I dont get down there so often.
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