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07-20-2010, 02:52 AM   #1
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Aha..my K20 cannot MF to Infinity..

Dear fellow Pentaxians,

I seemed to have been using my now almost 2 year old K20 without any problems for my type of photography.. until recently while doing some more landscape work and focusing for that purpose to infinity Manually I noticed that in the images the camera produced the apparent zone of best sharpness anywhere BUT infinity..

So I started testing.. The slice of pic attached here is taken on a sunny day with a camera with an FA50mm f1,4 on f4 set Manually precisely on a Infinity mark, cam locked on a tripod, MLU, SR-off, no filter on lens.

(I hope the picture shows up correctly)..

http://www.flickr.com/photos/tmt79/4811915726/sizes/l/

As it appears the dof of perfect sharpness lies somewhere in a zone from around 3 to 8 meters from the cam.. Remember, the picture was taken in the MF mode. Now the camera is AF focus-corrected to -6 (my other AF lenses also need around -5 to -6 correction!) and it focuses perfectly on the subjects in a close to medium distances. But again it had nothing to do with AF..

I've retested it numerous times with my other lens (FA20f2,8 FA35f2 DAstr.16-50, and A70-200f4) at various apertures and various settings. After observing images
the distance of zone of sharpness and its depth varies according to the focal lengths but I found NONE of my lenses to focus properly to infinity...Except perhaps my A70-200 which has a renowned focusing-zooming ring looseness and can apparently focus beyond the infinity.

So, what do you people think of this? Does anybody think my cam should be taken to repairs? Its sensor "shimmed" if at all possible??
Maybe somebody already had/has such a close-focus K20?

Does Your Pentax really focuses correctly to infinity...?

Any thoughs are extremely welcome.
Tom

07-20-2010, 03:24 AM   #2
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My K-7 could not focus to infinity with the FA 50 f/1.4. It was only noticeable with apertures wider than f/2.8. Since I almost never shoot at such large apertures at distant objects, I never found out earlier.
Stopped down, the DOF took care of that.

The lens AF correction custom setting couldn't solve the issue.
The FA 50 had been "perfect" on my old K100D, so I was pretty sure it wasn't a lens problem.

I sent back both camera and lens back to the Pentax distributor. The camera needed a physical adjustment, fortunately still under warranty.
I'm not too sure what they did to it, but it focuses very accurately on MF and AF now.

Last edited by kittykat46; 07-20-2010 at 03:30 AM.
07-20-2010, 03:50 AM   #3
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Thank you kittykat46 for sharing your experience.
Unfortunately my K20 is out of the warranty period, but it seems that nothing could really be done by a user to correct the issue..
I am considering sending it in for service, its just I'm curious at what price since I'm sort of reluctant to sending my precious FA50 along.. Although I understand It would probably be needed for replication of the problem and adjustments.
07-20-2010, 04:34 AM   #4
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QuoteOriginally posted by tmt79 Quote
... FA50mm f1,4 on f4 set Manually precisely on a Infinity mark
...
Any thoughs are extremely welcome.
Well, my first thought is that this infinity mark is not very accurate anyway. Can you rotate the ring any further? And does is autofocus at infinity? maybe I didn't read carefully enough, but I can't really get that out of your post.

07-20-2010, 05:22 AM   #5
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QuoteOriginally posted by urje Quote
Well, my first thought is that this infinity mark is not very accurate anyway. Can you rotate the ring any further? And does is autofocus at infinity? maybe I didn't read carefully enough, but I can't really get that out of your post.
Dear Urje,

thank you for your thoughts, I also think the infinity mark probably is not that accurate. The AF series were designed and produced for the film cameras, and maybe there could be some incompatibility - I by no means have sufficient knowledge to understand that.

No, the focus ring abruptly stops at the infinity mark, and I do not want to force the rotation any further as I'm afraid I might damage the lens..
The lens physically autofocuses to infinity and I get the AF confirmation, but when examining the pictures I very clearly see the in-focus zone to be closer than in infinity, more so when the lens is set to wider apertures (f2,8, f2).

Anyway, I am thinking of sending it in for servicing..I hope it will not cost me a lot..
Thanks for your input,
T
07-20-2010, 07:01 AM   #6
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I think you need to do more research on DOF and Hyperfocal distance, I don't think you really understand them. DOF at f4 is still fairly shallow. Try stopping down more and see what happens.
07-20-2010, 07:08 AM   #7
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It's likely that there is an adjustment shim behind the lens mount that needs to be replaced with a thinner one.
Obviously, stopping down will bring infinity into focus, but this isn't the same thing as being focused to infinity, which with your 50/1.4 should probably be around 50' or so.
07-20-2010, 09:26 AM   #8
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Generally, it is lenses that focus, not cameras. While there is the possibility of bad shimming, I think I would try your test with another lens before I fiddled with the body.


Steve

07-20-2010, 01:13 PM   #9
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Pretty much all AF lenses were adjusted to focus past infinity due to their looseness and imprecise nature. You really need to focus based on the focus screen to be certain the camera was faulty, if you haven't done so.
07-20-2010, 01:41 PM   #10
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I am sure that the focus stop in your 50/1.4 is out of adjustment. This problem should not occur. When you set the lens to the infinity mark, you should get focus somewhere from 17 or 18 meters out to the horsehead nebula.

With long lenses, such as my 400, you cannot rely on the infinity mark because the lens is designed to focus to infinity even when temperatures cause the lens to not be able to focus to the design distances. For that reason, lens designers allow focusing past the infinity mark so that you can always focus all the way out there.

Your problem does not fall into this class of trouble at all. The focus ring is not set properly. Have you checked at measured distances to see if the nearer focus marks are somewhere close to the actual distance? You have two possibilities with your lens. The first is that the infinity stop is not in the correct position. The second is that your focus ring itself is not in the correct position. The nearer focus distance check might be able to tell you which problem your lens has.

In any event, the problem can be repaired, as it is a simple adjustment problem. It has absolutely nothing to do with which camera you have it on, because you are scale focusing. I suppose it is remotely possible that the sensor is not at the correct registration distance from the mount, but that is something I have never heard of.
07-20-2010, 04:08 PM   #11
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Funny you should mention this issue. I have been dealing with exactly the same thing every since I dropped my K20D from a few feet....twice. The DA*300 was on it both times and it is fine. But the lens mount came off both times. The screws were reluctant to grab onto anything after being yanked out so violently. I ended up gluing them in. Holding fine so far.

But there were two consequences: I can no longer focus to infinity with any lens *except* the DA*300 (it is made to focus past infinity which gives it enough room to focus in this case); and the stop-down lever is now stuck so it is back to manual mode. So I have a close-focus manual camera for the most part. In bright enough light I can use small apertures and let DOF make up for the far focus problem.

The focal plane has shifted somehow. Its is almost as if I have stuck a very narrow extension tube on the mount. But it is flush with the front of the lens mount as it should be. Oh well, I will wait for the next Pentax DSLR iteration rather than invest repair money in this one which is way out of warranty anyway.

The one benefit in this is that it drove me back to my film cameras. Nice to re-experience the benefits of silver halide over digital.

Jack
07-20-2010, 06:28 PM   #12
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QuoteOriginally posted by stevebrot Quote
Generally, it is lenses that focus, not cameras. While there is the possibility of bad shimming, I think I would try your test with another lens before I fiddled with the body.


Steve
He did mention that none of his lenses hit infinity. I wouldn't fiddle with the body either, but I'd consider sending it in for calibration.
07-20-2010, 07:11 PM   #13
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QuoteOriginally posted by Wheatfield Quote
He did mention that none of his lenses hit infinity. I wouldn't fiddle with the body either, but I'd consider sending it in for calibration.
I must have been suffering from fact blindness when I read the OP...

Thanks for pointing the obvious out. This is a pretty strange story overall. I mentioned it to the camera repair guy earlier today when I dropped off some film and he also thought that it would be unusual to have the fault be with the body. Perhaps the camera has been dropped or the victim of a botched repair. (Remember the thread a while back where the mount screws were stripped?)


Steve
07-20-2010, 10:45 PM   #14
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QuoteOriginally posted by stevebrot Quote
I must have been suffering from fact blindness when I read the OP...

Thanks for pointing the obvious out. This is a pretty strange story overall. I mentioned it to the camera repair guy earlier today when I dropped off some film and he also thought that it would be unusual to have the fault be with the body. Perhaps the camera has been dropped or the victim of a botched repair. (Remember the thread a while back where the mount screws were stripped?)


Steve
Thank you for the comment, Stevebrot.
It is strange body-lens behaviour indeed. As I mentioned in my OP I have tried manually focusing all my other AF lenses as well.. the focal plane was definately infront of what should be the infinity. When I try to MF a bit closer than infinity-the dof gets to a closer zone as well. But I am able to focus my A70-200 to infinity correctly (I suppose this is possible because of its ability to focus past infinity). Of course when I set the aperture to say f8 - f9, the DOF corrects the issue.
The body was never dropped, and never serviced (as far as I know) - bought about 2 y. ago from B&H.
I'll probably go for servicing the cam/lens anyway, I'll also go back to my film equipment during that time..I love film btw.
Thanks everyone for your comments.
T.
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