Help with AF with Pentax 55-300 mm


aminstar

Link Posted 05/03/2009 - 12:40
Hi everyone

I came across this problem just by chance while trying to focus on the edge of a tree trunk.

What I suddenly realised was that it wasn't locking on unless I aimed slightly lower to the edge with the red focusing dot on my viewfinder, then it would lock and snap. And suddenly I found that the part of the tree trunk I was aiming to focus at in the beginning came out pretty sharper although that wasn't where I aimed at.

Then I decided to carry out some tests and this is what I found out.
So I am hoping someone could give me some idea of what is happening and what I need to do. I suspect this might require me digging in further into the manual but it would be great help and steping stone if someone could just briefly explain me what is going on.

All the images that are out of focus were aimed at the flower. Spot on in the middle of the flower and the images came out all out of focus.

The images that came sharper were all aimed at the bottom of the flower, right where the left leaf has grown. And I notice that these came out with the flowers sharpened.

I think this is an issue only with my 55-300 as the 100mm macro is the other one that I tend to use but I haven't noticed anything wrong with it. Having said that, I wasn't looking for any faults so I will go and check for the AF with that too.

Any help and advice is greatly appreciated

All images are 100% crop. To be sure that it wasn't just hand shake or breeze moving the flower I took 3 snaps at each focusing spot using the tripod






Amin Photo Gallery
Last Edited by aminstar on 05/03/2009 - 13:24

iceblinker

Link Posted 05/03/2009 - 13:06
I'm not sure what's going on in this particular case, so I'll just make a couple of general points.

1. Focus points can be slightly differently positioned from how they are indcated in the viewfinder.

2. Sometimes you have to find the edge of an object or a different part of the subject to get sufficient contrast for AF to work.

First of all, eliminate front/back focusing problems by testing with flat subjects.
~Pete

Anvh

Link Posted 06/03/2009 - 00:23
Amin I readed in a comment directed at me under the robin photo of Mike that you found the problem so what was it?
Stefan


K10D, K5
DA* 16-50, DA* 50-135, D-FA 100 Macro, DA 40 Ltd, DA 18-55
AF-540FGZ

bforbes

Link Posted 06/03/2009 - 00:51
I'm not sure if this is what you are describing amin.

This was taken with a 70-200 tamron. i was pointing at the perching birds but the picture seams focused on the lettering on the building to the left. i think what i did wrong was to leave the auto focus set to auto select the focus point. It's gone for the area of highest contrast and not what i was aiming at. i've tried other shots with the auto focus selection point set to the center of the frame and had no more trouble.



Barrie
Too Old To Die Young

http://www.pentaxphotogallery.com/artists/barrieforbes
https://www.flickr.com/photos/189482630@N03/

Anvh

Link Posted 06/03/2009 - 00:55
In the first shot Amin the branch in the upper right corner seems in focus but moved, could it be that you/the camera focused on that?
Stefan


K10D, K5
DA* 16-50, DA* 50-135, D-FA 100 Macro, DA 40 Ltd, DA 18-55
AF-540FGZ

aminstar

Link Posted 06/03/2009 - 07:02
Hi again,

This is really really strange, and I really dont have any logical explanation for this.

After reading page 106 of the manual and getting ready to adjust my AF, I got my stuff ready including my laptop to shoot tethered and make adjustments, put my camera on a tripod again and aimed at that flower. Position of the camera precisely at the same spot, angle, same setting, everything. Aimed at the flower again at the long end of the lens like I had done with the above images and the image came focused when I checked. Repeated this several times and none of the images came out of focus like the above one.


Barry,
I am using centre focus, tend to keep my camera always with that setting.


Then decided to aim at the top edge of a tree trunk like I had done before and which had prompted me to think that there was something wrong in the first place as it just wouldn't lock unless I aimed a few good inches lower, and again surprise as this time it was locking. I even managed to lock on a small splinter which was sticking up from the cut tree trunk which had no other background around it except clear sky, and the image came out sharply focused.

So I decided not to mess with the AF and left as it is.

All I had done was change from AF.C to AF.S and tried these shots, so thought I should go back to AF.C and check if this was the culprit, but this time all the images were sharp again at whatever I was aiming to focus.

Any explanation please? Or this could have been just a one off or one of those situations that electronic stuff sometimes can just have one of those moments and hard to explain.

Does this seem something to do with the lens (55-300) or with the camera itself?

Thanks

Amin
Amin Photo Gallery
Last Edited by aminstar on 06/03/2009 - 07:04

johnriley

Link Posted 06/03/2009 - 09:22
Autofocus is not infallible.

You can see in the viewfinder whether or not you are focused correctly, so with practice you shouldn't miss many.
Best regards, John

aminstar

Link Posted 06/03/2009 - 09:26
johnriley wrote:
Autofocus is not infallible.

You can see in the viewfinder whether or not you are focused correctly, so with practice you shouldn't miss many.

Thanks John

That's very kind of you explaining me that. I wasn't aware of that fact.

So in other words what happened is normal, sometimes a situation like that can occur and it doesn't mean that I have to readjust any settings? Specially after I noticed within a few minutes that it was focusing again pretty accurately?

Thanks

Amin
Amin Photo Gallery

johnriley

Link Posted 06/03/2009 - 09:30
Absolutely not a problem. Sometimes the system is fooled by something and we just have to reframe a little and try again.
Best regards, John

iceblinker

Link Posted 06/03/2009 - 10:02
Amin, The camera would behave the same way each time if everything really was exactly the same each time with the subject and camera. But the smallest change of aim or light can make the camera behave differently.

You may think you're aiming at the same part of the flower, but in fact have moved the aim a tiny weeny bit (this can even happen on a tripod), or the light has changed enough to affect contrast.

Just try again (preferably on a different part of the subject) if it looks like the AF has failed to focus on what you want. It's only when it repeatedly fails after several attempts with many subjects that you should suspect a problem with the equipment.
~Pete

aminstar

Link Posted 06/03/2009 - 10:09
iceblinker wrote:
Amin, The camera would behave the same way each time if everything really was exactly the same each time with the subject and camera. But the smallest change of aim or light can make the camera behave differently.

You may think you're aiming at the same part of the flower, but in fact have moved the aim a tiny weeny bit (this can even happen on a tripod), or the light has changed enough to affect contrast.

Just try again (preferably on a different part of the subject) if it looks like the AF has failed to focus on what you want. It's only when it repeatedly fails after several attempts with many subjects that you should suspect a problem with the equipment.

Pete

Thanks, thats very kind of you to really explain in detail, and reading what you wrote something tells me now that light must have been the culprit here.

I am much more comfortable now knowing that these issues are normal and I do not have to tweak anything. Its a daunting task for me at this stage .

Thanks very much for all your kind help
Amin Photo Gallery
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