Go Back   Steve's Digicams Forums > Digital SLR and Interchangeable Lens Cameras > Pentax / Samsung dSLR, K Mount Mirrorless

Reply
 
Thread Tools Search this Thread
Old Feb 27, 2006, 3:55 AM   #1
Senior Member
 
Join Date: Feb 2006
Posts: 150
Default

Hi all,

Recently I bought *ist DS with 16-45 F4 lens.

On weekend I was making some indoor photos (using AF-360FGZ) and noticed that my *ist DS has tendency to backfocus (15-30cm behind autofocus pointed object). It is not constant, however it happens quite often (~30% of shots).

Is it my camera's inaccurate autofocus system, is it my lense?

Unfortunately I can't test my camera with different lens (16-45 F4 is the only Pentax lense I have) and I can't give my camera to dealer for inspection -- I will travel in a couple of days and I really need my camera with me.

Is anyone here who experienced the same problems as me?

--
Edvinas


Edvinas is offline   Reply With Quote
Sponsored Links
Old Feb 27, 2006, 7:02 AM   #2
Senior Member
 
errno_gmm's Avatar
 
Join Date: Feb 2005
Posts: 518
Default

Try centre point AF only, if you are still having an issue your AF may be misaligned. Ive never heard of the DS having backfocus issues
errno_gmm is offline   Reply With Quote
Old Feb 27, 2006, 7:30 AM   #3
Senior Member
 
Join Date: Feb 2006
Posts: 150
Default

I was shooting using centre AF point all the time.

Also I've noticed that sometimes wrong focal length in EXIF information is displayed. Can't this be realated with backfocus?

--
Edvinas
Edvinas is offline   Reply With Quote
Old Mar 1, 2006, 4:01 AM   #4
Senior Member
 
Join Date: Jan 2006
Posts: 344
Default

> On weekend I was making some indoor photos (using AF-360FGZ) and noticed that my *ist DS has tendency to backfocus (15-30cm behind autofocus pointed object). It is not constant, however it happens quite often (~30% of shots).

Do you experience the same problem under normal daylight (not on overcast day)?

The BF and FF problem is actually dependent on the color temperature of the light source, AFAIK:-

http://www.geocities.com/ricehigh/DS...cy_on_KEV.html

> Is it my camera's inaccurate autofocus system, is it my lense?

My DA 16-45 is yet the most accurate focusing lens amongst my 12 original Pentax AF lenses including F, FA, FA*, FA Limited, DFA and DA lenses, under normal daylight. So, I bet it is the body :-) Wild guess, though.

> Unfortunately I can't test my camera with different lens (16-45 F4 is the only Pentax lense I have) and I can't give my camera to dealer for inspection -- I will travel in a couple of days and I really need my camera with me.

Again, can you accept the AF accuracy for shooting outdoor and with aperture closed down?

> Is anyone here who experienced the same problems as me?

Do a search on the net and it is not difficult for you to find more cases.

RiceHigh
http://www.geocities.com/ricehigh
RiceHigh is offline   Reply With Quote
Old Mar 1, 2006, 11:08 AM   #5
Administrator
 
Join Date: Jun 2003
Location: Savannah, GA (USA)
Posts: 22,378
Default

Make sure you're not contributing to the problem, too. I tend to see more BF problems reported with wider zooms from owners of DSLR models from other manufacturers.


http://www.mhohner.de/recompose.php?lang=e

http://www.outbackphoto.com/workshop...y06/essay.html


JimC is offline   Reply With Quote
Old Mar 1, 2006, 2:17 PM   #6
Senior Member
 
Monza76's Avatar
 
Join Date: Jul 2004
Posts: 4,095
Default

Thanks for the links JimC, I think we may be making a lot of an issue that is a little more universal than most people think. I have seen items on AF inaccuracy in the KM forum and the Olympus DSLR forum as well (along with a lot of talk about the Fuji S9000/9500 digicam). The only reason I have not heard more from Canon and Nikon is probably because of the fact that I stay off the Canon and Nikon forums. From experience I must say that the 20D I used was no more accurate than my DL.

Personally my biggest peave is with the focus of manual focus lenses, I have so much trouble getting it right without a visible focus aid (the little green light doesn't count).

Ira
Monza76 is offline   Reply With Quote
Old Mar 1, 2006, 2:47 PM   #7
Administrator
 
Join Date: Jun 2003
Location: Savannah, GA (USA)
Posts: 22,378
Default

I had to chuckle when I read the second article when he explains the problem with locking focus and reframing (at least in some conditons). Of course, that's going to depend on things like lens design, flatness of focus plain, focus distance, aperture, etc.).

Discussing recomposing:

Quote:
There's just one problem with this idea: it is 100% guaranteed to cause focus to be behind the intended center of focus.

http://www.outbackphoto.com/workshop...y06/essay.html
Personally, I've gotten to where I try to focus on the eyes of my intended subject with the closest focus point to miinmize any reframing with my KM Maxxum 5D. It seems helps with the "hit rate" versus using the center focus point to lock and reframe.

In very low light it gets tougher, trying to get the sensor you want to use for AF on a high contrast area that's closest in distance to your subject.


JimC is offline   Reply With Quote
Old Mar 1, 2006, 9:35 PM   #8
Senior Member
 
Join Date: Jan 2006
Posts: 344
Default

Here is another essay on the same "suspected" issue:-

http://visual-vacations.com/Photogra...pose_sucks.htm

Theoretically, it should be correct but I really doubt the significancy of this effect actually. I also second with JimC that for the point the idealness of the focal plane flatness, of the lens' optical design, does come into play too.

I myself found that in real practice, a SLR with accurate AF system will have no such error in an obvious manner no matter I recomposed or not, even with a fast prime tele lens like my FA* 85/1.4, say, on my MZ-S. I could say the whole case boils down to AF system accuracy and performance afterall.

RiceHigh
http://www.geocities.com/ricehigh
RiceHigh is offline   Reply With Quote
Old Mar 1, 2006, 9:49 PM   #9
Administrator
 
Join Date: Jun 2003
Location: Savannah, GA (USA)
Posts: 22,378
Default

It's not just the reframing, either.

If you have a habit of locking focus with a half press and lean any when shooting a subject where you've got very shallow DOF, you may get an out of focus image from leaning (changing your distance to your focus point, even if you don't reframe).

It doesn't take much if you're shooting at larger apertures and closer ranges (or filling the frame with many subjects using a longer focal length).

I like to shoot in dimly lit clubs with my KM 5D, and when shooting at wide open apertures with a bright prime, it can be a challenge due to a shallower depth of field for closer subjects. So, by using the focus point nearest to my subjects eyes, I minimize any camera movement before taking the shot.





JimC is offline   Reply With Quote
Old Mar 2, 2006, 1:09 AM   #10
Senior Member
 
Join Date: Jan 2006
Posts: 344
Default

> If you have a habit of locking focus with a half press and lean any when shooting a subject where you've got very shallow DOF, you may get an out of focus image from leaning (changing your distance to your focus point, even if you don't reframe).

Well, we should not move much after focusing anyway, right? :-)

> It doesn't take much if you're shooting at larger apertures and closer ranges (or filling the frame with many subjects using a longer focal length).

I do like to re-compose much when I shoot portraits and I like to use large aperture for some shots at f/1.4 or f/2.

> I like to shoot in dimly lit clubs with my KM 5D, and when shooting at wide open apertures with a bright prime, it can be a challenge due to a shallower depth of field for closer subjects. So, by using the focus point nearest to my subjects eyes, I minimize any camera movement before taking the shot.

I have another thought. Isn' t that at very shallow DoF, the AF system should be able to distinguish difference better and easier?

RiceHigh
http://www.geocities.com/ricehigh
RiceHigh is offline   Reply With Quote
 
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off



All times are GMT -5. The time now is 1:40 AM.