Pentax *istDs autofocus problem

[richard_ross1]

I have been using an *istDs for a week now and have come to the

conclusion that the autofocus is off by 1/2 to 1 inch, locking focus

closer than the subject. I am shooting portraits at wide apertures and

am definitely missing focus. I have since done some testing by

shooting a ruler at an angle and verified that it is not the lens.

 

Have other people had this issue with the Pentax DSLRs (or other

autofocus SLRS for that matter.) B&H will exchange the camera but I do

not want to be without for a week and half and end up with a body with

the same or similar problem.

[godfrey]

You don't mention with what lenses and apertures you are finding this to be a problem.

 

I have no problem with the AF on my *ist DS. AF on any camera has limitations, however,

and when you're using a very fast lens at close distances wide open, no AF system can

truly know what it is you want to focus on specifically. I normally switch my camera to

manual focus (or use a manual focus lens) when I'm doing portraiture with a 50/1.4 or

135/2.8 lens wide open as I need to be in explicit control of the critical focus point.

 

With slower lenses of shorter focal length, I find I need to switch to manual focus less

frequently ... depth of field covers a variety of issues, including making critical focus less

critical.

 

Godfrey

[mitchell_kirschner]

I haven't given my *ist DS a full workout yet, but so far autofocus seems to be working correctly with the kit lens (the only lens I have so far). I also just did some close up quick and dirty ruler tests @ f/4.5 (widest aperture on kit lens), which seemed to verify correct functioning.

 

Are you using a Pentax lens or some other brand?

 

Firmware version is probably irrelevant but you might want to upgrade to v1.02 just to see.

 

Please keep us posted as you learn more.

 

Mitch

[mdickerson]

There was a thread about a different camera with focus problems a couple days ago; the consensus there seemed to be that errors of a few inches at wide apertures are normal for AF. I haven't noticed on mine, since (like Godfrey) I would be using manual focus in that situation.

[richard_ross1]

I am using the Pentax kit lens 18-55, Tokina ATX/Pro (AF) 28-80/2.8 and Pentax A50/1.7 (MF). I have experienced the problem mostly at short distances (but also with portraits at a few feet) and large aperture. Focus locks over a short range of distances, which seems to be skewed on the near side of the subject, and the two AF lenses lock in focus on the near end of that range.

 

This is my first AF camera so I don't know if these are normal tolerances and if I am expecting too much here.

[jcuknz]

It could be worth experimenting with AF on 'spot mode' if you have that and pointing the focus area onto say the ear of the subject while holding half trigger and then once focus is set re-compose for the shot.

This test, if the ear is sharp, should indicate the camera is not at fault but it is an operator problem ... you could also use the eye as the target area.

Most casmera problems come from the operator not understanding how this modern gear should be used. It is quite different from film.

[richard_ross1]

I have done some more testing and real world shooting. I think what I am finding is what the guy at B&H mentioned, a slight 'backfocus' problem, that I now find was widely discussed with regards to both the Nikon and Canon DSLRs. Mine is off just slightly so is probably well within manufacturing tolerances. I suspect that exchanging the body is not going to be a wise move. If it really bothers me in the coming months, perhaps I will ask to have Pentax adjust it. Is Pentax service good about this sort of thing, or will they tell me to stop whining?

[rod c.]

I am an ist* D owner and just happened to scan by your question in this forum and can contribute some info on your problem. Item 1) when taking close-up images and using wide aperture it is important to " square up " to the subject if possible. If you don't and are using auto zoning on the auto focus, it can easily shift focus points before you take the picture, thus skewing the shot. Try either manual focus or single point auto focus. Item 2) Focal lengths like 2.8 and under are nice, but it usually isn't the best quality rendering for a lens as far as being sharp. Why not experiment and see if 2.0 or 3.5 will suffice.

One more thing that occurs to me is part of the trouble may not be a lack of accuracy, but a very high amount of it instead. If, for example, you wear bifocal glasses like me, you find out really fast, like I did, that the ist*D can see a lot better sometimes than the operator.

 

Just some thoughts I had. I hope they will help.