[mutley]

go to the Canon Forums section. Announce the fact that you converted to "Pentaxcostilism"

yada yada yada Our photos are expressions of ourselves, not our camera brands.

Chris

I don't think he was THAT serious...

Larry

[silverbullet]

I'm just trying to figure out how to slip 'Pentaxcostilism' into a conversation! One of the best new words I've heard in some time.

Chris

p.s. I'm still not going to use it at DPR

[benjikan]

I'm just trying to figure out how to slip 'Pentaxcostilism' into a conversation! One of the best new words I've heard in some time.

Chris

p.s. I'm still not going to use it at DPR

Thanks, made it up after all the years dealing with Ad Agencies...

[benjikan]

Time for a dumb question, perhaps......
Somewhere I have read that the extreme left and right autofocus sensors are vertical only, not crosses as the remainder of the sensore are. This means that they will be susceptible to error for certain types of images since they will work best on horizontal lines (normal camera orientation) or vertical lines (with the camera orientation you shot). Given the location of the sensor, and the image you have of the subject, is it possible that the focus was not where you thought it was?

p.s. The only really dumb questions are those not asked, but which should have been!

I often quadrupled my focus confirmation before pressing the shutter. I will feel much better once Pentax does their bench tests and corrections...

[amcinroy]

Benjamin,

Did you ever get any further info from Pentax on this.

I took my 21mm out for it's first outing at the weekend and shot a lot of close up family shots at f3.2 using autofocus. When I got home I found that a lot of shots were randomly out of focus.

So I did a little test, just to make sure that my fast moving 2 year old wasn't to blame!

I shot a test card at 2m at F3.2 (tripod obviously). I took three shots of the card with the lens initially set at infinity before the autofocus kicked in and then another 3 shots of the card with the lens set at closest focus before autofocus kicked in. I took one further shot at f16 to compare with.

The three shots that the autofocus took from close focus were all out of focus. The three that the autofocus took from infinity were all fine.

I will be interested in how Pentax addresses this.

Andy
Landscapes of Great Britain and Ireland
Andy McInroy Photography - Landscape Photography of Great Britain and Ireland

[sharpshoota]

My K10D does this as well. With all my lenses. Even my FA 31mm LTD. I fell like selling the whole system and going back to Canon. I love the camera otherwise but the AF is just not dependable at all. If I'm getting paid to shoot something I have to have confidence in the focusing.

[benjikan]

My K10D does this as well. With all my lenses. Even my FA 31mm LTD. I fell like selling the whole system and going back to Canon. I love the camera otherwise but the AF is just not dependable at all. If I'm getting paid to shoot something I have to have confidence in the focusing.

I sent the last two posts directly to Pentax and asked to send to R&D at Pentax. We'll see what happens..

[BarneyCG]

My K10D does this as well. With all my lenses. Even my FA 31mm LTD. I fell like selling the whole system and going back to Canon. I love the camera otherwise but the AF is just not dependable at all. If I'm getting paid to shoot something I have to have confidence in the focusing.

Intresting to see that the latest top of the range Canon has a menu to allow the adjustment of the AF system

[amcinroy]

Yes Barney,

But in this case, I don't think this sort of adjustment is going to help us. My own tests have concluded that the accuracy of the autofocus depends on which side of focus (infinity or close focus) that you are approaching your focus point from.

Andy

[BarneyCG]

Yes Barney,

But in this case, I don't think this sort of adjustment is going to help us. My own tests have concluded that the accuracy of the autofocus depends on which side of focus (infinity or close focus) that you are approaching your focus point from.

Andy

Wouldn't that be due to the way that the DOF tends to extend further behind the desired focus point than it does in front of it so when focusing from infinity there is a little more leway ? In which case you'll see exactly the same problems no matter which manufacturer you pick.

[amcinroy]

Wouldn't that be due to the way that the DOF tends to extend further behind the desired focus point than it does in front of it so when focusing from infinity there is a little more leway ? In which case you'll see exactly the same problems no matter which manufacturer you pick.

In an ideal world, the autofocus would focus perfectly on your subject. But in the real world, inaccuracy will cause the autofocus to stop just in front or just behind your chosen focus point. In a well designed lens/camera system this inaccuracy should certainly be less than the depth of field (at maximum aperture).

I'm not sure that I fully understand your point, but I agree that the non-symmetric nature of depth of field is important. In my opinion, a front focusing system is preferential to a back focusing one because in a front focusing situation the larger depth of field behind will more likely bring your subject into the zone of acceptable sharpness.

In the case of my 21mm, the depth of field at maximum aperture appears to be shallower than the K10D autofocus accuracy. I also wonder if that is why they made this lens a f3.2 rather than f2.8, 2.4 etc. They perhaps knew that their autofocus accuracy was right at the limit at this fstop and short focal length and that we might not notice !! I can't say that I've seen a problem on my 14mm/f2.8, but this is not the sort of lens I use for close shots at maximum aperture.

I still don't understand why Benjamin is having trouble at f9, f14. There should be bags of depth of field at these apertures. Look at the sums (e.g 1m subject at f9 results in a depth of field of 0.95m (0.71m to 1.66m). Also, if the scene is out of focus at these apertures then surely you would notice it in the viewfinder before stopping down?

Andy

[sharpshoota]

I can't get into the technical aspects but my dissatisfaction also stems from the fact that the situations that this happens in are well lit and should be relatively easy to focus on.

[Denis]

I can't get into the technical aspects but my dissatisfaction also stems from the fact that the situations that this happens in are well lit and should be relatively easy to focus on.

Which focus setting did you use, Spot, Sel or Auto? I find that Spot and Sel mode always give me good focus results. I find Auto to be good for scenery but not close up.

[WMBP]

Which focus setting did you use, Spot, Sel or Auto? I find that Spot and Sel mode always give me good focus results. I find Auto to be good for scenery but not close up.

Have others found this, too? Someone else here in this forum (sorry, can't remember who or in what thread) recommended using spot focusing most of the time.

I have been using the Auto setting, I think. My pictures are not always tack sharp. I've been willing to take the blame myself so far. But I think I'll switch to spot focus and see if it makes a difference....

Will

[benjikan]

In an ideal world, the autofocus would focus perfectly on your subject. But in the real world, inaccuracy will cause the autofocus to stop just in front or just behind your chosen focus point. In a well designed lens/camera system this inaccuracy should certainly be less than the depth of field (at maximum aperture).

I'm not sure that I fully understand your point, but I agree that the non-symmetric nature of depth of field is important. In my opinion, a front focusing system is preferential to a back focusing one because in a front focusing situation the larger depth of field behind will more likely bring your subject into the zone of acceptable sharpness.

In the case of my 21mm, the depth of field at maximum aperture appears to be shallower than the K10D autofocus accuracy. I also wonder if that is why they made this lens a f3.2 rather than f2.8, 2.4 etc. They perhaps knew that their autofocus accuracy was right at the limit at this fstop and short focal length and that we might not notice !! I can't say that I've seen a problem on my 14mm/f2.8, but this is not the sort of lens I use for close shots at maximum aperture.

I still don't understand why Benjamin is having trouble at f9, f14. There should be bags of depth of field at these apertures. Look at the sums (e.g 1m subject at f9 results in a depth of field of 0.95m (0.71m to 1.66m). Also, if the scene is out of focus at these apertures then surely you would notice it in the viewfinder before stopping down?

Andy

I was about 2 meters from the subject. I was lying on the floor of the studio shooting up at the model, focussing on her eyes. She is about 1m 80cm. Her feet were sharp and her eyes were out. That is why the kit has gone to Germany to be adjusted. Once I get the stuff back in about five days I'll share what I have re: diagnostic.