Seems you may have misunderstood me. What I am suggesting is that the sensor area is greater than the tiny red dot. So when you focus on something that has contours, it is possible for the sensor to focus forward or backward from what you intended. An example would be a face - where the sensor area covered most or all of the face. The nose sticks out a bit, the eyes are sunken a bit, the ears are even further back. So maybe you are thinking it should focus on the eyes, but maybe it picks out the nose instead. It still focused on something in the sensor area, so could be considered accurate. This is what I saw alot from my niece's D1X Nikon when I looked close at the images. I further stated that this effect is minimized by slow lenses - IOW, the greater DOF provided by the smaller aperture minimizes this problem to some degree.
In your case, Stopping down while focusing wouldn't have any better effect on exactly what gets focused, because when you open back up, it could still be on the wrong spot. Based on the picture you showed, I'm wondering why you don't manually focus anyway and get exactly the focus you want. -- Best regards, Bruce Tuesday, December 6, 2005, 3:09:04 PM, you wrote: JB> Bruce, are you suggesting that I should stop the lens down while focusing? JB> For actual shooting I need to use F.2.8 - F.4 in order to get enough speed. JB> For concert and theatre shots I'll be using ISO 800-3200. Very often a JB> shutter speed faster than 1/500 secs is necassary to "freeze" af performing JB> rock star. It's the old problem - choosing between shallow DOF or motion JB> blur. JB> Pictures like this ( 135mm + 1.7 x AF Adapter, 1/400 secs., ISO 3200 - JB> probably F.4) just doesn't cut it, even though this was my best of perhaps JB> 100-150 shots: JB> http://www.flickr.com/photos/bladt/41643010/in/set-877712/ JB> Regards JB> Jens Bladt JB> http://www.jensbladt.dk JB> -----Oprindelig meddelelse----- JB> Fra: Bruce Dayton [mailto:[EMAIL PROTECTED] JB> Sendt: 6. december 2005 00:07 JB> Til: Jens Bladt JB> Emne: Re: A better 70-200mm F. 2.8 JB> Hello Jens, JB> My personal feeling is that the sensor area is larger than we would JB> like to think. Achieving AF within the sensor bounderies (not visible JB> to us) is all the system is going to do. Classic problem is when the JB> sensor area covers something like eyes, nose and ear. One of them JB> will be in focus, but not necessarily the one you want. Stopped down JB> a bit, it may not show up so much, but with narrow DOF, it does. I JB> remember looking at some of my niece's work from a Nikon D1X+AF JB> 80-200/2.8 lens. I was surprised on close examination how many shots JB> were off just a touch from what would have been ideal. JB> I operate from the concept that I only us AF when it can be more JB> reliable than me. That equates to a very small percentage of the JB> time. Mostly fast moving subjects that are unpredictable in course. JB> In those cases, there is enough slop in my tracking of the subject, to JB> more than compensate for the very minor miss by AF due to the large JB> sensor area. JB> Anyway, those are my thoughts. JB> -- JB> Best regards, JB> Bruce JB> Monday, December 5, 2005, 2:29:04 PM, you wrote: JB>> Small? 2104 x 1468 can easily fill you screen! JB>> And yoyu're right - it's probably not the lens (although I had great JB>> problems getting sharpness at all at F 2.8 - F3.5: JB>> It may very well be the AF system. JB>> Yes, I use the term when the lens/camera is focusing further away than JB> it JB>> should - that is behind the point where the red square in the viewfinder JB> is. JB>> ASome times at the top or above the little circle that confirmes focus JB> in JB>> the *ist D. JB>> In this shot, I focused at the white streamer (stensikkert.dk) - but JB> look JB>> how sharp the brick wall looks here JB>> http://www.jensbladt.dk/Test/rooms.html JB>> Compared to the second and third shot (made with the FA lens). It can't JB> be JB>> that the Tokina has a LOT better DOF at F. 2.8, can it? It must be JB> because JB>> the focusing is off. Behind the white board. OR this sample og the lens JB> is JB>> not good. JB>> I find the SMCP FA 2.8/80-200mm ED(IF) quite good, when focused JB> manually. JB>> Regards JB>> Jens Bladt JB>> http://www.jensbladt.dk JB>> -----Oprindelig meddelelse----- JB>> Fra: graywolf [mailto:[EMAIL PROTECTED] JB>> Sendt: 4. december 2005 22:30 JB>> Til: [email protected] JB>> Emne: Re: A better 70-200mm F. 2.8 JB>> The photos are rather too small to tell anything from. However, it is JB>> most likely an auto-focus problem, not one with the lenses. JB>> Try manual focus. After you have the proper part of the image in what JB>> you think is correct focus, go look out the window for 5 minutes or so, JB>> then come back and tweek your focus as quickly and exactly as you can. JB>> Now you can take your photo. Do that with the other lenses. Get back to JB>> us with the results. JB>> I would look at every other posibility with focus problems before JB>> blaming the lens. JB>> And by the way, from a technician's point of view back focus problems JB>> means the lens is not focusing the image exactly the flange to film JB>> distance from the lens mount. I do not think that is how you were using JB>> the term. JB>> graywolf JB>> http://www.graywolfphoto.com JB>> "Idiot Proof" <==> "Expert Proof" JB>> ----------------------------------- JB>> Jens Bladt wrote: >>>I have been trying out three different lenses in the 70-200mm range. >>>Sigma EX 2.8/70-200mm APO >>>Tokina AT-X Pro 2.8/80-200mm >>>SMCP-FA 2.8/80-200mm ED(IF) >>> >>>All of them seem to suffer from Back Focus. The Sigma not much, though. >>>Had I been offered a used Sigma, I probably would have bought it. >>> >>>I published a small test showing the problem. >>>Due to Back Focus (i BELIEVE), my SMCP F-4-5.6/70-210mm is the sharper one >>>at F. 5.6. >>> >>>Please take a look and feel free to comment: >>>http://www.flickr.com/photos/bladt/sets/1509814/ >>> >>>Regards >>>Jens >>> >>> >>> >>> >>> >>>
