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06-14-2012, 03:05 PM   #16
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I have had a quick go a re-calibrating my lens and it seems OK against the chart at 2/3 meters (200mm f2.8). Then went into the garden to shoot objects further away and am still having problems beyond 10m. Need to do some more scientific test with proper targets though when I have time. One thing I did notice is that at 200 and f2.8, if I turn the zoom ring slightly, the f number changes to f3.2 (in Av mode). I can still set it back to f2.8 but seems odd. My other constant aperture lenses don't behave like that.

If you want to have a look, the RAW file for the high jump shot is here:

http://www.npl-photo.com/Test/IGMP1440.dng

BTW, as I said before, I was using the center point and AF-S (and yes I know about keeping the shutter button pressed when recomposing. I may not be the most gifted photographer but I would like to think I kind of know what I'm doing (most of th time):

Flickr: mark woolcott's Photostream

In there you will find loads of shots with my 55-300 over 200 mm at f5.6 and no AF issues. For example, here is one such image where I ficussed on the face and recomposed:

All sizes | Woburn Safari Park | Flickr - Photo Sharing!

06-15-2012, 09:16 AM   #17
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QuoteOriginally posted by woolcotm Quote
I have had a quick go a re-calibrating my lens and it seems OK against the chart at 2/3 meters (200mm f2.8).
Wasted time on a red herring. Again, there is absolutely no possible way a miscalibration would have produced the results shown. You need to accept that before you can even begin to solve the problem. Right now, believing there is a calibration issue and spending time attempting to "fix" it is just taking time and effort away from actually getting anywhere.

QuoteQuote:
I was using the center point and AF-S (and yes I know about keeping the shutter button pressed when recomposing.
good. You have to understand, we don't know you and have no idea what you do or don't know.

Right now, my beat guess is what I already stated - you don't realize just how large the AF sensors are (they are much larger than the red dots), and managed to lone up your shot with the red dot where you waned but part of thebackground still within range of the AF sensor, and for whatever reason, the camera thought the background made a better target.

You need to do some experimenting to learn how big your AF sensors are (eg, by trying to focus on a single black dot on a white piece of paper with the dot in various positions near but not directly under the red dot in the viewfinder). Then you will be better able to choose appropriate focus targets.

QuoteQuote:
In there you will find loads of shots with my 55-300 over 200 mm at f5.6 and no AF issues
In case you needed proof that there is no camera/lens issue, there you go. It's all a matter technique. 100%, guaranteed.
06-15-2012, 11:51 AM   #18
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Of course I am not so arrogant to discount user error but this is the kind of shot I have taken on many occasions (with the 55-300) and I certainly cannot see anything set wrong on the camera so you can perhaps understand why I am baffled.

I have done a bit more testing but am still having issues (but not with my 55-300). I will post my results when I am done (hopefully this weekend).

Anyway, any ideas about the "moving aperture" when zooming out from 200mm? Maybe a communication issue between the lens and the camera? Certainly not something I have ever seen before with any of my other constant aperture lenses.
06-16-2012, 07:50 AM   #19
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Read my post again. Note I said nothing about having something "set wrong". It's not about settings. It is about fully understanding just how big the AF sensors actually are, and taking that into account wher lining up your shots. If you line up your shot such that part of the background is within range of the selected AF sensor, then there is always a risk the camera will focus there instead. Often, the camera will guess right, which is why you don't always see problems. But Your pictures are absolutely classic textbook demonstrations of what can happen when you don't take this onto account.

06-16-2012, 09:02 AM   #20
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Marc, i get what you are saying. Maybe I did mess up with the high jump but i had my battery grip on so I reckon its more likely i might have touched the other shutter button on the grip accidentally. There is no way I messed up 130 of 150 shots in that way though. My "keeper" rate is normally pretty high (90%+). Also, this was my 6th althetics meet and have taken some great shots for the club previously.

Anyway, problem sorted. I said I had re-callibrated and moved the AF adjustment from 0 to 3. Have done this again with a different chart and found I need 9 or possibly 10. WIth this setting, it seems to be working much better. When I re-callibrated in the week with my old chart at just over 2m, I was finding it hard to decide how much adjustment was needed because (to my eyes at least) the near out-of-focus numbers blur in a slightly different way to the numbers the other side of the sweet spot. And even with f2.8, at over 2M away, the DOF is relatively large (for callibrating). My new chart has horizontal dotted lines between the numbers and I noticed it was easier to compare them as opposed to the numbers because as they blur, they look like a faint solid line. An adjustment of 9/10 seems a bit much but my 55-300 needs 8 and my primes need 3/4 so I guess the body could do with an adjustment.

The lens had been working well enough for me previously but I do seem to remember that I had to ajust for back focus but it is possible I may have un-set the adjustment at some point prioir to using it again because I haven't had the lens out for a few months.

So, embarassing lesson learnt : be more carefull callibrating.

Last edited by woolcotm; 06-16-2012 at 09:08 AM.
06-16-2012, 12:13 PM   #21
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QuoteOriginally posted by klh Quote
I never updated the posting, so let me share a few things I learned along the way. First, I should thank Marc, and encourage you to pay attention to what he is saying here. In general, the problem was the user (me). I don't see it much anymore because of the following:

...........

3) I switched to using the AF button to focus (in AF/C mode), then letting off when I am sure I have the target. I also pre-focus a lot more.

.
Hello, I read a book called Sharp as a Tack recently and it also recommended using the AF button to focus, I'm struggling to understand why this should make a difference to image quality , your advice on this would be appreciated, Thanks
06-16-2012, 12:35 PM   #22
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QuoteOriginally posted by TylerD Quote
Hello, I read a book called Sharp as a Tack recently and it also recommended using the AF button to focus, I'm struggling to understand why this should make a difference to image quality
Don't get that one either unless its means using LiveView.
06-18-2012, 05:13 PM   #23
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QuoteOriginally posted by TylerD Quote
Hello, I read a book called Sharp as a Tack recently and it also recommended using the AF button to focus, I'm struggling to understand why this should make a difference to image quality , your advice on this would be appreciated, Thanks
I wouldn't say it improves image quality so much as it improves my chances of getting the shot. Here's the advantage as I see it. By using the AF button, I choose where and when the camera autofocuses (as well as when it meters). With AF-C, I get to track an object when I want, but then freeze the focus (like AF-S) when I choose. When there is a lot to choose from, I can hold the AF button until I catch the subject in focus, then let it go.

By the way, this technique taught me a lot about how the camera AF logic works.

An example where this is very helpful is in soccer. I focus on my player as he/she moves around the field, but when another player (or line judge) cuts across the camera view, I can let off the AF button and still get my shot without the focus hunting. It's also a great technique for focus/recompose, or anytime I want to prefocus and take shots later (without having to hold down the shutter button).

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