Does my 70200g work as it should ?

[lock]

I never really had the time to take a very close look at my 70200g I bough second hand last November, until the winter holidays.

My initial impressions during testing when I bought it were very positive. But I have to admit that after checking these pictures, I did not really challenge the lens AF-wise. And I do not mean speed, but accuracy.

Now that I had plenty of time to try out the lens it struck me that it had a hard time getting some detail in the pictures. I did see the occasional AF-mistake but I thought hat was the usual error with a more contrasty object fooling the central sensor. But after some more testing pictures I really got a bit worried cause even at high shutter speeds and good light detail in the center crop was poor.

So I started tested the lens in a more controled environment with a tripod, this time checking the position of the central AF sensor. I put on the wall a white sheet of paper with a block dot in the middle. I positioned the camera right in front of it. At 70 mm with the central square dead on the black dot, the lens only hunted. Then, moving the lens a bit to the right and bringing the dot just at the boundery of the central square, it locked. It disappearedat about 90 mm. I stopped any further detailed testing because the tripod broke is at failed to keep the heavy lens in position....

So I took some pictures without the combo. These two examples are displaying some pencils in a metal basket. The elips on the pencil shows the spot I focussed on with the central sensor. The one pencil to left with the red top is about 3 inches behind.
It's at F2.8, iso 400,1/60th sec, 70 mm and at aprox. 2.5 meters:


Another picture of the same scene:


At first glance, I thought "back focus". But then I thought about the hunting .... Could it be that the central sensor is not perfectly centered ?

[stevecim]

hi Lock

I guess the first image is croped?

Cheers, Stephen

[lock]

Stephen, both images are 100% crops form the original picture.


lock

[stevecim]

a700 body? I can't say what's happening, just thinking of what extra info someone else may need to help you

[David Kilpatrick]

The central sensor is BIG - much bigger than the mark in the finder. Think of the entire centre circle. It just picked out the best lit, highest contrast part of the area it covers. That's all.

All the focus sensors cover much more area than the marks in the finder would have you believe.

David

[lock]

David, you are of course correct and I'm aware of that. But the circle covered the left black pencil on the white background as well. That surely had the highest contrast and it was in front of the one that seems sharp. Why did the AF not pick that pencil ? And, why does the AF not focus on the black dot when there is nothing else but a black dot on a plain white paper, unless I move the camera to the right....

[David Kilpatrick]

Half close your eyes and the H on the focused pen is the clearest target in the scene. Of course there was an equal chance the AF might have focused on the grid of the pen tub, but it can only focus on one distance. There's no point in asking why it focused on one point not another, as it is all down to the level of illumination and the way the target interacts with the AF sensor lens and phase detection module. I don't see a black pencil on a white background, I do see a black pentop against a darkish shadowed background, with no detail visible on the pentop - this would be a very poor focus target.

When you fit an f2.8 lens and enable the central sensor on an A700 or A900, you switch automatically to using the f2.8 sensor, which is a line-type with two 'ends' (like two small bricks end to end). I guess you might manage to place a dot on white paper in between these. It will always be better with a subject slightly offset, or larger than the sensor. The double cross sensor (f5.6 sensitivity) should also be active, but only operate if the f2.8 sensor fails to find a suitable target.

And it is possible for the entire sensor array to be offset relative to the markings in the finder. The screen itself can be moved slightly in the frame (the A900 is not prone to this, the A700 and others all are, they lack the same position locking design). Even the AF markings can be shifted relative to the other markings on the screen, as it is a two-part assembly. I installed a Haoda Fu screen in my A100 and spent some time making sure the microprism coincided with the focus aid marking, the two could easily be separated. I have no idea whether the result, which looks better lined up, means the AF marking no longer exactly matches the sensor position.

Basically, I think you are asking the impossible of the system - a cluster of objects with a depth, all within the focus zone of the central sensor; or a dot on paper. The dot on paper does not resemble any normal target the system would be designed to focus on. Try sticking a postage stamp on paper the same way - something with some detail contrast for the phase contrast system to detect. AF may be off centre but I'll bet it doesn't hunt as much.

David

[lock]

Half close your eyes and the H on the focused pen is the clearest target in the scene.

The focus was on the pen to the right with the barcode on it, as shown in the first picture.

I don't see a black pencil on a white background, I do see a black pentop against a darkish shadowed background, with no detail visible on the pentop - this would be a very poor focus target.

I meant this one: http://farm4.static.flickr.com/3533/319 ... 4f95_o.jpg" onclick="window.open(this.href);return false;

And it is possible for the entire sensor array to be offset relative to the markings in the finder.

But it's possible to fix this, isn't it ?

Try sticking a postage stamp on paper the same way - something with some detail contrast for the phase contrast system to detect. AF may be off centre but I'll bet it doesn't hunt as much.

David, I will do that. In stead off stamp I will use a square, though small, with diagonal lines in it. And I will check the other sensors too, but first I need a new tripod that handles an a700 and the 70200g well. Do you have any suggestion which tripod to pick but without spending all my money ?

Thanks

lock

[David Kilpatrick]

Don't use a target with ruled lines. This is the most certain way to get OOF results, as the phase detection can easily go out of phase (it can happen on fabrics, fences, brick walls - lots of subjects). The best focus target is more random in texture but with some low frequency element. If you must rule lines, rule them at varied intervals and do not make them parallel.

For a tripod at reasonable cost I would always recommend Manfrotto, as long as you stick to around $200 cost including the head, as a minimum. I use Gitzo, but they are very expensive - I have an original Studex Reporter which must now be 25-30 years old, and I've just got a Traveler 6X Carbon Fibre. That is strictly there for extreme light weight and portability. Manfrotto has some more or less equivalent carbon fibre models, all of them cost about twice as much as alloy versions; I think they are worth it for the extra rigidity.

David

[lock]

Well, i've checked the Manfrottos and compared various 055 versions with about 15 other tripods. Than I found out the Vanguard Alta 363 is by all means a worthy contender. I bought it cause it holds my A700 with 70-200g very, very well and it's a very versatile yet reasonably light tripod.

But I bought it, amongst other things, to test my 70-200g. And unfortunately, I was right about my worries.
At 70 mm from left ro right there is backfocus from going from 3.5 to 3 cm (distance from the flat targets 1.7 meter). At 135 mm again from left to right this is 1.5 cm to 1 cm backfocus (at 2.5 meter distance ). At 200 mm I could not find much. If it would have backfocussed all through the focal range then I would have done the adjustment with the three screws myself. But there's a lens issue here...

This is a real let down cause after the AF repair I would not have thought I would have to suffer a new round of waiting for repairs to finalize. And this time it's not under warranty !