[lol101]

OK guys, I am confused here.

I did a few tests with my 50-135 @50mm and was getting affraid I might have a BF or FF problem with it but only at shorter focal range.

Explanation:
@135mm, I am getting perfect focus but sometimes, @50mm I get not so good focus.

So today I did a few tests... and got even more confused!

Here's what I did:

First, take a picture @50mm.
Then zoom to 135mm, lock AF, zomm back to 50mm and re-take the shot.

The results I have gotten are pretty inconsistent.

Depending on the distance where I am focusing I get different results.

On close distance (<5m), I get pretty much the same results with the two focusing techniques and everything is OK.

But when I focus closer to infinity, things are getting very stange.

Let me show you what I am talking about:

Here's the (ugly as it should) test picture:

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Last edited by lol101; 01-24-2008 at 03:58 AM.

[lol101]

And here are comparisons of different 100% crops from both focusing techniques:

First one is by focusing directly, second one from focusing @135mm, locking and zooming back:

Center crop, focus point was on the house's number.

[lol101]

So it appears the direct AF @50mm is pretty bad, maybe I have FF or BF.

Let's look at details in front or back of the focusing point:

In front should suffice (back is the same):

[lol101]

OK, there I am stuck.


It appears it's not FF or BF but the pics focused @135mm and then zoomed back are clearly sharper all across the board than the one directly focused @50mm.


Again, it's not something I see @ all focusing distances but it seems to appears when I focus on far away things (>5-10m).


I am suspecting it has something to do with floating lenses that are supposed to optimize image quality at various distances for various focal lengths but it is the case, it appears they are not doing their job on my unit!

Could some of you guys with the DA* 50-135 try to test your unit for this behaviour so that I know if mine is faulty?

Many thanks for any advice!

[lol101]

I also checked that the focusing mark didn't move between the two shots and was indicating near infinity in both cases.

If I set the focus to continuous, focus on the number @50mm, and zoom in, I can clearly see, as I zoom that I am not in focus and the camera will adjust focus slightly until I get a perfectly focused 135mm picture.
If I zoom back, still in AF-C, I feel at some point something moving and the picture @50mm will not be sharp.


When it happens, it looks like the point of focus is nowhere in the picture and everything is oof...

[Wethphotography]

I just did a similar test and got the same results you did.
I do not have time to download images to computer and upload and post now, but from zooming in on the k10d lcd screen, the differences are apparent. The images are far sharper when focuss is locked at 135mm and zoomed back to 50mm than those images focussed directly at 50mm. Also, I find no apparent BF/FF issue.
And, like your tests, this only seems to be an issue at greater distances. When I tried the test on an item 6 feet away, there was no difference. I will post pictures later this evening.
Perhaps this is an issue that affects all these lens.....
But at least in the short term, I know to focus at 135mm, lock focus, and zoom back to 50mm if I did a distance shot taken at 50mm. My lens serial # is 9002629.

OK, there I am stuck.


It appears it's not FF or BF but the pics focused @135mm and then zoomed back are clearly sharper all across the board than the one directly focused @50mm.


Again, it's not something I see @ all focusing distances but it seems to appears when I focus on far away things (>5-10m).


I am suspecting it has something to do with floating lenses that are supposed to optimize image quality at various distances for various focal lengths but it is the case, it appears they are not doing their job on my unit!

Could some of you guys with the DA* 50-135 try to test your unit for this behaviour so that I know if mine is faulty?

Many thanks for any advice!

[Wethphotography]

I just duplicated my tests at f3.5 and f5.6 instead of f2.8 and the problem is gone by 5.6; the two images are of equally good focus quality. At least as far as I can tell zooming in to x16 or x20 on the LCD camera screen. At f3.5 the difference is less pronounced than at 2.8 but not as good as f5.6.

[codiac2600]

What I've noticed with all the DA* lenses and different K10D's I've tried is that they will bring out FF & BF errors more than a standard lens cause they are more accurate. Setup a focusing chart, a yard stick, and pick an inch marking and aim dead on to it at a 45 degree angle. Test your results at different focal lengths and different lenses. You may have a centering issue if it's only at the wide end and not the tele end. Give it a try and see what hapens.

[blende8]

What you experience is pretty normal for all zooms.
AF is better if you zoom in.
It is an old rule that for best sharpness zoom in, focus and then zoom back.

[lol101]

What I've noticed with all the DA* lenses and different K10D's I've tried is that they will bring out FF & BF errors more than a standard lens cause they are more accurate. Setup a focusing chart, a yard stick, and pick an inch marking and aim dead on to it at a 45 degree angle. Test your results at different focal lengths and different lenses. You may have a centering issue if it's only at the wide end and not the tele end. Give it a try and see what hapens.

Hi Codiac,

It's obviously not a centering defect since all the 100% crops posted above are done @50mm

Also, it is not BF nor FF since on the direct focus, I can't seem to find a sharper point in the picture than the one I aimed for, the only problem is that the other focusing technique gave a better result all across the frame.

Again, the only difference is the technique I used for focusing. Everything else is the same.

[lol101]

What you experience is pretty normal for all zooms.
AF is better if you zoom in.
It is an old rule that for best sharpness zoom in, focus and then zoom back.

I know, it's actually the reason I tried to focus @135 and zoom back when I saw that my 50mm shots were not acceptably sharp.

I can alway do that if it is a "normal" behaviour for this lens but this is somewhat disapointing from a zoom of this class (and price) not to be able to focus accurately from 90mm down to 50mm on far away targets.

I will try the DA 50-200 zoom tonight or tomorrow to see if it's lens or body related.

[lol101]

I just duplicated my tests at f3.5 and f5.6 instead of f2.8 and the problem is gone by 5.6; the two images are of equally good focus quality. At least as far as I can tell zooming in to x16 or x20 on the LCD camera screen. At f3.5 the difference is less pronounced than at 2.8 but not as good as f5.6.

Thanks for taking the time to do these tests.

I hope some other people can try it too and confirm this is "normal" behaviour for this lens.

No point in returning the lens if all of them work the same.

To me, these results are clearly unacceptable but I'll try with some other lense (FA 135, FA 50 DA 50-200) to see if I get the same results (which would mean it might be a body/firmware issue) or not.

[blende8]

I did a small test here with my copy.
I could do it only up to about 10m, because it is dark outside already.
The effect basically non existant at this distance, see below.
I will test it tomorrow morning at true infinity.

EDIT One question:
Where is your focus if you focus with 50 mm?
Is it backfocus? Beyond infinity?

100% crop, well, ISO 1600 only:

[lol101]

I did a small test here with my copy.
I could do it only up to about 10m, because it is dark outside already.
The effect basically non existant at this distance, see below.
I will test it tomorrow morning at true infinity.

EDIT One question:
Where is your focus if you focus with 50 mm?
Is it backfocus? Beyond infinity?

100% crop, well, ISO 1600 only:

Thanks for your help.

Actually, since I am seing this close to infinity, I am wondering if the point of focus with the direct 50mm focusing isn't past infinity.

I can't quite find it when looking at the picture, it's like evrything is blurred.

[jeffkrol]

for a couple of reasons 1) are you saying that there is no sharp point of focus at 50? ie: cannot manually focus at 50 to be sharper and have to zoom /focus/ un-zoom to get a sharp image?
2) since the AF "zone" has a much larger area at 50 then 135 then naturally AF "errors" could arise due to AF on things that are not planer.. ie much farther and or closer all in the same frame of AF reference?. I assume you were attempting to focus on the post w/ the address, which at 50mm is considerably smaller then at 135..... and the AF area could include things in the foregroud/background.
Personally it is a "good technique" to maximise the AF area of interest and then back zoom.
And a good thing that the lens "holds" as I believe some would not. If you back zoom the optics change enough to lose AF........
EDIT: You seemed to have cleared up one point, the 50 has no focus at all at least according to your last post.. That's not a good thing and frankly would be hard to explain since the lens groups should be the same if natively at 50 or backed off to 50....