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07-04-2010, 05:49 PM   #1
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OPINIONS PLEASE: Focus Test (DA* 16-50mm SDM)

Hello Everyone,

Backstory: When I first got my K-7 and slapped on the DA* 16-50mm SDM, it was back-focusing a bit, so I adjusted in the menu and all seemed well.

Lately, I've been noticing more and more shots wherein I'm off my intended focus mark by just enough to be noticeable on anything larger than a 5x7 print, and it's really irritating me! Thinking I maybe needed to revisit the back-focus adjustment, I re-ran the testing and found something that looked a bit odd to me...

Below is the best adjustment I could manage:



I'm not going to say just yet what my concern(s) are because I want to see if it's just my paranoia or if any of your keen eyes actually spot something.

Any and all thoughts/comments are welcome and appreciated.

Thanks,
-Paul

07-04-2010, 05:58 PM   #2
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You really need to align the camera well. In this case, it is slanted. It's back focused on the left and perfect on the right side.
07-04-2010, 06:32 PM   #3
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QuoteOriginally posted by hangu Quote
You really need to align the camera well. In this case, it is slanted. It's back focused on the left and perfect on the right side.
Correct...those were my thoughts initially as well, only the camera is NOT slanted.

The "Focus Here" bar is lined up perfectly square relative to the frame (I made sure of this). You can easily overlay any kind of square or grid tool to see for yourself.

Look a little more closely though too...the right side isn't "perfect" either...

Assume then for the moment that it is not an alignment/slanting issue...thoughts then?

-Paul
07-04-2010, 06:51 PM   #4
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Paul, it's close, but it's not perfectly aligned. It's off by a tiny bit. Check the blurry 2" mark at the bottom left and right.

I have a few more conjectures but they're highly unlikely. Before exploring them, you should probably square away the alignment issue first. It makes no sense to explore highly unlikely situations when the most likely one is starring you straight in the face.

Honestly, it seems like the focus is pretty much spot on. It's not 100% perfect but it's not bad enough to cause an issue. I see posts like this a lot and more often than not, it's just a matter of user error.

07-04-2010, 07:19 PM   #5
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QuoteOriginally posted by hangu Quote
Paul, it's close, but it's not perfectly aligned. It's off by a tiny bit. Check the blurry 2" mark at the bottom left and right.

I have a few more conjectures but they're highly unlikely. Before exploring them, you should probably square away the alignment issue first. It makes no sense to explore highly unlikely situations when the most likely one is starring you straight in the face.

Honestly, it seems like the focus is pretty much spot on. It's not 100% perfect but it's not bad enough to cause an issue. I see posts like this a lot and more often than not, it's just a matter of user error.
The 2" discrepancies at the lower left and right is due to a slight paper curl on the lower right, however the position of the curl in no way affects the center focus line, which is the point of concern here. From my understanding, this is the vital area - so long as it is square relative to the frame, it shouldn't matter if I even cut off the top and/or bottom of the page, should it?

Nonetheless, let's assume for a moment your assertion is correct and the alignment is off by a hair. There is no way this should result in what appears to be a 1mm-2mm front focus on the right and a 6mm-7mm back focus on the left. That would mean my alignment is off by a total of 7mm-9mm, which would CLEARLY be visible as a slanted center focus bar.

Am I making any sense here or is my understanding of the mechanics involved completely off base?

Believe me, hangu, I would love for the answer to be something as simple as user error...

Please understand I'm not trying to be argumentative to prove I'm right...I'm just trying to satisfy my concerns. I do appreciate your help.

-Paul
07-04-2010, 08:05 PM - 1 Like   #6
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The only other explanation I can think of is that the glass in your lens is not absolutely parallel to each other, causing it to focus correctly on one side and back focus on the other. It could also be your sensor that's doing it. Both are highly unlikely in my opinion.

What's more likely is that this test is not perfectly aligned. Like you say, the paper is knurling on the lower right corner and I notice that the triangular negative space on to top left and right corners are not equal. Just because the square lines are parallel to the edge of this photograph does not mean it's perfectly aligned. We're dealing with 3 dimensional space here and there is a lot of room for errors. You're accounting for tilt but what about perspective? An error along the y-axis can compensate for a z-axis error, giving you the impression that it's perfectly aligned. That's probably what's happening here as the right side of this paper is farther away from the camera than the left side:



Now, I don't think it fully explains the 2mm back focusing on the left side (if at all), but I also don't think this is what's causing your photos to be blurry. The center black bar is sharp as a tack and you're probably focusing at minimum distance, where the depth of field is thinnest. In real world conditions where the subject is farther away, it should be absolutely negligible. I'd be very happy if I got results like this.
07-06-2010, 07:08 AM   #7
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I have also discovered some problems focusing with my brand new da*16-50.

It tends to back-focus from 16-24mm, and the extent of back-focusing increases with stopping down of aperture.

However, from 28-50mm, the focus is spot on.

I was under the impression that the degree of back focus remains constant, allowing it to be calibrated easily within the camera.

Any other users of this lens experienced similar issues?
07-06-2010, 08:31 AM   #8
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QuoteOriginally posted by Sid_SG Quote
I have also discovered some problems focusing with my brand new da*16-50.

It tends to back-focus from 16-24mm, and the extent of back-focusing increases with stopping down of aperture.

However, from 28-50mm, the focus is spot on.

I was under the impression that the degree of back focus remains constant, allowing it to be calibrated easily within the camera.

Any other users of this lens experienced similar issues?
Back focus should be far more evident from 28-50mm and less so from 16-24mm. Also, stopping down aperture should hide back focusing even more. Are you positive this isn't a result of user error? You might get more help if you posted some examples.

07-07-2010, 06:59 AM   #9
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Yea you were right after more testing i came to the conclusion that stopping down did improve the focus accuracy.

Today I made a trip down to the Pentax Service centre. I was informed that indeed there was nothing wrong with the lens, it is common for most wide angle zooms to mis-focus at the wide end. The solution would be to zoom in on the subject, lock focus then zoom back out.

Hope this helps
07-07-2010, 06:57 PM   #10
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QuoteOriginally posted by Sid_SG Quote
Yea you were right after more testing i came to the conclusion that stopping down did improve the focus accuracy.

Today I made a trip down to the Pentax Service centre. I was informed that indeed there was nothing wrong with the lens, it is common for most wide angle zooms to mis-focus at the wide end. The solution would be to zoom in on the subject, lock focus then zoom back out.

Hope this helps
The 16-50 is a varifocal lens, if you zoom in and lock focus and then zoom out you will be out of focus. The only zoom lenses that stay in focus right through the zoom range are called parfocal lenses and they are as rare as rocking horse poo and a lot more expensive.
07-08-2010, 12:54 AM   #11
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QuoteOriginally posted by baldrick Quote
The 16-50 is a varifocal lens, if you zoom in and lock focus and then zoom out you will be out of focus. The only zoom lenses that stay in focus right through the zoom range are called parfocal lenses and they are as rare as rocking horse poo and a lot more expensive.
Almost all of the IF (internal focusing) lenses are parfocal, so they are not rare. For example DA 17-70 is parfocal.
07-08-2010, 09:21 AM   #12
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I dont quite understand how the focal point can change with varying focal length(zooming) with this lens....

Imagine if an object i wish to focus on is 2m away. I zoom in and lock focus, and the focus scale shows 2m. After I zoom out, the focus scale still will be at 2m(assuming the focus was locked and i did not adjust the focusing ring). At this point the focus will still be on the same point, wouldn't it? Focal distance remains constant @ 2m.
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