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11-20-2013, 06:56 PM   #1
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Lens adjustment

I've done some very preliminary test shots with my K-50 and 40 and 70 Limiteds. It seems the lenses are quite sharp at f5.6 and get steadily worse down to wide open, where focus is way off. I dialed the 70 to -10 in the fine tuning, and it was better, but still not sharp. Of course, with the adjustment, f5.6 was then a bit off. Live view focus is much better.

I need to do more tests, but does that seem right? If the lenses are only off at certain focal lengths, then is it possible to dial in correction?

11-20-2013, 07:23 PM   #2
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QuoteOriginally posted by DennisH Quote
Of course, with the adjustment, f5.6 was then a bit off.
Why of course? If you finetune at the widest aperture, it should also be good at f5.6.
11-20-2013, 07:26 PM   #3
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Because it was sharp at 5.6 to begin with. That's what I'm asking: if the lens is only off at certain focal lengths, how can I adjust it?
11-20-2013, 07:38 PM   #4
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QuoteOriginally posted by DennisH Quote
if the lens is only off at certain focal lengths, how can I adjust it?
That's almost an ontological question

I'd probably examine my methodology for consistency. What tool(s) are you using for focus adjustment?

M

11-20-2013, 07:44 PM   #5
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QuoteOriginally posted by DennisH Quote
off at certain focal lengths
I guess you mean at certain aperture setting....

I agree with
QuoteOriginally posted by sterretje Quote
If you finetune at the widest aperture, it should also be good at f5.
When in focus at f/2.8, the same subject will stay in focus at f/5.6, there just will be more DOF in front of and behind it.
11-20-2013, 07:50 PM   #6
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I definitely have to do real testing, but I've done these adjustments before, and I don't like the way this looks.

But to be clear, they should be accurate or inaccurate across all focal lengths, correct?
11-20-2013, 08:29 PM   #7
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QuoteOriginally posted by DennisH Quote
But to be clear, they should be accurate or inaccurate across all focal lengths, correct?
I best understand this question in the context of a zoom lens, so a 16-50mm lens should be set at 50mm and the benefits apply at the widest foal length. If you made a typo and mean apertures, yes, setting the lens at the maximum aperture will apply to the smaller apertures.
I've been using this free tool and approach for years; though it is written towards a Canon audience it is equally applicable to other brands.

M
11-20-2013, 08:46 PM   #8
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Thanks, I did mean apertures. I think I'm being unclear though.

What looks like is happening is that with no adjustment, f5.6 is in sharp focus; f2.4, however, is way out of focus. Now, if I dial in adjustment so it's in focus at 2.4, it will throw off the others.

So, what would cause this, and is there a way to correct it?

11-20-2013, 09:00 PM   #9
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QuoteOriginally posted by DennisH Quote
Thanks, I did mean apertures. I think I'm being unclear though.

What looks like is happening is that with no adjustment, f5.6 is in sharp focus; f2.4, however, is way out of focus. Now, if I dial in adjustment so it's in focus at 2.4, it will throw off the others.

So, what would cause this, and is there a way to correct it?
Maybe you're just missing the focus to begin with? You should make all adjustments wide-open.

The lens won't be as sharp at F2.4 as it is at F5.6 anyway, so you can't expect too much from it.

I recommend following this procedure with a lensalign or similar tool:
Fixing Front and Back Focus - LensAlign MkII - PentaxForums.com

Adam
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11-21-2013, 03:22 AM   #10
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<br><a href="http://diglloyd.com/articles/Focus/FocusShift.html" target="_blank"></a>
11-21-2013, 03:53 AM   #11
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@markkuk
I don't think that focus shift is an issue at /2.4. That applies to fast lenses (f/1.2, f/1.4 lenses).
11-21-2013, 05:49 AM   #12
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QuoteOriginally posted by sterretje Quote
@markkuk
I don't think that focus shift is an issue at /2.4. That applies to fast lenses (f/1.2, f/1.4 lenses).
Focus shift can happen in slower lenses as well. The Photozone review of DA 70/2.4 says it has focus shift: Pentax SMC DA 70mm f/2.4 Limited - Review / Lab Test - Analysis
11-21-2013, 12:54 PM   #13
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Thanks for your help, everyone. I donīt know what my problem was yesterday, but I played around with the lenses today (under better conditions), and they are SHARP. I did dial in -5 on the 40mm, so everything is a-ok now.

Thanks again.
11-28-2013, 09:57 AM   #14
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QuoteOriginally posted by DennisH Quote
Thanks for your help, everyone. I donīt know what my problem was yesterday, but I played around with the lenses today (under better conditions), and they are SHARP. I did dial in -5 on the 40mm, so everything is a-ok now.

Thanks again.
Similar things used to happen to me.Once I cleaned up the mount and contacts the issue disappeared.

Cheers

Randy
02-27-2014, 04:57 PM   #15
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I keep reading about this "fine tune adjustment of the lenses". Does this require opening the bottom of the camera & adjusting those 2 screws? I don't know how anyone could "play" with this part of the camera. For sure this would void warranty & possibly ruin the camera if not adjusted correctly. How can someone want to take that risk? I like "tinkering", but not with a $900.00 piece of equipment, that is so delicate.
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