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02-19-2011, 01:28 PM   #1
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DA* 55mm/1.4--Erratic focusing

I have just finished testing my second copy of the DA* 55/1.4, and both have had significant backfocusing issues that could barely be corrected by +10 AF adjustments. What is more troublesome is that both lenses behaved erratically--sometimes accurate and sometimes backfocused. This is on consecutive shots at f1.4 and f2.0 with the camera remaining stationary on a tripod. I am using the LensAlign focus calibration device, and my camera is a K-7. I have also checked all of my other lenses by the same method, and none exhibits this problem.

Am I the only one to have such an experience with the DA* 55, or is it a known issue? In any case, I consider this performance to be unacceptable, and the second lens will be returned to Amazon for replacement. Perhaps the third copy will be OK. If it is not, I will return it and wait several months for a new batch to come in, hopefully with better quality control--that is unless I hear from other owners that they have the same problem with their lenses.

Rob

02-19-2011, 02:00 PM   #2
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I have the lens and correction is +2, so in my former K-7 and also now in my K-5. I think most copy off DA*55 need correction (as reported in 2009, my copy's productiontear), but yours is off the chart.


The erratic you are talking about is called Focus Shift and this occurs with lenses with IF. My copy has almost no trouble with that, but my FA*85mm/f1.4 is a real Focus Shift champion.
02-19-2011, 02:18 PM   #3
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QuoteOriginally posted by RonHendriks1966 Quote
I have the lens and correction is +2, so in my former K-7 and also now in my K-5. I think most copy off DA*55 need correction (as reported in 2009, my copy's productiontear), but yours is off the chart.


The erratic you are talking about is called Focus Shift and this occurs with lenses with IF. My copy has almost no trouble with that, but my FA*85mm/f1.4 is a real Focus Shift champion.
Ron,

How much focus shift is acceptable, and how does one deal with it, except by taking multiple shots of every image with narrow DOF? Also, how much lens microadjustment is acceptable? All of my other lenses require between +2-4.

BTW, Amazon's return service is amazing. With both free returns, they are giving me free overnight delivery of the replacement lenses. If the third copy is a clunker as well, I will assume that Amazon got a bad batch, wait for the dust to settle and then re-order.

Rob

Rob
02-19-2011, 04:36 PM   #4
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My DA * 55 back focused just a touch, need a focus adjustment of +3, I think. Otherwise, has no focus shift or other problems.

02-19-2011, 04:37 PM   #5
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QuoteOriginally posted by robgo2 Quote
Ron,

How much focus shift is acceptable, and how does one deal with it, except by taking multiple shots of every image with narrow DOF? Also, how much lens microadjustment is acceptable? All of my other lenses require between +2-4.

Rob
Well for f1.4 to f2.0 it is not wise to just rely on AF, either focus several times and see that is on the spot you want or do just a little fineadjustment with manual override of focus.

You can see on page 2 http://www.pentax.de/de/group/111/product/21790/body/downloads/media/1cc86d6...att%2055mm.pdf just how narrow your aerea of sharpness is. The chance that you will nail that by simply hitting AF isn't very good
02-20-2011, 01:22 AM - 1 Like   #6
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QuoteOriginally posted by RonHendriks1966 Quote
The erratic you are talking about is called Focus Shift and this occurs with lenses with IF.
Focus shift occurs with very fast lenses that have spherical abberrations wide open. It is not erratic. The term describes the phenomenon that one needs to adjust the focus when the aperture changes.
02-20-2011, 04:20 AM   #7
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QuoteOriginally posted by Class A Quote
Focus shift occurs with very fast lenses that have spherical abberrations wide open. It is not erratic. The term describes the phenomenon that one needs to adjust the focus when the aperture changes.
Wow thank you for this article. That makes a lot of sence. As in the article is stated it also differs between lenses, as I noticed myself in difference between my 55mm and 85mm lenses.
02-20-2011, 10:43 AM   #8
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QuoteOriginally posted by Class A Quote
Focus shift occurs with very fast lenses that have spherical abberrations wide open. It is not erratic. The term describes the phenomenon that one needs to adjust the focus when the aperture changes.
That is a very informative article, but I'm not sure that what it describes as focus shift is what I have experienced with two copies of the DA*55/1.4. In my case, there was extreme backfocus at maximum aperture of f1.4, and the focal plane shifted from one shot to the next without any change in aperture.

What I want is a lens that can be calibrated with only slight to moderate microadjustment and that consistently focuses on the same point at a constant aperture.

Rob

02-20-2011, 01:05 PM   #9
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QuoteOriginally posted by robgo2 Quote
That is a very informative article, but I'm not sure that what it describes as focus shift is what I have experienced with two copies of the DA*55/1.4.
I'm pretty sure it doesn't and that was the purpose of my post. Maybe I should have made that clearer. Whatever causes the erratic focusing, it is not focus shift.

QuoteOriginally posted by robgo2 Quote
What I want is a lens that can be calibrated with only slight to moderate microadjustment and that consistently focuses on the same point at a constant aperture.
Unfortunately, Pentax don't seem to be able to match their not-so-affordable-anymore-prices with good quality control. DPReview had to go through three copies of that lens and still didn't get one that was free of optical defects. This, even though they ordered from Pentax UK directly and it was obvious that they should have sent the best copy they had in stock.

You'll just have to keep trying. If I were Amazon and had a lot of returns, I'd remove the lens from the catalogue but apparently it isn't that bad.

There is, of course, also a chance that your testing approach causes the erratic focusing. What is your procedure? You may want to consider my AF adjustment hints.
02-20-2011, 02:33 PM   #10
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QuoteOriginally posted by Class A Quote
I'm pretty sure it doesn't and that was the purpose of my post. Maybe I should have made that clearer. Whatever causes the erratic focusing, it is not focus shift.


Unfortunately, Pentax don't seem to be able to match their not-so-affordable-anymore-prices with good quality control. DPReview had to go through three copies of that lens and still didn't get one that was free of optical defects. This, even though they ordered from Pentax UK directly and it was obvious that they should have sent the best copy they had in stock.

You'll just have to keep trying. If I were Amazon and had a lot of returns, I'd remove the lens from the catalogue but apparently it isn't that bad.

There is, of course, also a chance that your testing approach causes the erratic focusing. What is your procedure? You may want to consider my AF adjustment hints.
I use the LensAlign system for focus calibration and follow their guidelines exactly. It has worked well with all of my other lenses, but I have never used it on a f1.4 lens before. The 31/1.8 and the 77/1.8 passed with flying colors. I, too, am surprised by the apparently loose quality control at Pentax. A 55/1.4 lens is fairly standard and should be dead on perfect, especially if it costs more than $600.

What are the alternatives to the DA*55/1.4, assuming that I am unsuccessful in obtaining one that works? The Sigma 50/1.4 HSM is highly regarded, but it is very large and heavy. It is also said to be closer to 45-47mm than to 50mm in true focal length, in which case, I would just as soon get the 43 Limited. I would actually love to own that lens, but it is not that far from my 31 Limited, which is why I am so interested in the DA*55.

Rob
02-20-2011, 05:09 PM   #11
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QuoteOriginally posted by robgo2 Quote
I use the LensAlign system for focus calibration and follow their guidelines exactly.
That should rule out user error.

QuoteOriginally posted by robgo2 Quote
What are the alternatives to the DA*55/1.4, assuming that I am unsuccessful in obtaining one that works?
In that focal length range, I personally could recommend the K55/1.8 and the Nokton 58/1.4, both MF lenses but you probably want AF.

QuoteOriginally posted by robgo2 Quote
The Sigma 50/1.4 HSM is highly regarded, but it is very large and heavy. It is also said to be closer to 45-47mm than to 50mm in true focal length, in which case, I would just as soon get the 43 Limited.
Is the Sigma really larger than the DA* 55/1.4?

Regarding focal length: Note that the stated focal length is for infinity focus only. If someone "measured" the Sigma for closer focusing distances, it is no wonder that the focal length was lower than 50. Same would hold true for the DA 55/1.4. Since DPRreview reckons the focal length might only by 47mm there might be some truth to it, though.

However, there is still quite a difference between 43mm and 47mm. Not trying to talk you out of the FA 43 which surely is a heck of a lens.

I regard the difference between 43 and 55 as very large. The 43 would not be as ideal for close portraits as the 55 is.

P.S.: Perhaps you can write to Pentax and ask them what's up with their QC. If you end up not buying the lens because you couldn't get a good copy, I'd definitely tell them about it. Constant dripping, wears the stone...
02-20-2011, 07:12 PM   #12
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QuoteQuote:
Is the Sigma really larger than the DA* 55/1.4?
Sigma 50/1.4 EX DG HSM:

Filter Size 77mm [Wow!]
f/Stop Range 1.4-16
Minimum Focus Distance 1.48' (0.45 m)
Magnification 1:7.4
Angle of View 46 (35mm format)
Groups/Elements 6/8
Length 2.7" (68mm)
Maximum Diameter 3.3" (85mm)
Weight 1.1 lb (505g)

Pentax DA* 55/1.4:

Filter Size 58mm
f/Stop Range 1.4-22.0
Minimum Focus Distance 1.5' (45cm)
Magnification 1:5.8
Angle of View 28.6
Groups/Elements 8/9
Length 2.6" (66mm)
Maximum Diameter 2.8" (70mm)
Weight 0.83 lb (375g)


QuoteQuote:
Regarding focal length: Note that the stated focal length is for infinity focus only. If someone "measured" the Sigma for closer focusing distances, it is no wonder that the focal length was lower than 50. Same would hold true for the DA 55/1.4. Since DPRreview reckons the focal length might only by 47mm there might be some truth to it, though.
Dpreview estimates the true focal length of the Sigma to be about 47mm, but I have read lower estimates. Just how these were determined, I cannot say.

QuoteQuote:
However, there is still quite a difference between 43mm and 47mm. Not trying to talk you out of the FA 43 which surely is a heck of a lens.

I regard the difference between 43 and 55 as very large. The 43 would not be as ideal for close portraits as the 55 is.
I agree that the 55mm focal length is better for my purposes, which is mainly portraiture, although I expect it will find other uses as well.

QuoteQuote:
P.S.: Perhaps you can write to Pentax and ask them what's up with their QC. If you end up not buying the lens because you couldn't get a good copy, I'd definitely tell them about it. Constant dripping, wears the stone...
I just may do that. Still, I'm hoping eventually to get a good copy of the DA* 55. It can happen, you know.

Rob
02-20-2011, 07:45 PM   #13
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FWIW, I got one from Amazon a few months ago, and it's dead-on on my K-5. Sorry to hear about your troubles.
02-21-2011, 05:39 AM   #14
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QuoteOriginally posted by robgo2 Quote
Sigma 50/1.4 EX DG HSM:

Filter Size 77mm [Wow!]
f/Stop Range 1.4-16
Minimum Focus Distance 1.48' (0.45 m)
Magnification 1:7.4
Angle of View 46 (35mm format)
Groups/Elements 6/8
Length 2.7" (68mm)
Maximum Diameter 3.3" (85mm)
Weight 1.1 lb (505g)

Pentax DA* 55/1.4:

Filter Size 58mm
f/Stop Range 1.4-22.0
Minimum Focus Distance 1.5' (45cm)
Magnification 1:5.8
Angle of View 28.6
Groups/Elements 8/9
Length 2.6" (66mm)
Maximum Diameter 2.8" (70mm)
Weight 0.83 lb (375g)




Dpreview estimates the true focal length of the Sigma to be about 47mm, but I have read lower estimates. Just how these were determined, I cannot say.



I agree that the 55mm focal length is better for my purposes, which is mainly portraiture, although I expect it will find other uses as well.



I just may do that. Still, I'm hoping eventually to get a good copy of the DA* 55. It can happen, you know.

Rob
It is a nice lens if you find a good copy (they do exist!). Good luck.
02-21-2011, 12:38 PM   #15
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More thoughts on focus shift and lens testing

After reading more on the subject of focus shift, including a piece on the LensAlign website, (Do I need to test at more than 1 aperture? / LensAlign / FAQs - Michael Tapes Design Support) it seems that it might be smarter to test the lens at f2 or f2.8, rather than at f1.4, assuming that it does exhibit focus shift. This is because I am much more likely to use the lens at f2 and above than I am at f1.4. An alternative is to test at two different apertures and dial in the appropriate AF adjustment according which one I am using and how crucial accurate focusing is likely to be in a given situation (i.e. DOF.) That approach, I think, is going to prove cumbersome in actual practice.

In any case, I will still face the question of whether the lens can be focused accurately and consistently using AF microadjustments. If it cannot, then the lens goes back.

Any thoughts on this?

Rob
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