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06-24-2009, 03:34 PM   #1
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FA 50mm f1.4 jagged aperture - is this normal?

I got my FA 50mm f/1.4 the other day (along with the Da* 16-50 ) and have been playing around with it - great lens overall. I was just cleaning it and noticed that the aperture blades on one side do not form a smooth curve when the lens is stopped down to f/2.0 - see the pictures and you'll know what I mean. This problem is only prominent at 2.0 and 2.4, it smooths out when the lens is stopped down further.

Is this normal? Anyone got their FA 50mm handy to peer into? It doesn't seem to show up in pictures in any identifiable way, but the lens is brand new and I'm going to be returning it if it has a manufacturing defect.





06-24-2009, 03:40 PM   #2
emr
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Yes, mine's the same. Guess that's what the structure's like.
06-24-2009, 04:14 PM   #3
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Cheers, emr - I was concerned that the opening was only jagged around some of the aperture blades, implying it was a flaw rather than by design. I would have though an asymmetrical aperture opening would cause all kinds of funny effects on the image.

Then again, I hardly know Sonnars from Planars
06-24-2009, 04:23 PM   #4
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Pentax lenses have been like that since the FA series. It could be due to loosely made parts or not precisely adjusted. For some samples, the shape will change too as you rotate the lens. It can be opened and and have the blades rearranged and adjusted to your liking. I have done this to my own FA43 & FA77. You can of course exchange for another sample, but there is no guarrantee the next one will be any better (or worse). The worst sample I have seen was one FA135/2.8 where the aperture looked like an egg.

06-24-2009, 04:57 PM   #5
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QuoteOriginally posted by wlachan Quote
It can be opened and and have the blades rearranged and adjusted to your liking. I have done this to my own FA43 & FA77.
Hmm, I've taken my SMC-M 50mm apart for cleaning and had no problems (besides screwing the plastic front bezel back into the banged-up filter thread - yes, it was an e-bay purchase). Is this much harder with an FA lens? I've got access to a class 100 cleanroom and all the tools I need. I would not take apart a brand new lens, but it's good to know such things.

Actually, I just checked and both my old 18-55 and 50-200 kits look fairly similar aperture wise when stopped down just a tad. The 16-50's 9 blades align nicely though, as they bloody well should considering its price.

You seem to have seen a lot of these defects, wlachan. As far as aperture misalignment goes, is my sample reasonable or would you say it warrants an exchange?
06-24-2009, 05:39 PM   #6
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The FA50 is pretty much the same as the M50. You can unscrew the filter retaining ring from the front, remove 3 screws and the filter ring, then u can unscrew the front lens group. After that u will hv full access to the aperture blades. Based on my experience, urs is not bad really. But whether u will be able to live with it is another matter. The truth is if u r capable of fixing it, it might be a hxll lot easier than exchanging lenses (unless u can exchange it locally).
06-24-2009, 10:19 PM   #7
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I just did a quick survey of a few of my lenses and the here is the jaggy poll:

Have Jaggies
  • Pentax-M 200/4
  • Pentax-M 50/1.7
  • Pentax-A 50/1.7
  • Sigma 50/2.8 EX DG Macro
  • Pentax-FA 35/2
  • Tamron 28/2.5
  • Zenitar 16/2.8

Lack Jaggies
  • Vivitar 200/3.5
  • Vivitar 135/2.8
  • Jupiter-9 85/2
  • Pentax FA 77/1.8
  • Auto Mamiya/Sekor 55/1.8
  • XR Rikenon 50/2
  • Auto-Rikenon 50/1.7

A few of the lackees have a very mild case, but I won't hold that against them.

I will also note that the jaggies (when present) are symmetrical for all my lenses, not one-sided as in your example.

Steve
06-25-2009, 02:27 AM   #8
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Thanks, Alan - I think I'll live with it for now - exchanging would be a pain and I think I can suppress my perfectionism enough not to disassemble an absolutely fine lens for the sake of a relatively small flaw which is only evident at F-stops I rarely use.

Steve, the Pentax glass is not stacking up too well in that survey of yours. That's quite a few classic primes you've got there!

06-25-2009, 06:38 AM   #9
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Another with jaggies.

Just checked my Pentax-A 50/1.4
As you would expect, asymmetrical jaggies.

Al
06-25-2009, 07:25 AM   #10
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In looking at many of my lenses, I find that there are a lot of different things happening, all of which I am sure are much more related to the "bokeh" and behavoir under flare conditions than anything else.


most of my pentax lenses have the "jaggies" and this is a function of the aperture blade design. It seems to be present on not only 6 blade but 8 blade diaphrams.

I also note that not all apertures remain symetrical as you stop down, and some 6 blade designs are heading towards a triangular opening, or 8 blade designs heading towards square. This to me implies that there are minor asymmetries in the assembly or slight angular errors in the pivot points for the blades.

if you want really distorted apertures, look at an adaptall 2 lens iwht 5 blade diaphram, the opening in no way resembles a pentagon.
06-25-2009, 04:00 PM   #11
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QuoteOriginally posted by wlachan Quote
It can be opened and and have the blades rearranged and adjusted to your liking.
But that will only address any asymmetry, not the jaggies, right?

Pentax apparently thinks the jaggies aren't a big deal. I don't like them. My FA 50/1.4 has them too. My Sigma 28/1.8 has a perfectly smooth and round aperture at all f-ratios.

Last edited by Class A; 06-25-2009 at 06:28 PM.
06-25-2009, 04:14 PM   #12
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QuoteOriginally posted by Class A Quote
But that will only address any asymmetry, not the jaggies, right?
True. The jaggies are part of the designs and cannot get removed.
06-25-2009, 07:06 PM   #13
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QuoteOriginally posted by Lowell Goudge Quote

if you want really distorted apertures, look at an adaptall 2 lens iwht 5 blade diaphram, the opening in no way resembles a pentagon.
I guess I have a good one...my Tammy 28/2.5 has the 5-bladed diaphragm and is quite symmetrical. I will admit that the lens had a defect in the aperture mechanism when new and was factory serviced at that time. It may be that they took special care when reassembling to make sure everything was lined up.

Steve
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