[bygp]

Here is the challenge :

The first lens I bought after starting with my K200D is an Auto CHINON MC 50mm 1.7, I got it for around 45$ on e-bay and it is in mint condition, lenses are clear, no dust, no fungus, I love it because it takes beautifull colors, it's very fast, sharp.

BUT, if I focus infinity, all the photo is sharp excepted right and bottom starting to get blurry (focus before infinity) after 3/4 of horizontal and vertical dimensions. Just like if I used 2 linear gradient filters (blur) on right and bottom sides.

Someone told me one lens element could perhaps not be in the focal plan. Ok, but if so, I should also get blur on top and left (beyond infinity), no ?

I now have a bit more experience in M or A lenses disassembling, but how to find the faulty part ?

Any tips or tricks on how to inspect deeply a lens would be welcome. I know I can buy another 50mm, but I have a low budget and I just want to keep this lens and fix it, I will fix it, no matter when, just because I'm a f.... hard-headed celtic

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Last edited by bygp; 09-12-2015 at 10:09 PM.

[spencerskelly]

Stubborn Irish, here.
I have had bad results with Chinon lenses, I'm afraid.
The adjustments can sometimes be made, but I'm not sure it would ever match the M 50 1.7 from Pentax.
You can find it for about the same price, I think, less from a thrift shop or a similar situation.
If it's almost a gradient blur, It might even be the lens mount...
Good luck, let me know if you are able to fix it.
Thanks.

[Na Horuk]

Don't know that lens specifically, but here is what I'm thinking.
a) possible the lens is decentered, unaligned. No, this doesn't mean both opposite corners are blurred. It just means the DoF will be more diagonal, that part of the photo will be unsharp, and that CA will be inconsistent. This is possible, this its a bit odd that this problem would only be noticeable at infinity. And its a bit odd that a 50mm lens would exhibit this (usually wide angle lenses are more susceptible to this)
b) I think its more possible the lens has a fault where its focusing mechanism moves the glass elements in an odd way. I think this might explain why the IQ is normal at close range (actually.. is it? can you make tests near the MFD?) but gets worse at longer distance, as the focusing mechanism is not moving all the parts correctly. Now, this is not a problem that all lenses are susceptible to. This depends on the lens design, so your lens might not have this problem.
c) Its possible the lens has grime on the back element or somewhere. Make sure it is clean. Sometimes things like that become more apparent at certain aperture and focus range

Anyway, the problem is that if you send the lens in for repairs, you would end up paying as much as you paid when you bought it. And the repair might not even be 100%. Maybe you can send it to a trusted repair person. Or upgrade to a DA 50mm f1.8 for infinity shots. These are super affordable lately, cheaper than some older 50mm lenses.. And it has full automation, warranty, rounded aperture blades, good modern lens coatings.. more contrast than older lenses, good colours, great flare resistance.

[bygp]

Don't know that lens specifically, but here is what I'm thinking.
a) possible the lens is decentered, unaligned. No, this doesn't mean both opposite corners are blurred. It just means the DoF will be more diagonal, that part of the photo will be unsharp, and that CA will be inconsistent. This is possible, this its a bit odd that this problem would only be noticeable at infinity. And its a bit odd that a 50mm lens would exhibit this (usually wide angle lenses are more susceptible to this)
b) I think its more possible the lens has a fault where its focusing mechanism moves the glass elements in an odd way. I think this might explain why the IQ is normal at close range (actually.. is it? can you make tests near the MFD?) but gets worse at longer distance, as the focusing mechanism is not moving all the parts correctly. Now, this is not a problem that all lenses are susceptible to. This depends on the lens design, so your lens might not have this problem.
c) Its possible the lens has grime on the back element or somewhere. Make sure it is clean. Sometimes things like that become more apparent at certain aperture and focus range

Anyway, the problem is that if you send the lens in for repairs, you would end up paying as much as you paid when you bought it. And the repair might not even be 100%. Maybe you can send it to a trusted repair person. Or upgrade to a DA 50mm f1.8 for infinity shots. These are super affordable lately, cheaper than some older 50mm lenses.. And it has full automation, warranty, rounded aperture blades, good modern lens coatings.. more contrast than older lenses, good colours, great flare resistance.

Thanks very much for your sharp analysis, the focus offset is quite tiny because I inspected closely some photos and yes, you can see this effect before infinity but it is harder to spot it. When focusing infinity, the right side is focusing 10-20 meters, while center, left and top are perfectly infinity sharp, so I have a very good feeling with your 1st hypothesis : one lens is not well centered. Now that I've seen several rear blocks build, it sound possible that with a shock, one lens moved a bit out of its support ring, so I'll digg this way. I don't think it is a focus mechanism issue because the build quality of this body seems excellent and damaging this mechanism (almost always using the same helicoidal thread with grease) seems impossible, even if the lens falls on the floor. You gave me ideas and perhaps the good path. I'll take a pic of disassembled suspect part to show it here. Oh yes, I also love to fix things so I won't ask someone to do it for me When it comes to my health I go see my doctor, but when it is hardware (I'm a polytechnics engineer) I rarely fail to fix something (but often short in good tools), from electronics to heavy mechanic, not forgetting software and O.S. issues.

[bygp]

It seems fixed, but I now know this is not a top lens. The rear block is made of 3 lenses with 2 of them sticked together with a resin. Very strange engineering, separation rings or hosting cylinder shape are really weird so that lens positionning is very approximative IMO, there is loose untill you screw firmly the top ring. I now only have a tiny borders "normal" blur mainly on left and right side, top and bottom stay almost as sharp as center. That said, I can now use it for landscapes after an infinity recalibration (fixing the rear block issue seems to have offset a bit infinity focusing, not as sharp as it was before). Thank you again Na Horuk for your guidings, it helped me a lot investigating

[Na Horuk]

Great! And thanks for posting the mini explanation about what you did. Such lens disassembly tutorials can be very valuable to people who encounter a similar problem.
And thanks for the thanks, but I did next to nothing Good work

[manntax]

It seems fixed, but I now know this is not a top lens.

I wouldn't go as far as that - it just means your lens was either dropped or disassembled (at least by you now) so it cannot perform as it should and was originally meant to. Some of the best photos I saw were from various chinon lenses, 50, 55, 28 and 135 and they really can match or exceed sometime the quality you'd normally expect from pentax. So don't try draw too broad conclusions out of one case

[bygp]

I wouldn't go as far as that - it just means your lens was either dropped or disassembled (at least by you now) so it cannot perform as it should and was originally meant to. Some of the best photos I saw were from various chinon lenses, 50, 55, 28 and 135 and they really can match or exceed sometime the quality you'd normally expect from pentax. So don't try draw too broad conclusions out of one case

Yes you are right, i'm pretty sure this lens fell on the floor. It gives beautifull colors and is very fast and sharp, mine looked mint but was not, now it should be

Some shots I've taken with this lens (and I'm not yet a pro, got my K200D 1 month ago) :