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03-29-2009, 03:41 PM   #1
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Auto Chinon 50mm f1.7 - Dust - Repair

hi Guys

First time poster long time browser here.. and a recent migrant to Pentax already afflicted by LBA..

I digress...

I managed to pick up a reasonably priced (for Aus anyway) Auto chinon 50mm f1.7 lens yesterday. All looks very mint condition except dust inside the lens. The dust looks almost like glitter (im pretty sure its not mould as its in the centre of the lens) it doesnt seem to effect the pictures but it is visible through the viewfinder.

Anyway me being me, im looking at taking the lens elements out and cleaning it up. i think from what i can see all the lens elements come out of the back (certainly a couple do!) is there any secrets to taking these lens's apart? other than powder free latex gloves / cotton gloves to stop sticky finger prints and a good lens cleaning solution?

Any hints would be greatly received!

Thanks

Steve


Last edited by 5teve; 03-30-2009 at 04:39 AM.
03-30-2009, 04:39 AM   #2
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Any ideas how to get the front elements out? i have the rears out. it looks like there is a screw threaded end holding the lens in place but there is nowhere to grip it and its doesnt seem to budge.. any help would be very much appreciated!

Thanks

Steve
03-30-2009, 04:39 AM   #3
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Hi Steve and welcome!

I wonder what you are actually seeing in the viewfinder, cause you can't see particles within the lens through the viewfinder. Most probably this is dust on the matte screen surface.

You probably know already that specks within the lens do not affect image quality except very harsh lighting.

If you are still keen to take that lens apart (you seem to be :-) I would do just like you proposed. For cleaning, I use clean cotton swabs (no 'aloe vera') and alcohol from the pharmacy in 70%alc./30%distilled water and for finish my breath. Make sure to always have the swabs softened via the alcohol or breath film, don't rub with dry swabs on the lens, but rather dry up through the sweeping process.

Most particles inside lenses (specially in the center) are almost always in the middle of the lens on either side of the aperture. There are two concave surfaces, try to access these without further assembly of the lens groups. Then, put the lens back together only to find more dust than before .-)

Here's good info on basic lens design,
History of fast 35mm film optic lenses

You've been warned!
Georg (the other)
03-30-2009, 04:43 AM   #4
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Thanks Georg...

i have it in a few peices and have the rear out ready for cleaning, the major specks are in the front 2 elements and look like hefty lumps of dust, hence the reason i can see them as a black splodge in the viewfinder. it goes when i change lens so i know its not the matte screen.

any pointers for the front would be good tho

i do have a lens cleaning kit which contains what you suggest so all is good there, i also have some light cotton gloves and a rocket blower to dispell everything!

once again thanks... i'll go look at the link now!

Steve

03-30-2009, 04:45 AM   #5
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edit: You have been faster than me writing. As for the front, try to move the barrel all the way out to the near limit. Then, see if there is some tiny screw in the barrel to get at.
With the screw undone or without any, try to unscrew the front barrel on infinity position (don't put too much force on it, just have a try)
Other than that, you could try to loosen the name ring via friction. A back lens cap with double side sticky tape on the rim helps with this
G'luck!
03-30-2009, 06:46 AM   #6
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OK! finally!

Turns out that behind the name plate are 3 screws, the name plate is glued on!

once i worked that out it was all plain sailing, all lenses are nice and clean and DUST FREE due to some very careful placement and blowering all at the same time! im pretty happy with the result, just have to try the lens out now...

the focus has gone a little stiff toward infinity, i think i may of removed some grease from the threads.. i'll get round to redo-ing that soon.

as for the splodge... i was WRONG!

it must be on the matte screen, but it doesnt want to budge even with the rocket blower doing its best... any ideas?

i dont want to introduce any more issues by blowing more dust in

Thanks again for you hints and tips... much appreciated!

Steve
03-30-2009, 07:03 AM   #7
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what camera? Pentamirror or prism?
Is not so difficult to remove the Matte screen. For me was more difficult to put it back.
Try this KatzEye site, and choose your camera.
Basically is easy but take care not to scratch the screen.

Focusing Screen Installation Guides - Katz Eye Optics
03-30-2009, 07:07 AM   #8
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Thankyou for the link

sorry i forgot to say it is a k20d...

im sure they are pretty much the same so will have a look.

Thanks

Steve

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