just recieved and interesting (to me) J-8... any input appreciated!

[PatrickCheung]

I just recieved a J-8 I ordered a few weeks ago! I'm super excited as I've heard great things about this lens, however... things don't look good already.

First, there appears to be two small airbubbles trapped under the first element. I don't know if they're air bubbles or if they're just... hollow particles. Right next to those bubbles appear to be some residue under the glass. I've tried to take a photo but this is the best I can do for now... I'll try and take a better one some other time...

Second, the focus ring turns way past the 1m mark... so far that the lens doesn't really engage the rangefinder cam anymore... does anyone know what's up with that? I've attached a photo of how far it turns

I've "tested" the focusing (focused on an object with the camera attached to tripod) against a Canon 50/1.8 LTM and a Nikon D7000 + 35mm lens (the canon LTM and Nikon match up)... and it appears to be a bit off, off so that (if the scales are correct), all shots shot at f/5.6 should be in acceptable focus.

I'm just wondering... how much would the air bubbles (They seem pretty big relative to the glass) would affect the images... and if this lens is worth fixing.

I'm kinda afraid to shim it myself... would anyone know someone who would shim and CLA it for a reasonable price, preferably located in Toronto? I wouldn't mind sending it out if the price is right. But just incase I feel brave one day... would anyone have a guide with photos? I've looked at a few links, threads, and guides, but they all seem to be for J3s or just don't seem to be too detailed!

thanks for your time!

 

[jdos2]

Jupiter-8's are a funny beast.
Some fantastic, some not so much. Yours seems pretty dirty- and I would hope that the reason is because it's so well used that the person wouldn't part with it.

Bubbles in the glass, at one time, meant that the glass was of such quality that it couldn't sit in the crucible for long because of chemical reactions that would happen- so was considered a sign of "good glass."

Dirt and smudges, on the other hand, are signs of... Dirt. Smudges.

So... I'd not worry about the lens until it's been in for a CLA.

Please know too that many lenses focus "too close" for the rangefinder to track. Just keep your eye on the rangefinder patch when you're close focusing and be aware of when it decouples.

Sorry- don't know about anyone who can CLA the lens anymore. I think my Russian guy is out of business these days- and he's in Russia anyway.

 

[Sonnar Brian]

Air bubbles are typical in lenses from this era. The focus is supposed to go past 1m, that is be design. The RF pickup of the FSU cameras allow closer coupling than Leica style roller wheels.

How far off is the focus? Usually adding 0.1mm to the shim corrects most back-focus. The optics module unscrews from the focus mount, you will see the shim. The worst part is re-aligning the aperture ring. The process is the same as the J-3.

Kim Coxon has a J-8 guide here:

http://pentax-manuals.com/repairs.htm

 

[PatrickCheung]

oh! I was looking at those guides earlier, i assumed the shimming process only applied to the J3!

I'm kinda scared to work on this lens though, the screws seem extremely fragile. I'm wondering if they can be replaced with I50 screws? And... sorry or all the questions but where would i find a shim?