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Optomax 300mm f/5.6
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PostPosted: Tue Aug 28, 2018 7:00 am    Post subject: Optomax 300mm f/5.6 Reply with quote

I picked up this T2-mount lens this morning for £1, so no great loss if it can't be fixed, but if possible I'd like to separate the rear lens group as there's haze/fogging between the elements.

I've figured out that the rear end of the lens assembly unscrews behind the aperture, so I can get at both sides of the rear group, but there doesn't appear to be any slots or other means of unscrewing the internal lenses.





The aperture has 13 blades, so it might be a nice lens to use on occasion, but at the moment it's just a little too soft Sad

I realise there should be a tripod mount, but for the price I couldn't really complain Wink

Any help would be gratefully received Smile


Last edited by kypfer on Fri Sep 14, 2018 10:02 pm; edited 1 time in total


PostPosted: Tue Aug 28, 2018 7:49 pm    Post subject: Reply with quote

It'll be easy enough to clamp a tripod-mount ring on it,

Can't offer anything on disassembly though, sorry. Rolling Eyes


PostPosted: Tue Aug 28, 2018 10:18 pm    Post subject: Reply with quote

Haze on the rear group of many these presets (300mm f5.6's; 400mm f6.3's; 500mm f8's etc) is bad news - it's usually deterioration of the glass, can't be cleaned.

http://forum.mflenses.com/hazy-lens-element-not-responding-to-normal-cleaning-methods-t66075.html


PostPosted: Tue Aug 28, 2018 10:59 pm    Post subject: Reply with quote

marcusBMG wrote:
Haze on the rear group of many these presets (300mm f5.6's; 400mm f6.3's; 500mm f8's etc) is bad news - it's usually deterioration of the glass, can't be cleaned.


Thanks ... not what I was hoping to hear Sad

Unfortunately, without knowing how to disassemble the rear group I can't test the haze for solubility or "cleanability". Given that the group appears sealed, whatever caused the problem was probably put there at assembly time. I'd guess either degraded optical coating, though I'd expect to see that on all surfaces if that were the case, or deposit from evaporated locking sealant or maybe even the paint.

Here's hoping for bit more information Smile


PostPosted: Fri Sep 14, 2018 6:10 pm    Post subject: Reply with quote

Well, failing any other information and working on the principle that "as it's already broke I can't hurt it", I got brutal and managed to partially disassembled the rear group.

With the T2 mount removed, the retaining ring inside the rear of the lens proved to be plastic, which actually unscrewed easily once I'd drilled a couple of holes in it to get a purchase with a pair of needle-nose pliers!

With that loosened the rear element was loose and could be removed. The plastic ring won't actually come out of the rear tube, but it will rotate longitudinally to allow the rear element to be extricated. There is a spacer that may also fall out at this point ... don't lose it!

The mistiness/fogginess proved to be only on the inner (flat) surface of this rear element and would appear to be degraded optical coating. It won't wash off with alcohol or acetone, but, as it's a flat surface, it's practical to re-polish the surface with some metal polish. Several minutes later there was an obvious improvement in the clarity of the element so the whole has been reassembled (with a bit of fiddling to get the lens and spacer aligned).

At the next opportunity I'll make a few tests to see how it performs. It probably won't be outstanding, it's only a 3-element lens after all, but I think it's clear enough to be usable and that multi-blade aperture does look enticing Smile


PostPosted: Fri Sep 14, 2018 10:27 pm    Post subject: Reply with quote

I finally found my post on a very similar lens I have (somewhere, in a box in the garage)

http://forum.mflenses.com/rubica-300-5-6-what-the-heck-is-that-t6798,highlight,%2Brubica.html

A "Rubica" brand on mine, otherwise the front barrel styling seems updated on your Optomax, with a rubber grip and a "step" in the barrel, probably because the maker reduced the barrel dimension for the sake of the new grip. Less material and machining without the knurled portion, etc.

Its probably also a three element lens.

Please ignore my comments on it, this was in my more speculative days!
The shots were taken at 6mp on a Pentax K100D and the lens is quite sharp on that sensor at f/8, not quite so at f/5.6 - see the bird.


PostPosted: Sat Sep 15, 2018 7:38 am    Post subject: Reply with quote

luisalegria wrote:
I finally found my post on a very similar lens I have (somewhere, in a box in the garage)

http://forum.mflenses.com/rubica-300-5-6-what-the-heck-is-that-t6798,highlight,%2Brubica.html


Thanks for the link Smile If the results from my lens are anything like as good as yours I'll be well pleased ... even if I too rely on the Shake Reduction in my Pentax Wink