[stevebrot]

Pentax M 50-1.7 and M 28-2.8 do not have cemented elements.(the 50 has a cemented rear pair). Most likely bad dust, fog or start of fungus. See lens diagrams at Bojidar Dimitrov's Pentax K-Mount Page

Six elements in five groups???



The fourth group is cemented.


Steve

[Tomm]

Six elements in five groups???



The fourth group is cemented.


Steve

Seems like that's exactly the element that is 'dirty/bubbled/dusted' at my 28mm lens! Hmm, interesting.

@ Bratt: I don't think it is fungus though. The lenses are in the same bag in the same place for more than 25 years so why should the fungus come up now and not earlier? Dust is possible but I can't see it without a LED light, that's strange...

[bratt]

Sorry I was not clear in making my point. Poor communication skills on my part.

If both of Tomm's lenses have a similar symptom/appearance, the 50 M has a cemented pair but the 28 M does not. Therefore the likelihood of both having separation is 0% .

@ Bratt: I don't think it is fungus though. The lenses are in the same bag in the same place for more than 25 years so why should the fungus come up now and not earlier? Dust is possible but I can't see it without a LED light, that's strange...

It is only noticeable when I use a bright LED light on it.

Did you check it earlier with a LED light?

I received a Takumar lens from Ebay which looked real similar to your lenses. The seller carefully placed the lens in the original leather case, wrapped in bubble wrap and a box. During shipment the lens pulverized the old dried foam padding of the case into a talcum like powder that penetrated into every area of the lens. It is on my to do list. Pristine before shipment

[macTak]

I've seen this to some degree on many of my Takumars and M-series lenses. I don't think many notice it since it's only observable (usually) shining a light from the back of the lens to the front, something hardly everyone does. You'd be surprised how bad it makes most of the older lenses look! Given my experiance that it always seems to occur by the aperature blades, I'm inclined to think that it is dust and/or haze (either from the aperature or migrated from the focus helicoid). When I did research on this, I read that there are two types of haze: "smoky" haze that looks like a layer of smoke deposited on an element, and this "droplet" type haze. I did end up with a lens with the "smoky" type haze (A28/2.8), so there definatly is a differece. I'd expect this droplet type of haze/dust to be easy to clean if one desired, but I've never found it to impact IQ. In my opinion, I wouldn't worry about it until it's clearly visible shining a light through from the front of the lens (as the light travels to the sensor anyway).

[stevebrot]

... shining a light from the back of the lens to the front, something hardly everyone does...

...and which I routinely do with all used lenses that I purchase. I have a pretty good number and can honestly say that I have never seen anything like the OP's image.


Steve