SMC coating colours.

I have two identical Pentax lenses. SMC-M 135mm f3.5s. Excellent, compact teles, much discussed and admired, on this forum.
Identical, except for the coating colour, that is!
One has a purple tint, to the front element. I thought they all had. I have had a few pass through my hands over the years, and as far as I recall, they were all purple. The second I have now, is green!. A lovely deep emerald green tint. I was careful to hold both lenses up to the light at the same angle, and position. The second one is without doubt, green. I never noticed, until I had them side by side, today.

Has anyone else noticed this, in other Pentax lenses?. Maybe Asahi changed the coating recipe for a while, like Coca-Cola famously (and disasterously) did, a few years ago.

all my pentax lenses are green, maybe its the sp coating.
SP coating to block dust and dirt
The front surface of these lenses is treated with Pentax-original SP (Super Protect) coating, which applies a special fluorine compound to the lens surface through a vapor deposition process. This SP coating not only effectively repels dust, water and grease, but it also makes it easy to wipe off fingerprints and cosmetics.

but im not sure, could be compleatly wrong.

SP is only applied to the newer lenses so you won't find them on the older lenses.

Could UV radiation or something like that change the composition slightly off the coating?

I think colour will vary with angle of perception.

I looked at them very carefully. Exactly at the same angle of view, in the same light, from the same distance.

This one is undeniably, definitly, beyond any shadow of doubt... GREEN.!

The SMC coating often reflects a definite shade of green.

The coating on the glass is acting as a dichroic filter, which without getting into details of the physics, is making it appear as green or purple depending on the angle you are looking at it. Note how on your lens picture above, it goes from green to purple as the glass curves away from you at different angles (top to bottom).

Yes, flossie. The colour seems to change if you move a lens around, in the light. But, there is a predominant colour, and in each of these lenses, it is different.
I will photograph both lenses, in the same light, from the same distance and from the same angle, now.

Watch this space!

Damn.! - I can't get a decent image to show you, but I swear, they are different colours!... I'm not going mad, really I'm not!

I have a Pentax F 50mm f1.7 which is a noticeably different colour from all my other lenses and has been since new - definitely greenish/yellowish

I keep saying that to the bunny who sits on my shoulder, but he doesn't believe me!

Er - can you have have a concensus on this.

Right.!... You bunch of doubting Thomases, you!..






You see!?... Bleedin' green, innit! - The first one has a purply blue tint, like all M series lenses, but the second is clearly green. GREEN, I tell you!. Stone the flamin' crows! it's obvious. Even Mongooses bunny would say so.

Y'know, I think it may be a Willy Wonka type thing.
Maybe Pentax made a few special green ones, and when the finders of them all are identified, they will get a tour of the factory. All the bad ones would be bumped off during the day, but the good boy.. (me) would be given the company. - Yes, that must be it.
Unfortunately, I didn't spot the difference until yesterday. I put the GREEN lens on ebay a few days ago, and it has bids now. - It will belong to somebody else, tomorrow.

Agreed - it's green.


Unless it's not.

Or should that be :mrpurpletint:

What is the serial number of the lens in the second picture? Looks like it begins 69 to me (compared to 72 for the other one) - maybe they are from a few years apart (not suggesting that the first two digits are year of manufacturer, but rather than these are seemingly several batches apart) and Pentax changed the precise composition of the coating between times?