A/R Coated lenses ?????

[harry a saake]

chuck marks

yes, we just had the same problem with chuck marks and we now put the round blue surfacing tape on both sides, seems to work

[Tibor]

Why do spider marks happen?

SIMPLE, the spider marks are what we call crazing, it is when the AR part of the coating cracks up very lightly.

You have to remember that the AR part of the coating is made up of metalic oxydes which don't have the same flexibility as the hard coat or the lens substratum. So when we edge the lens, the pressure that we apply on the lens will flex it (remember high index lenses are much thinner than regular CR39 on center) and that pressure will break up the Ar part of the coating. The coatings are usualy .5 micron thick and approx half of it is the hard coat. Now the hard coat is a plastic type material that can easely support stretching, but the AR is a metal which even though it has a elasticity factor, it is much less than the hard coat's.

The solution, lower the edgers blocking pressure as much as possible and also when deblocking the lens, do not tear it away or use a deblocker as those tools will also bend out of shape the lens, simply use a pair of pliers and twist of the block.

This crazing also happens when the lens is edged too thight in the frame, as the lens wil bend out of curvature (increase) and the ar coating will not be able to support the tension.

If and only if the AR coating is applied with a buffer coating underneath ( in order to comply with security standards in the USA), these problems will most probably not show up. Also remember that whatever procedure that creates an increase in curvature ( Heat, pressure, very rapid temperature changes etc...) can and most probably will affect the AR coating on the lens.

I which to say that this should not prevent you from using the AR coating as the benefits that they provide are much greater than the little problems that can happen with them. Furthermore with a little care this will not happen. And I can vouch for this as we coat and edge a huge number of lenses everyday.

Tibor Martz

Marketing and Technical consultant
Nikon Optical Canada Inc. :)

[Steve Machol]

Originally posted by Tibor
This crazing also happens when the lens is edged too thight in the frame, as the lens wil bend out of curvature (increase) and the ar coating will not be able to support the tension.

I'm glad you mentioned this Tibor - it's always been one of my pet peeves. I recommend that after edging and inserting into the frame, people shine a small halogen light into the edge of the lens. If there's any stress induced crazing, it'll show up under this lighting.

[Specs]

We won't use Crizal stock lenses because of the block marks. We figured we 'd try them, we had the problem with the block marks on each of three consecutive jobs. All were Poly w/Crizal. What's that rule about three strikes...

[Tibor]

Change sports...

Try football, they give you a fourth down!

On a more serious note, not withstanding the manufacturer, the main reasons for crazing coming from edging is or chuck pressure or when deblocking the lens. If care is taken (I'm not saying that it is not ) there should be no problem relative to crazing. I know that it takes some special care to do it but it is well worth the effort.

Tibor ;)

[Woofman]

Thanks for all of the great a/r cleaning options and recommendations ! This should go over really well at a store meeting with my associates. Who would have ever thought a/r cleaning would lead to other issues with a/r lenses then poly cracking issues. What a great place of information. My experience seconds the recommendations on blue chipping at least the back of an a/r lens ;however, I would not leave the lens blocked up overnight with a blue chip on. I have seen some a/r lenses (non-stock a/r) have the a/r come off !!

GO GATORS !:D

[sandeepgoodbole]

Try coke..

Originally posted by Woofman
Thanks for all of the great a/r cleaning options and recommendations ! This should go over really well at a store meeting with my associates. Who would have ever thought a/r cleaning would lead to other issues with a/r lenses then poly cracking issues. What a great place of information. My experience seconds the recommendations on blue chipping at least the back of an a/r lens ;however, I would not leave the lens blocked up overnight with a blue chip on. I have seen some a/r lenses (non-stock a/r) have the a/r come off !!

GO GATORS !:D

One of my customers reported that Coke is a better shampoo for AR lenses .
Why not try Scotch as well ?

[Pete Hanlin]

Heat pans, blowers, etc., can also cause crazing for the reasons mentioned above. Here in Florida, I've also been able to trace some crazing to the glasses being left on the dashboard in the summer heat.

As to the "imprint" left behind by the blocking pads, I've asked this question before, and I'm almost positive it comes from the stickiness of the pad- not the pressure.

[Tibor]

I cannot say for sure that the glue on the block cannot create a crazing, but it would have to be mighthy sticky to cause a crazing.

As mentionned before, AR coating crazing will happen whenever the AR coating's elasticity factor is reached and passed, and this will happen in all of the aforementioned situations that this forum put forward, excessive heat, pressure, bending, deblocking etc...
I have to this day, see a crazing cause by unglueing a block from a lens.

The only time I can see this happening is when the multilayer coating was not designed as a one coat system, disparate coatings are not ment to create a molecular bond and at that time it can maybe happen?

Tibor

[Calvin Loh]

Well it might sound outrageous here, but I have heard some car polish gives a good protective clear wax coating on top of the AR coated lenses. Any idea about this suggestion?

[Tibor]

This is interesting, because most multilayer coatings offer a hydrophobic layer that acts exactly like a wax.

In fact we could compare it to a wax coat which is under extreme tension and doesn't offer griping capability to water (this is why it beads on the lens. It also works as a protective coating on top of the AR .

Tibor :)