Haze Problem. sub group dissasembly question

bokeonthenet · Joined: 29 Dec 2015 Posts: 3 Location: UK

Hi

First post on the forum, I've come here as it seems to be the best place on the net for DIY lens fixers.

The lens; I have a canon 28mmf1.8 autofocus lens.. the AF is fine, but it's hazy.. fractionally noticable with a cloudy sky, very noticable shooting into the sun.

I've tried dissasembly, cleaned all accessible lens surfaces and reassembly, whilst this helped it didn't fix the haze. I've also tried a vacuum chamber.. again without success.

I've had a price from Canon for replacement of the offending part, but they want to replace half the lens so it would cost ~£140.

Please see the canon "museum" for the block diagram: http://www.canon.com/c-museum/en/product/ef331.html

I've isloated the fogging to the second or mid group, which I believe contains only elements 3&4 (No.1 being the biggest front element). If I'm right then it can't be a problem of "interlens" bonding.. is that the right term?.. as both elements in that group are single elements.

The problem I have is the two elements are bonded to either end of a plastic tube with a mounting flange on it. This is what I'd like to get apart and reassemble... my problem is I don't know how to dissolve the glue.

Some images of the offending group, and a sample image.

https://dl.dropboxusercontent.com/u/44641599/28f1.8_1.JPG
https://dl.dropboxusercontent.com/u/44641599/28f1.8_2.JPG
https://dl.dropboxusercontent.com/u/44641599/28f1.8_3.JPG

any and all suggestions would be extremely helpful.

thanks in advance.

bokeonthenet · Joined: 29 Dec 2015 Posts: 3 Location: UK

odd stuff happening.. the lens diagram link doesn't always seem to show.. so here it is again.

http://www.canon.com/c-museum/en/product/ef331.html

visualopsins · Expire: 2021-06-22

Welcome bokeonthenet

What a lens!

Perhaps you can purchase the problem assembly from Canon parts...they may have old parts...

170 doesn't sound like a lot to me for fixing this fast wide lens.

SXR_Mark · Joined: 12 Jun 2010 Posts: 506 Location: England

This lens is not especially expensive so I appreciate your reluctance to pay �140 for a repair at Canon. However, I would say that a DIY repair would be risky. You are correct in saying that the fault in that group is not due to cementation failure since the two elements are separate. But, extracting those two elements from the plastic carrier in a way that allows them to be replaced with the required alignment precision will not be easy, i think.

My guess is that the elements are not glued in but �clinched� into the plastic. I did extract an element from a plastic carrier like this by cutting the rim of plastic around the lens away with a sharp knife. Once the rim was removed the lens came out easily. You could try this with one of the elements. The inner faces of both can then be cleaned.

You will then have the problem of fixing the element back in. This will have to be done with glue, and you will have to hope the fit in the plastic carrier is sufficiently close to ensure the element is properly centred and spaced from the other element.

The final risk is that you may find the haze is due to coating deterioration and cannot be cleaned away. I would say this is not a high risk, since the lens is quite modern and should have quite robust coatings.

bokeonthenet · Joined: 29 Dec 2015 Posts: 3 Location: UK

Thanks for the feedback. the lens is "only" worth ~£300, it's mid range glass, perhaps if it were an "L" series lens then everything would be screwed into place.

I hadn't concidered that the lenses might be held into the plastic tube by some kind of frictional arrangement, but it would make sense.

could gentle application of heat help?.. i.e. a temperature controlled heat gun run at something like 70C?