Focusing problem with FA 24-90mm


aliengrove

Link Posted 13/07/2012 - 15:31
I have a minor niggle with my FA 24-90mm. It's razor sharp at infinity focus, but at less than infinity it seems very slightly out, more so at f5.6. At f8 it's good at across the board. It's only a very small amount, but it is noticeable at 100% zooming into an image. Occasionally the focus "chatters" at infinity at 90mm. Any ideas?
Flurble

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fritzthedog

Link Posted 13/07/2012 - 16:12
Hi

I had a similar issue with an FA 28-90 - including the "chattering". I think the construction of these 2 lenses is similar although I am not 100% certain and am aware that the 24-90 is superior quality which may mean the design and element grouping is different.

On the FA 29-90 - I removed the front element group which was straightforward using a standard lens opening tool. On doing this, I discovered that in fact the front element had worked a little bit loose which may or may not have been part of the problem. On removing the front element group - you have a clear view down the barrel to the gearing. On mine I found that there is a plastic cog and sort of ratchet assembly - same sort of principle as a canal lock gate and this had become slightly out of alignment and was not fully engaging. I corrected this with mild pressure and it clicked back in to place. I re-assembled the lens after taking the opportunity to use the rocket blower and remove an annoying hair whilst I was in there - and it has been perfect ever since.

Carl
No matter how many lenses I have owned - I have always needed just one more

aliengrove

Link Posted 13/07/2012 - 17:20
Thanks Carl, I have never opened a lens up before, but am willing to give it a try. What is a lens opening tool? I will have to find one of those.
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fritzthedog

Link Posted 13/07/2012 - 18:30
There are many slightly different designs

Here is an example of one: link

I actually have a precision made tool that I bought from a well known guy in HK who makes them to order - beautiful quality but not worth it if you only want to open one lens

Many people actually make their own out of old calipers, paint scrapers and other assorted hardware, Basically all you need are 2 arms with a spacer in between and at the end of the arms you need a thin flat head - like a precision screwdriver.

On the outer ring of the front of the lens you will see 2 grooves opposite each other - what you are doing is lining up an arm with each groove and then twisting you unscrew the front of the lens.

Sometimes these are quite tight and need pressure - hence the need in my book for having something that is not going to slip - even with the tools I use - I still protect the glass before starting.

It is also worth trying just putting a precision screw driver in one of the slots and seeing of you can move the ring - as conversely sometimes they are quite loose.

If you were in the UK I would offer to lend you mine if you paid postage both ways but that nay be a tad expensive to Doha!

If you decide to give it a go and want any advice - just pm me - I have opened many lenses - mainly to clean internal elements and so far have not lost one!

Carl
No matter how many lenses I have owned - I have always needed just one more
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