Cleaning the insides on a lens

While cleaning my Sigma 24-135mm I noticed a small dot on the last but
one lens element. On close inspection it seems that a tiny insect
crawled into the lens and died on the glass element Now its stuck
there!!

Any idea how easy/difficult it is to reach that spot? I found several
screws on the back of the lens but am not enthusiastic about opening up
this lens as it is my primary lens unless I know what I am getting
into. Also, I doubt if there are any Sigma service centres in India.

Thanks for any help,

- Siddhartha

 

Live with it! Any attempt to dis-assemble the lens has a very high
chance of ending in tears. The effect of the bug on image quality is
almost certain to be immeasurably small.

David

 

Don't mess with it. There are just far too many little parts in there
that all need to be returned to their exact original positions. That little
bug is not likely to impact on the images (a few test will show that, try
photographing a gray card). If there is a real problem, get an estimate
from a camera repair person.

 

All I see is a small black dot on my photographs so I am not in panic
yet. I guess I will just live with it and replace the lens with
something better and faster in a few months.

- Siddhartha

 

It is highly unlikely that the small black dot on your photographs is
caused by a speck inside the lens; the latter would be so far out of
focus as to be wholly invisible. Far more likely (assuming, from the NG
you have posted in, that it is a DSLR) that there is a quite separate
dust spot on the sensor.

David

 

It's Karma!

 

It sounds like something on your sensor. If you have another lens, I'll
bet you will see the same spot. Lenses don't focus on bugs inside the lens
very well. At most it is likely to show up as a very fuzzy difficult
to see blob.

 

Yeah, I have a substantial speck of dust inside the front element on my
70-200L f/4 and it's nowhere to be seen in my photos.

Greg

 

A lot of people in this newsgroup have said that Sigma lenses
tend to have bugs.

 

You are getting plenty of warnings not to try. (and it might be just
sensor dust causing the spot - as pointed out).
I'm not disagreeing with them.

But if you do want to give it a try, then it might not require removal
of the mount. Rear element assemblies usually unscrew as one piece in
their barrel - you may need a lens spanner - don't try just jamming
something down to the notch to try and lever it around as you will
likely slip and break something.
Sometimes the gap is not wide enough between the rear element group
barrel and the inner edge of the mount - and the mount will need to be
removed.
To remove the mount, as well as the obvious screws, there are usually a
couple of very small screws (assuming lens is positioned front element
down) under the electrical contacts. Unscrew and remove these - as
otherwise you risk damaging the ribbon cables as you attempt to remove
the mount. The electrical contacts stay with the body of the lens as
you remove the mount.

Don't drop anything down the lens once you have removed the rear
element. If you do, it will likely end up in the middle of the lens
around the diaphram - and you may have turned a relatively simple job
into a big one.

Note that there are probably spacer washers with positioning lugs under
the mount. The screws holding the mount on are not usually positioned
symetrically - they only go one way. Make sure when reassembling that
all washers are replaced and positioned in the correct rotation. If
something doesn't want to go back into place, then don't force it.

If everything turns to custard, then the following method should be used
only as a last resort:
http://www.hermes.net.au/bayling/repair/lens_20b.jpg


Good luck.

 

Nice choice of tool, but I like this one:
http://www.sledge-o-matic.com/images/Fan1-30-2004b.jpg
-tom

 

If it doesn't bother the pictures - leave it!!

I once opened a lens just to see what was in it, and now it's a big dust filled
mess! Remember the kid that took apart the alarm clock and never got it back
together? That was me... although I'm better at it now!

The lens I played with was just a scrap lens from a copy machine, but there is
no way to keep the dust out once opened, unless you have a clean room...

 

Take it to a camera repair operation,. They don;'t need to be Sigm
experts just experts at cleaning lenses. If you take it apart, even i
you got it back together properly, you will likely have more dust and jun
in the lens than you have now. If it is truly a bug, then I would ge
rid of it. Organic matter in the lens is just likely to attract mor
organic matter which over time will make a mess of this lens. --
Ji

 

Unless you have a particularly dust-free environment, and the idea of
replacing precision-fit lens elements in aluminum tube doesn't bother
you, I'd avoid it. The saddest thing will be if someone tries to
"drop" a lens element back into place only to have it lodge sideways.
Then the real fun begins. Having disassembled and reassembled lots of
these things, it's not something you'd want to do if you have no
experience. The worst thing is that if the lens has any flint (it
will) or ED elements. They are fragile and can get edge chipped
easily.
-Rich

 

is the bug more or less flat black? If so, how could it have *any*
effect apart from reducing the T number by about 0.001%? (not that it
would have much effect if it was a brilliant white apart from increasing
flare by 0.001%)

 

sorry, I just re-read the OP and he mentions the bug is close to the rear
element and he sees a black spot. I think I have read that scatches on
rear elements (especially wide angles) can be imaged, so it sounds like
it is time to take it to a good repair shop (if the black spot only
appears with this lens and is not a sensor blob).

 

Hmm .... that's something I've been dreading - dust on the sensor. Need
to run some tests to figure out where and how much dust is on the
sensor. And with all the responses I've got regarding the lens, I guess
I am not attempting to open it or anything of the sort.

Thanks,

- Siddhartha

 

It is most unlikely that the "bug" is on an inside surface of the rear
element group - as the tube itself is effectively sealed. It is probably
on the front element of the assembly, and should be reasonably easily
accessible - without disassembling the rear element group itself.

 

Oh, now you disassemble and reassemble lenses? Jeez, what a moron!

 

He could also take a photo with no lens at all. If it's dust on the
sensor, it'll still show up.

Mark