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Old Aug 20, 2006, 10:23 PM   #11
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Hi again John,

It sounds like it's viewfinder dust to me. I usually ignore it and write it off as a relatively small inconvenience compared to the great feature of a user removeable interchangeable focusing screen. I usually will let it get to the point where I can no longer ignore it before I take mine out and clean it. It's really doubtful that dust on the focusing screen is causing your AF problems.

If you do choose to clean the screen, be very careful because it's very fragile (easily scratched) -- believe someone who's had the experience -- the lines caused by scratching the screen will be a much bigger PITA than any dust.

Here are some tips from Rachel Katz, from Katz Eye, the company that makes replacement split-prism focusing screens for most DSLRs:

"As you probably know, focusing screens (of any brand) are quite delicate
and must be cleaned very carefully. Here are my recommendations:

In the camera, about all you can really do is blow the dust off with a
bulb blower. I don't recommend canned air, as the possibility exists to
blow freezing liquid into the camera. The only other thing that can be
done in-camera, is that you can loosen any stubborn dust particles with
a very soft brush - a sensor brush or something similar.

With the screen out of the camera, you have a few more options. Again,
the bulb blower is the preferred option because it is the safest.
Sometimes dust will adhere to the screen by a static charge; here's a
little trick to help with that - just give the screen a breath of warm
air, as if you were trying to fog up a window, and immediately blow the
screen off with the bulb blower. You may need to repeat the process
several times. I know it sounds a little strange, but the slight
moisture in your breath will actually help to dissipate the static
without contaminating the screen. Of course, the sensor brush is just
as valid out of the camera as in-camera. In addition, the canned air is
safe to use outside the camera, although you should still try to avoid
letting the freezing liquid hit the screen. Don't panic if it does - it
probably won't hurt the screen; but avoid it if you can.

If the screen has become fingerprinted, contaminated with oil, or
heavily soiled somehow, it is possible to wash the screen. Avoid this
if at all possible, but if you have serious dirt or oil that won't
respond to the gentler methods, here is what you should do. Get a very
soft artist's brush
- red sable works nicely. Wet the screen with warm tap water; place a
drop of liquid dish soap (like Dawn) directly on the screen. Wet the
brush and work the soap thoroughly into the screen. On the matte
(prism) side, you can use any motion you'd like. On the other side,
there is a Fresnel lens that has a series of concentric grooves; to
clean them properly, you will need to work in circles starting at the
middle and getting progressively larger. When you're done, rinse
thoroughly in warm water using the brush to help to soap get off. Then,
follow with another thorough rinse, this time in dead cold water.
Finally, blow the water off the screen with canned air (making sure not
to freeze it) or a big bulb blower (like a rocket blower). The key is to
blow the water off the screen before it can evaporate and leave water

As a final bit of advice, here are some things you should NEVER use on
the screen. DO NOT USE alcohol, nail polish remover, paint thinner,
Windex, commercial lens cleaning solutions, or any other sort of
solvent. Some of them might be safe, but you use them at your own risk
- the plastic of the focusing screen can be softened and smeared by
solvents. Also, DO NOT dry-rub the focusing screen with a lens cloth or
lens tissue - it will almost certainly scratch. And finally, although
it might seem obvious, DO NOT touch the either face of the screen with
bare fingers - fingerprints are very difficult to get off. Hold the
screen only by the edges or by the tab and if you want to be really
safe, put on some latex gloves before you do any serious handling (like
the washing procedure above).

If you have any questions or need more clarification about cleaning,
please feel free to email or call - I'm in the office 8am-6pm EST,
Monday through Friday.

Best regards,
Rachael Katz
Katz Eye Optics "

Good info, IMO I've corresponded with her on a number of issues, and she's very knowledgable concerning focusing screens.


John Hill wrote:
One of the things with my camera is that there are a few dust specks that show in the viewfinder, now I have been of a mind that I should leave well enough alone so I have never tried to find and clean these specks as they do not show in the captured images.

Potentially though they could be messing up the auto focus? Maybe? My little rubber bulb air blower did not dislodge them all, there is one little speck in particular right close to the middle of the view I would like to eliminate but I cant decide which surface it is on. Releasing the lens lock and gently turning the lens while looking in the view finder confirms this speck is in the camera but where? On the mirror? On the focus screen? If it is on the mirror can I clean that with anything other than air? What about the focus screen, is it OK to use lens cleaner swabs on that?


snostorm is offline   Reply With Quote
Old Aug 21, 2006, 1:34 AM   #12
Join Date: Aug 2006
Posts: 45

Check the contacts between the lens and the camera. I had this problem once, and I pulled the lens off, and put it back on and everything was fine. I'm thinking that perhaps not all of the contacts were properly lined up or working as they should.
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