Tim,
Oily blades eventually cause unpredictable overexposure. Over time, the oil
picks up dust which makes the blades slow to respond.
Example:
========
You have an early 50mm f/1.8 and select f/8. At the moment you release the
shutter, the mirror comes up and the lens tries to
"reach" the selected aperture. The sludge on the blades slows them down so you
might only get f/2 at the beginning of the exposure.
As the vibration of the shutter shakes the lens slightly, the blades close down
a bit more so at the end of the exposure the lens
might be at f/4. It might never reach f/8. Result: over exposure that is not
predictable.
Worst case scenario:
====================
If you shoot wide open, you get correct exposure. If you shoot at f/16, you'll
eventually get more than 5 stops of over exposure if
it's locked open entirely.
How to tell:
============
It's very easy to check a lens for oily blades. With a reading light (bright,
contrasty) shining over your shoulder, dismount the
lens.
1. Stop it all the way down.
2. Find the "aperture preview" button and hold it down.
3. Look into the front of the lens carefully. The lens should show you a
small opening. The word aperture means "opening" or
"hole". It should not be "stuck" all the way open. (Be sure you're not
pressing the very similar-looking "lens dismount" button.)
You'll see there are several fingers or "blades" that close in a pattern to
form the aperture. Logically enough these are called
"aperture blades".
Now we get to the heart of the matter. The face of the blades should not be
oily. If they are it means that some oil has traveled
("migrated") onto them. If you keep it out of dusty environments you may be
able to use it the rest of your natural life.
If you shoot macros with the 50/f3.5 though, focussing the lens up close
actually drags lots of external air (and pollen, dust,
nicotine, desert sand) into your lens.
Solution:
=========
If you just bought the lens, send it back immediately and get a refund.
If not, you can live with it or send it to a camera repairman for disassembly
and cleaning. Some lenses are not worth cleaning.
(They are "beyond econmoic repair", or "BER".)
For example, if I had an early 50mm f/1.8, I would not pay to have it cleaned.
Instead, I'd buy the later 50mm f/1.8, labled "Made
in Japan" on the filter ring. People on the list who test lenses say this is
the sharpest general-purpose 50 for OM. The only
problem with the "MIJ" run is that they have oily blades more often than other
lenses. Now that you know what to look for, you can
buy a "MIJ" 50 on ebay and examine it for oily aperture blades.
Prevention:
===========
Store lenses upright, not on their sides. The theory is that gravity will not
pull the lubrication onto the blades.
Remember, grasshopper, use your new knowledge for good, never for evil.
Lama
From: Tim
> >Where in the lens does the oil come from?
> >Can the oil be cleaned off the blades? and how hard is this to do?
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