Further to the recent thread regarding stiff aperture rings, some may
like to know my experience with a brand new Sigma 70-200 2.8, so if this
is too off topic for you, please go no further. However, there is a
Pentax link...
Notice how most lenses, including Pentax offerings are sturdy affairs,
with lots of metal used. Take my A* 85mm 1.4, er well - not literally -
it has a fine aperture ring, made of metal, and is very smooth to operate.
Using my new Sigma, I was conscious that a lot of effort was needed to
get the aperture ring to move, but seemingly only when mounted on a
camera. Ho, you my think it was rubbing on the lens mount? Not a bit of
it. But why was is so darned difficult to move? Upon closer inspection, I
discovered that not only is the aperture ring not made of metal (rather
some sort of hard plastic or poly-carbonate), but it is basically an
interference-fit with the lens flange metalwork underneath, as it should
be, keeping the aperture ring good and stable, with no movement other
than in the proper directions.
But - what happens when you squeeze a metal aperture ring? Answer,
nothing. The fact it is metal means that it holds its shape. When you
squeeze a plastic/poly-something aperture ring, it changes shape from a
true circle, very very slightly, but just enough to cause the lens mount
metalwork underneath to act like drum brake on the aperture ring. The net
result is that the more you squeeze, the less it wants to turn.
Unfortunately, the less you squeeze, the less it also wants to turn
because of the small spring-loaded ball dropping into the tiny detents,
giving the aperture the 'clicks'. Brilliant Catch-22.
The solution was very simple and worked beautifully: tiny amounts of
grease carefully pasted into the interference-fit gap between the
aperture ring and the lens mount metalwork. I cannot believe the
difference it has made.
With modern lenses using increasingly more cost-effective designs and
materials, situations like this are bound to rise in frequency. I am
surprised that design testing of this lens has not revealed this obvious
flaw - or is my example unique, and everyone else's lens working just
fine?
Cheers,
Cotty
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