PRACTICALPHOTOGRAPHY.COM 73
Know Your StuffIs dust inside a lensa problem or not?If camera lenses are
round, why are pictures
square or rectangular?
Sean Proctor, DerbyshireRichard says: That’s a really
interesting question, Sean. Lenses
are traditionally round because it
was easier to grind a round lens
than any other shape. With modern
technology it’s possible to make
a lens in almost any shape, but it
would incur unnecessary expense
and complexity.
Round lenses do, in fact, produce
round images but, as the extreme
edges of a lens deliver the poorest
image quality, this area is cropped
out by the image-making material,
be it film or a digital sensor. If you
were to draw an imaginary square
inside a circle, that’s what’s going
on inside your camera.
In the early days of photography,
film size was not regulated and
many cameras required you to
cut film to a unique size. But from
the earliest days the format was
generally square or rectangular as
that shape is traditional for image
presentation. There is, of course,
nothing to stop you cropping your
pictures to a circle in the edit...Why are imagesnot circular?Browsing through the
classifieds for used lenses,
I see many advertised
as ‘perfect’ condition, but in the
description it states that there is some
dust in the lens. Surely this will affect
performance and is not ‘perfect’?
Oliver Jameson, Manchester
Louise says: Lenses are precision
instruments and manufacturers take
great care to assemble them in the
cleanest of conditions to ensure that they
leave the factory free of dust. But even
the most pro-spec lenses are not entirely
sealed from the outside world, and over
the course of time some dust will get
inside, with zoom lenses and cheaper
models being particularly prone to the
ingress of particles.
Dusting down the facts
So, is this dust going to affect the image
quality? And are secondhand retailers
stretching the definition of ‘perfect’?
In short, the answer is no, and no.You
will find it almost impossible tobuya
used lens that does not have dust inside
it. Indeed, the fact that a retailer makes
the point of drawing your attention to it,
is actually a good indication that they
are trustworthy.
The effects from small amounts of
dust in the lens will be negligible. After
all, the air is full of dust and therefore
there will always be dust floating around
between the scene you’re photographing
and your sensor, both inside and outside
the camera. As a general rule of thumb,
any dust or marks on the front element
can be safely ignored, while the effects
can be more noticeable as you move
further back along the lens construction.
Buying secondhand lenses can offer
a very good financial saving compared to
buying new, so don’t let a little bit of dust
put you off when considering a purchase
if everything else about it is tempting.Totally protecting kit against
dust will never be possible as
we don’t use our camera gear in
sterile environments. But while we
necessarily take our kit to places
full of dust and dirt, there are some
simple precautions to follow in
order to limit the impact.
First, reduce the amount of dust
that your gear is regularly exposed
to by cleaning your camera bagafter every field trip. And if you
commonly visit dusty locations,
keep your gear in resealable food
bags inside your camera bag. Most
cameras have an option to clean
the sensor of dust. Locate this in
your menu and use it regularly.
Finally, whenever you change a
lens, switch off the camera’s power.
A charged sensor will attract dust
when exposed to the elements.How to keep your gear dust free