On Sunday, August 06, 2006 at 8:23,
Chuck Norcutt <olympus@xxxxxxxxxx> wrote:
> An extension tube doesn't change the focal length. It simply moves
> the lens further away from the camara so that it can focus on things
> closer than what the lens is designed for on its own.
>
> On modern zoom lenses the focusing action may be hard to recognize
> since many do the focusing internally by sliding the lens elements
> around.
...
And some zooms close focus by changing the focal length as well.
Macro lenses, as well as having more focus travel, are designed to be sharp
when close, while
normal lenses on an extension tube or bellows lose some sharpness.
> Another means of doing close-up photography is to use a supplemental
> close-up lens which is attached to the front of your lens like a
> filter.
> In fact, most people call them close-up filters altough that's a
> misnomer since there's no filtering going on. The close-up lens
> does work by altering the focal length of the lens.
Simple close-up "filters" introduce some colour fringing and reduce resolution
unless they are
expensive and consist of two+ elements, eg,
http://www.amazon.com/gp/product/B000050M6M/103-1673610-7727858
They are sold in dioptres, +1 to +10. The bigger the number, the closer, but
the more bad optical
side-effects.
Cheap, though, and light and useful.
tOM
p
--
tOM Trottier, Ottawa, Canada
758 Albert St, Ottawa ON Canada K1R 7V8
+1 613 231-6115 N45.41235 W75.71345
http://groups.yahoo.com/group/ottawa-photo-clubs
"The moment one gives close attention to anything,
even a blade of grass, it becomes a mysterious,
awesome, indescribably magnificent world in itself "
-- Henry Miller, 1891-1980
==============================================
List usage info: http://www.zuikoholic.com
List nannies: olympusadmin@xxxxxxxxxx
==============================================
|