I've re-read that twice now this morning, and yep, I'd agree with you Mr.
Crosby. Even *I* can understand this..
Supplimental to the outstanding piece of work I present:
http://www.nicobastone.com/Kiron_105_macro.htm
this is also a heck of a good description of the Kiron lens' capability, looks
like a good all-round lens to have in the kit.
My thanks and Cheers... Tim
Winsor Crosby <wincros@xxxxxxxxxxx> wrote:
Damn. You're good.
Winsor
Long Beach, California, USA
On Aug 6, 2006, at 5:23 AM, Chuck Norcutt 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.
> However, if you have a simple prime lens (one with a single focal
> length
> such as a 50mm), when you turn the focusing collar you'll notice that
> the lens gets longer or shorter. The lens is shortest when it's
> focused
> at infinity and longest whan focused at its closest focusing distance.
> The reason for the change in length is the focusing collar is simply
> moving the entire collection of lens elements (as a group) toward or
> away from the camera.
>
> If you're using a 50mm lens it will be in focus for objects at
> infinity
> when the optical center of the lens (not the mechanical center) is
> 50mm
> from the focal plane (where the film/sensor lives). As objects come
> closer to the lens it's necessary to move the lens further away
> from the
> focal plane to maintain focus. The problem is that this distance
> relationship is far from linear. The OM Zuiko 50mm f/1.8 can focus
> down
> to 45cm and can do so with a focus travel of only about 6mm. If you
> want to focus closer than 45cm you will need an extension tube to fit
> between the lens and camera to move the lens further away from the
> focal
> plane. Once the extension tube is in place the lens will focus on
> closer objects than without it but it will no longer focus to
> infinity.
>
> Apart from a different optical design to optimize for close objects
> the
> major difference between a normal and macro lens is that the macro
> lens
> typically has a very long focusing travel so that it can focus on
> close
> objects without requiring an extension tube. For example, the OM
> Zuiko
> 50mm f/3.5 macro will focus down to 23cm without the aid of an
> extension
> tube. At that focusing distance images at the focal plane will be 1/2
> life size. But going beyond that degree of magnification will require
> focusing even closer and require an extension tube for the macro
> lens as
> well. Some macro lenses such as the 105mm Kiron can focus down to
> lifesize without using an extension tube. Others, such as the 90mm
> Vivitar Series 1, use a form of teleconverter (which does alter the
> focal length) to get to lifesize from their normal range of 1/2
> lifesize.
>
> 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.
>
> Chuck Norcutt
> pusadmin@xxxxxxxxxx
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