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[OM] Re: Unexpected photo opportunity - bugs

Subject: [OM] Re: Unexpected photo opportunity - bugs
From: Winsor Crosby <wincros@xxxxxxxxxxx>
Date: Sun, 06 Aug 2006 09:38:01 -0700
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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