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[OM] Re: Unexpected photo opportunity - bugs

Subject: [OM] Re: Unexpected photo opportunity - bugs
From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxx>
Date: Sun, 06 Aug 2006 08:23:42 -0400
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

Tim Randles wrote:

> I'm a bit confused about the extension tube thing, I sorta understand
> the extension tube changes the focal length, so how does that allow
> you to take close up images like this with a 50MM..
> 
> I'm using a stock E500 lens that came with the kit, I think it is a
> 14-45MM but I cannot seem to get a closeup like this on some of the
> wildflowers here.


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