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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