Velocity Reviews > Calculating lens capability?

# Calculating lens capability?

Digital Rebel XT EOS
Guest
Posts: n/a

 06-01-2006
A friend gave me some Kenko Extension Tubes - 36mm, 20mm, and 12mm -
for doing macro photography using regular lenses.

How do I determine which extensions or combinations of extensions to
use? I've got a 28-135mm, 75-200mm, and 18-55mm lens...

Neil Ellwood
Guest
Posts: n/a

 06-01-2006
On Thu, 01 Jun 2006 07:52:08 -0700, Digital Rebel XT EOS wrote:

> A friend gave me some Kenko Extension Tubes - 36mm, 20mm, and 12mm -
> for doing macro photography using regular lenses.
>
> How do I determine which extensions or combinations of extensions to
> use? I've got a 28-135mm, 75-200mm, and 18-55mm lens...

Try them - use a spare morning, afternoon or evening.
--
Neil
Delete l for email

tomm42
Guest
Posts: n/a

 06-01-2006

Digital Rebel XT EOS wrote:
> A friend gave me some Kenko Extension Tubes - 36mm, 20mm, and 12mm -
> for doing macro photography using regular lenses.
>
> How do I determine which extensions or combinations of extensions to
> use? I've got a 28-135mm, 75-200mm, and 18-55mm lens...

Extension tubes don't work well with zooms, try it they may work, but
you may have focus problems. Generally you need a 2 x focal length
extension of the lens for 1:1, this is including the lens own
extension. Also be sure you can meter with the tubes as the exposure
will change.
I expect you will have problems, but it is worth trying, and if you get
the mag you need, thats great. Let us know how it works out.

Tom

Dankwart
Guest
Posts: n/a

 06-16-2006
The extension tube allows you to move closer to your object. It does not
depend on the type of the lens but on the focal length you are working with.
The smaller the focal length you are using, the more effective is a given
extension tube.

I did a quick calculation:
When lens is set to infinity: D = F((F+E)/E)
where D is the object distance, F is the focal length, and E the extension
tube length, everything in the same dimension, like mm. When E=0, D is
infinity, when E=F, your distance is twice the focal length F.

When the lens is set to a closer distance you get even closer. (The equation
becomes a bit more complex - I could give it to you.)

In practice you may want to start out with the 12 mm if you want to work not
too close. Then you use the longer extension tubes as you get closer. If the
extension equals the focal length you get 1:1 life size, even when the lens
is set to infinity.

"Digital Rebel XT EOS" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) oups.com...
>A friend gave me some Kenko Extension Tubes - 36mm, 20mm, and 12mm -
> for doing macro photography using regular lenses.
>
> How do I determine which extensions or combinations of extensions to
> use? I've got a 28-135mm, 75-200mm, and 18-55mm lens...
>