# Extension Tube + 400mm lens question

Discussion in '35mm Cameras' started by Eric Miller, Sep 1, 2004.

1. ### Eric MillerGuest

Okay, got the Canon 400 5.6L in today. Though its primary use will be
mounted on my 10D for bird shots at distance, if I wanted to take
hummingbird photos from about 10 feet, what size extension tube would I
need? Is there a formula for figuring this out? The minimum focus distance
on this lens is 3.5 meters. Additionally, I see that Kenko makes a set of
extension tubes 12mm, 20mm and 36mm. Should I just get a set or will just
one tube satisfy my need/want? Additionally, am I correct in thinking that
the magnification from one of these tubes will be the same as just adding
the length of the tube to the focal length of the lens?

TIA

--
Eric Miller
millereric at bellsouth dot net

Convictions are more dangerous enemies of truth than lies. - Freidrich
Neitsche

Eric Miller, Sep 1, 2004

2. ### DaveGuest

I have a Canon 12.5 and a 25 mm extension tube. Both work fine, both are
pretty cheap. I use them only rarely, but your use may vary depending on
interest.

I am not sure if I understand the second question. I get improved short
focus, but the extra magnification is mostly just secondary to being able to
get closer to the subject. There are no lenses in those extension tubes,
just air and electrical connections for the motor/camera. When I used them
for stacking lenses, they were just added to allow the lenses to fit
together.

Dave

Dave, Sep 1, 2004

3. ### Colin DGuest

Ok, basic formula: 1/u + 1/v = 1/f, where u=object distance from lens,
v=image distance from lens, f=focal length of lens.

Using a rearrangement of this formula, at the closest focusing distance
(cfd) of 3.5 meters, the effective lens extension is 51mm.

Adding a 48mm extension tube (12mm + 36mm), the effective lens extension
is 48mm with the lens at infinity, and 99mm when the lens is at cfd.

Referring to the formula again, object distance with 48mm extension is
3.733 meters, and 2.016 meters with 99mm extension.

So, focusing range with a 48mm tube is from 3.7 meters to 2 meters more
or less.

Colin D.

Colin D, Sep 1, 2004
4. ### Ray CrevelingGuest

--

I see that Kenko makes a set of
I really like the Kenko set of tubes. use them quite a bit. I'm to the point
now that I have to consider a macro lens because of the time it take to
figure out the right extension for a project. I would get the Kenko set over
individual Canon tubes. You have no glass to worry about

Ray Creveling
http://www.blackcatblog.com

Ray Creveling, Sep 2, 2004
5. ### Roy G. BivGuest

If my calculations are correct, a 25mm tube will allow the 400 to focus from
8 to 22 feet. Someone posted a formula for the math below. But remember
that internal focus lenses shorten the focal length at closer distances. So
focal length plus extension at close up is always equal to focal length at
infinity. I suggest that you forget the off brand tubes and buy the Canon. I
haven't checked all the different brands but as far as I can tell the Canon
will attach to the 400 much more securely - you can check this yourself in
the store.
Sorry due to spam I shall not provide a valid e-mail address.

Roy G. Biv, Sep 2, 2004
6. ### Colin DGuest

OK, an update on my previous.

I belatedly realized that the 400mm probably uses internal focusing, so
instead of racking the lens out by 51 mm from the film plane, the focal
length of the lens is shortened sufficiently to focus on 3.5 meters.
The question becomes, what reduction in focal length will cause a 400mm
lens to focus at 3.5 meters?

Since the object distance = 3500mm and the image distance remains at
400mm, then 1/f = 1/u +1/v, or 1/3500 + 1/400 = 1/358.9, so the
effective focal length shortens to 359mm approx.

If we add 48mm of tube in now, the image distance becomes 448mm, so now
1/u = 1/f - 1/v, or 1/u = 1/359 - 1/448, = 1/1807.1

Therefore, object distance at cfd is 1.8 meters approximately, so with
an internal focusing lens, the range is 3.7 meters to 1.8 meters
approximately.

Colin D.

Colin D, Sep 2, 2004
7. ### David LittlewoodGuest

What exactly does this mean?

David Littlewood, Sep 2, 2004
8. ### Roy G. BivGuest

If I remember the math correctly, (it's been a while since I worked this
out) at the near focus distance (about 11.5 feet) the 400mm canon lens has a
focal length of about 360 mm. So in effect you actually have a 360mm lens
with a 40mm extention tube. Hope this helps.
Sorry due to spam I shall not provide a valid e-mail address.

Roy G. Biv, Sep 3, 2004
9. ### David LittlewoodGuest

Well, I think I understand what you are saying. You seem to be asserting
that the focal length of a lens with internal focussing is always equal
to focal length at infinity minus the amount of extension form the
helical focussing mechanism. This implies that the position of the rear
nodal plane (the point from which image distance is measured) is
unchanged relative to the position of the lens elements. However, I
believe this is unlikely to be true in most cases.

It also assumes the lens has internal focussing only, i.e. no helical
focus mechanism to bodily move the lens elements away from the body.
There are lenses of this type, but not all are. If there is a helical
focus system, the position becomes more complex.

In all, the rule-of-thumb you put forward may work when it works, but
could not be taken as being applicable in all cases.

David

David Littlewood, Sep 3, 2004