Extension Tube + 400mm lens question

In article uiwZc.577$HY1.98@trnddc02, Roy G. Biv
writes
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.

What exactly does this mean?


--
David Littlewood

In article uiwZc.577$HY1.98@trnddc02, Roy G. Biv
writes
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.

What exactly does this mean?


--
David Littlewood

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.
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"David Littlewood" wrote in message
...
In article uiwZc.577$HY1.98@trnddc02, Roy G. Biv
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.

What exactly does this mean?

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.
Please reply to group with questions or comments.
"David Littlewood" wrote in message
...
In article uiwZc.577$HY1.98@trnddc02, Roy G. Biv
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.

What exactly does this mean?

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.
Please reply to group with questions or comments.
"David Littlewood" wrote in message
...
In article uiwZc.577$HY1.98@trnddc02, Roy G. Biv
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.

What exactly does this mean?

In article BIRZc.1300$xr1.221@trnddc04, Roy G. Biv
writes
If I remember the math correctly, (it's been a while since I worked this
out) at the near focus distance (about 11.5 feet) the 400mm canon lens has a
focal length of about 360 mm. So in effect you actually have a 360mm lens
with a 40mm extention tube. Hope this helps.
Sorry due to spam I shall not provide a valid e-mail address.
Please reply to group with questions or comments.
"David Littlewood" wrote in message
.. .
In article uiwZc.577$HY1.98@trnddc02, Roy G. Biv
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.

What exactly does this mean?


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