Lens Focal lengths not as advertised?

I was doing a lens test for my site with the 100-400 -vs- 70-200. The later having a 1.4 TC and 2.0 TC on it... But the interesting thing is when shooting at in between numbers in their zoom range, or even at the ends, the images are waaaaaay off.

Here's a few examples:

jacobsen1 wrote in post #256694
400mm, 100-400 left, 70-200 f4 w/ 2.0 TC right:
notice the 70-200 w/2.0TC @ 400 is MUCH longer than the 100-400!

f8

that's a 100-400 @ 400 and a 70-200 w/2.0TC @ 400... that's not even CLOSE to the same IMHO.... Which is really 400?

jacobsen1 wrote in post #256658
100mm (70-200 on left, 100-400 on right):
(also not, the 70-200 is at 97mm, but CLOSER than the 100-400 @ 100)
f4.5

again at the wide end... the 70-200 was at 97mm (my best guess as to where 100 was) and the 100-400 @ 100 was actually WIDER?!

I'm assuming this is because the FLs used in lens naming are more approximate than exact, but does this fall in line with what anyone else is seeing? Which one of these setup is closer to 400 out of curiosity?

if you want to see the full image test, go here (way too many posts to repost here, sorry):
http://www.newschoolof​photography.com/forum/​showthread.php?p=25665​8

for me the 100-400 wins, but all 3 lenses tested are fantastic.

Try the comparison at infinity focus. Does the 100-400 change lengths on zooming?

a 2" X 4" isn't really 2 inches by 4 inches...i think it's along the same lines as that...

DreDaze wrote in post #7164970
a 2" X 4" isn't really 2 inches by 4 inches...i think it's along the same lines as that...

but all 2x4s are the same size.

I've seen stuff like this with my lenses, but it's not an issue to me. The differences are so small.

A couple of points.....

The focal length is always specified at infinity....focussing more closely will reduce the focal length.

Secondly, the 100-400 is normally around 380mm max and the stated Fl is "nominal".

Bob

It also has to do with whether the lens extends on zooming. My Sigma 18-200 extends, whereas my 200 f/2.8L does not. At infinity their fov is the same; inside that the Sigma's fov is wider.

bob-e wrote in post #7164995
but all 2x4s are the same size.

But are two 2x4's nailed together the same exact size as a 2x8 or 4x4?

I doubt the TC is exactly 2x to the mm, it is probably a 1.98 or a 2.05 or something. Adds up to a few mm when you times it by 200mm

tkbslc wrote in post #7165184
But are two 2x4's nailed together the same exact size as a 2x8 or 4x4?

Sounds like a Dr. Suess kind of riddle

bob-e wrote in post #7164995
The differences are so small.

but this difference at 400mm isn't what I'd call small....

Canon Bob wrote in post #7165090
The focal length is always specified at infinity....focussing more closely will reduce the focal length.

good point, and this was all at <10'... Plus with the 100-400 extending, that probably makes it worse.

tkbslc wrote in post #7165184
But are two 2x4's nailed together the same exact size as a 2x8 or 4x4?

yes if you do it right. 4x4s and 2x8s are off in the same way a 2x4 is.

jacobsen1 wrote in post #7165961
yes if you do it right. 4x4s and 2x8s are off in the same way a 2x4 is.

not quite...a 4X4 is typically 3 1/2 X 3 1/2...and a 2 X 4 is 1 1/2 X 3 1/2...so it'd be 3 X 3 1/2...

back to the topic though...i'd say the difference is fairly small especially if that's at less than 10 feet...