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cango Friday 29th July 2016 13:25

extension tube lenght alters focal lenght?
2 Attachment(s)
at the moment I have approximatedly 10-11 cm extension tube lenght (80mm and pushed out a bit, as well as the camera mount adapter out a bit.

one of the photos below was taken at 300mm (zuiko 75-300) the other with the 600mm celestron 80ED, and sitting at the same spot. Sensor plane difference might have been a few centimeters, give or take. While upsizing the 300mm to match the other it took 255% - giving me approx 765mm.

Am I doing something wrong here? or can the small variance of a few centimeters me holding the camera give me so much of a diff? I mean, I sat on the same spot and just switched lens.

DanC.Licks Monday 1st August 2016 17:25

Hi Carlos,
Sometimes the "focal length" of zooms is not what it says it is, and maybe that is the case here. Maybe less than 300mm. I know the Sigma 150-600 is actually only about 560mm and I think even the Canon 100-400 II is a little short of 400mm. Also, we don't know if the focal length of the 80/600 is exactly 600mm. Some testing with a ruler and some simple math might give the answer.
Going by the samples I also get about 255-260%. At any rate, focal length is calculated at infinity, so maybe the distance to the subject is throwing things off. Wish I knew more about it...

Just did a quick test on the neighbors chimney, about 50 meters away. I have to magnify the image from the 75-300 II 215% to match the image from my 90/600.
Another test about 10 meters away, came out to be 230%
Another test about 200 meters away came out to 210%, so there seems to be a pattern here.

DanC.Licks Monday 1st August 2016 18:21

Found this
"The focal length marked on a lens is only right when it's focussed on infinity and is the distance from the rear nodal point of the lens to the focal plane in the camera.

...or at least close enough, there's usually a little difference between quoted focal length and actual focal length.

The focal length of some lenses changes significantly with focus though - e.g., Canon 100mm macro is apparently closer to 70mm when it's at minimum focus."

"Many modern lenses, both primes and zooms, have components that move relative to one another during focusing. On a prime lens this may be to control aberration, but often it is, or is also, to effect focusing by reducing the focal length of the lens rather than by moving it rigidly. "
"With modern zooms, the focal length shortens as you focus closer."

DanC.Licks Monday 1st August 2016 18:36

And this
the key phrase is "a telephoto lens is a specific type of a long-focus lens in which the physical length of the lens is shorter than the focal length."
So the 75-300 is a "modern telephoto zoom" and the scopes are non-telephoto lenses, and thus behave differently.

cango Monday 1st August 2016 19:18

Many thanks Daniel!

Not sure I got any wiser, though..haha, but I'll have to let it sink in a bit I guess :-)

" Some testing with a ruler and some simple math might give the answer. " Do you have some web page in mind or what to search for, so I can learn how to do that (calculate)? many thanks again

DanC.Licks Monday 1st August 2016 20:02

I wouldn't worry about it. With the scope it is very simple due to the pure optical construction. Telephotos and zooms get so complicated that all kinds of weird things go on in the optical path. But there seems to be a general rule or two. Focal length is only more or less what is printed on the barrel at infinity, and the closer you focus, the shorter the focal length becomes. That does not seem to be the case with a scope, so your original question was actually backwards. The zoom is the culprit and not the extension tubes on the scope.

Tord Tuesday 2nd August 2016 13:40


The extension tube does not alter the focal length, since the optical system remains the same throughout the range of operations. The extension tube is needed so that the camera sensor is placed where the rays are collimated to focus.

To find out the actual focal length of the scope:
Focus the scope on infinity.
Measure the distance from the rear vertex of the telescope lens assembly to the sensor. For exact measurement you would need to un-screw the front lens assembly from the SW80, or read the specs. The flange distance for m43 is 19,25 mm which is more or less same as the thickness of the front lens from what I remember last time I had it removed for cleaning. This means that you should get a quite accurate estimate within a few mm by measuring the length of the scope from the rear plane of the extension tube to the front of the lens assembly.

You could for fun do the same when shooting at other distances and will get a longer focal length the closer you focus.

With this measured value as a baseline you should be able to calculate the focal length of the zoom lens with the method you used, by shooting objects and comparing the relative sizes rendered on photo.

Hope this helps

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