Join for FREE
It only takes a minute!
Magnifying the passion for nature. Zeiss Victory Harpia 95. New!

Welcome to BirdForum.
BirdForum is the net's largest birding community, dedicated to wild birds and birding, and is absolutely FREE! You are most welcome to register for an account, which allows you to take part in lively discussions in the forum, post your pictures in the gallery and more.

Reply
 
Thread Tools Rate Thread
Old Friday 29th July 2016, 13:25   #1
cango
Registered User

 
Join Date: Jul 2008
Location: Tyresö
Posts: 707
extension tube lenght alters focal lenght?

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.
Attached Thumbnails
Click image for larger version

Name:	pole-300.jpg
Views:	52
Size:	418.9 KB
ID:	593421  Click image for larger version

Name:	pole-600.jpg
Views:	59
Size:	447.4 KB
ID:	593422  
cango is offline  
Reply With Quote
Old Monday 1st August 2016, 17:25   #2
DanC.Licks
AKA Daniel Bradley
 
DanC.Licks's Avatar

 
Join Date: Jul 2012
Location: Austria
Posts: 1,713
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.

Last edited by DanC.Licks : Monday 1st August 2016 at 18:05.
DanC.Licks is offline  
Reply With Quote
Old Monday 1st August 2016, 18:21   #3
DanC.Licks
AKA Daniel Bradley
 
DanC.Licks's Avatar

 
Join Date: Jul 2012
Location: Austria
Posts: 1,713
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."

More..
"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. "
and
"With modern zooms, the focal length shortens as you focus closer."

Last edited by DanC.Licks : Monday 1st August 2016 at 18:33.
DanC.Licks is offline  
Reply With Quote
Old Monday 1st August 2016, 18:36   #4
DanC.Licks
AKA Daniel Bradley
 
DanC.Licks's Avatar

 
Join Date: Jul 2012
Location: Austria
Posts: 1,713
And this
https://en.wikipedia.org/wiki/Telephoto_lens
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.
DanC.Licks is offline  
Reply With Quote
Old Monday 1st August 2016, 19:18   #5
cango
Registered User

 
Join Date: Jul 2008
Location: Tyresö
Posts: 707
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
cango is offline  
Reply With Quote
Old Monday 1st August 2016, 20:02   #6
DanC.Licks
AKA Daniel Bradley
 
DanC.Licks's Avatar

 
Join Date: Jul 2012
Location: Austria
Posts: 1,713
Carlos,
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.
DanC.Licks is offline  
Reply With Quote
Old Tuesday 2nd August 2016, 13:40   #7
Tord
Registered User
 
Tord's Avatar

 
Join Date: May 2006
Location: Malmö, Sweden
Posts: 1,985
Hi

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
Tord
__________________

My Gallery on 500px
Tord is offline  
Reply With Quote

BF Supporter 2013 2014 2015 2016 2017 Support BirdForum With A Donation

Advertisement
Reply


Thread Tools
Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is Off
HTML code is Off

Similar Threads
Thread Thread Starter Forum Replies Last Post
Eyepiece focal lenght ..Did i change it? mayoayo Spotting Scopes & tripod/heads 2 Sunday 16th November 2014 15:17
Lenght of T2 adapter musti Digiscoping Adapters 1 Monday 3rd December 2012 06:44
Extension tube question Cashie Canon 10 Wednesday 27th May 2009 19:18
35mm equivalent focal lenght????????? Paulyoly Spotting Scopes & tripod/heads 3 Saturday 5th April 2003 18:16



Fatbirder's Top 1000 Birding Websites

Help support BirdForum

Page generated in 0.17141509 seconds with 19 queries
All times are GMT. The time now is 19:35.