50mm with 1.4x teleconverter vs 70-200mm at 70mm - weirdness

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I am considering an 85mm f/1.8 so thought I would try my 50mm f/1.4 with my Kenko 1.4x teleconverter for a bit as this should give a similar-ish 70mm at f/2. I ought to point out that I am shooting on a 7D which has a crop sensor of course. As part of this I compared this set up with my 70-200mm at 70mm by taking the same shot (tripod mounted etc) at f/4 and f/5.6. The 50mm X 1.4 compared favourably I thought but the weird thing is the field of view is different, yet both should be at 70mm from exactly the same place:


70mm teleconv test 009mid
by jimi-the-fish, on Flickr
70-200mm at f/5.6

70mm teleconv test 010mid
by jimi-the-fish, on Flickr
50mm x1.4 at f/5.6

Can anyone explain?

cheers
J

(for those few who might be interested this is how the 50mm x 1.4 combo performs wide open i.e. f/2 - slightly soft still:

70mm teleconv test 012mid
by jimi-the-fish, on Flickr)
 
It'll probably be one of those 'smack in the face' moments when it gets explained, I can't for the life of me think why it would be so different so I'll be watching the thread until the clever guys come out and explain.
 
Thanks Dave, very interesting but surprising for a canon L lens to show such variation. Perhaps I should try the same idea but at a different focal point.
I will Google focus breathing and see how my lens stacks up.

J
 
I'd heard of focus breathing and thought it was the slight change of size just as the lens comes into focus on the subject ie the object goes from being smaller than actual size to larger than actual size as you get close to the point of focus, at which it changes to the actual size of the subject. I just didn't think it would affect two properly focussed images at the same focal length it has in the pics above.

From googling it I've found this explanation "Focus breathing" is the name often given to the change of focal length (and hence angle of view and magnification) when the focus distance of a lens is changed." but I'm still confused as the pics were taken, presumably, from the same focus distance. Methinks I'll have to do some testing and investigating into this, just for fun though.

Thanks for the links though Dave, very interesting reading.
 
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its how non video leneses often behave,as its much cheaper and easier to have breathing. Some leneses are also abit off spec too, eg 48mm rather than 50mm
 
Some leneses are also abit off spec too, eg 48mm rather than 50mm

a bit? Many 70-200s are more like 80-180mm in practice, but that wouldn't sound cool, would it? 24-70mm is probably 26-65mm, not cool number either... You have to look into their patents to see this clear in writing.
 
Focus breathing. Focal length is only true at infinity focus (and f/number too for that matter) and most tele-zooms are notorious for it. Nikon 70-200mm f/2.8 VR is the worst I've seen, new Tamron VC not much better though the older non-VC version is pretty good. Canons are actually not so bad.
 
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