Longer lens or extenders

[Neil1000]

Hello,

my first post.  Recently I got a Canon 5D and gave my 350 rebel to my new son in law.  I did not realise that with a full frame sensor my 70-200 f2,8 IS would look a lot smaller.  Having done the deed I was thinking of either a 400mm or 100-400mm to compensate.  I have a 1.4II and 2II extender so have been running some experiments whilst I think about the longer lens.

When coupled to the 70-200mm f2,8 IS lens I see degradation (though slight) with the 1.4II extender that becomes more noticeable with the 2II extender - enough to make me think that the f5.6 400 or the 100-400mm might be better.  But being curious I put the 2II and 1.4II extenders together and got some surprises.

a) as calculated the 2II extender puts up the f stop 2 units to f5.6 but coupling the 1.4 and the 2 together keeps f5.6 and not f8.  Curious. 
b) A further surprise is that coupling both extenders together not only gives me 560 mm but improves the sharpness to the 1.4 extender alone.

Now I have 560 mm with not so bad sharpness and f5.6 but it gets even sharper at f7.1.  Now my physics is really confused.  Are my eyes confused or is there something useful happening?

Any clues from the educated would be much appreciated.

best wishes,  neil

[Bob Atkins]

Whatever the camera readout says, if you put a 1.4x and a 2x on an f2.8 lens you end up with an f8 lens. That's just the physics. The camera readout depends on how the electronics of the two extenders interact and in this case they obviously do so in a way that doesn't report the actual f-stop of the lens combination.

I'd expect slight image degradation with a 1.4x, a bit more with the 2x. That's perfectly normal.

As for things getting better when the 1.4x and 2x are combined, I'm a bit skeptical. My guess is that there is something about your testing/focusing that's not right. While it's theoretically possible that the aberrations of one multiplier exactly cancel out the aberrations of the other, it's highly unlikely.

I'd do the tests again, this time taking multiple shots (both using AF and MF) with each multiplier and with the two combined.

[Neil1000]

Thanks,

I appreciate the sound advice,

best wishes, Neil

[marcfs]

Neil,

In thinking about your consideration of alternative lenses, my question to you is what are you shooting? 

I use the 400L 5.6 and the 100-400L 4.5-5.6 with a 40D.  The 400 is tack sharp wide open and excellent for birds in flight.  The 100-400 is a great all around lens, although not as sharp wide open or very fast.  However it does have IS.  If I close down to a 6.3 or 8, the results are excellent.  I use this lens as a walking around lens primarily when bird watching, capturing images of flowers that are beyond the rage of a macro, details within a landscape, etc.  The convenience and flexibility of the zoom is hard to beat.

Hope these comments help with your assessment.

Good Luck!!

Marc Schoenholz 

[whizkid]

Seeing your post moved me to try that combo myself. I have a 5DMk2 and a 70-200IS lens plus both the 1.4x II and 2.0x II TC's . You are correct that the trio mounted together reports an f/stop of f/5.6. However, I noticed the reported shutter speed dropped by a factor of 2 when adding the 1.4 x to the 2x TC. Exposure was fine but some softness was noticed by me when the trio is combined.