I'm going to the Galapagos next month, this time (unlike the trip with Michael on the Amazon), I've got myself the Canon 100-400. A buddy suggested the Canon 1.4 teleconverter and I'm wondering if this would be a good purchase and combo with this lens (I'll take a monopod too) or over kill. Last trip I only had (ugh) the 80-300 which wasn't up to snuff. FWIW, this will be on a 5DmII. Opinions welcome and thanks.
When I bought my first Canon DSRL many years ago, I also bought the 100-400 and 1.4x converter at the same time. I've carried out many tests during those years, comparing images at 400mm cropped to the same FOV as the 560mm shot with extender, then rezzed up to the same file size as the 560mm shot.
The best I can say is that it is possible, if the subject is contrasty in very strong light, to get a very marginal, pixel-peeping advantage in detail and resolution using the 1.4x extender with the 100-400 at 400mm. However, often there seems to be no advantage whatsoever, and sometimes the results with the converter seem to be marginally worse.
Investigating how this could be, that the image using the converter could actually be worse, I came to the conclusion that the quality of the lens at the real
aperture (ie. the aperture through which the light passes before reaching the converter at the other end) was the determining factor.
I believe that my copy of the 100-400 is at least of average quality, and perhaps better than average. At 400mm it's sharpest at F8, very marginally less sharp at F11, but quite noticeably less sharp at F5.6.
When you fix a 1.4x converter to the lens, the maximum aperture becomes F8 but the aperture blades in the lens are still wide open at F5.6. To get the sharpest result with a 1.4x extender you therefore have to stop down to F11. Even at F16, with extender, you will get a sharper result than you will get at F8, if your copy of the 100-400 is like mine.
When you consider the disadvantages of loss of autofocus capability and the slower shutter speeds required (or higher ISO), the use of an extender with the 100-400 is simply not worth the hassle.
In my opinion, you'd be far better off getting the longer reach by using a 7D.
Below are some comparisons showing the noticeably sharper results at F11 compared with F8, at 560mm with the 5D and the 20D. These were taken several years ago. The 20D has the same pixel density as the 5D2, yet still shows slightly better resolution at F11.
However, if your copy of the 100-400 is actually sharpest at F5.6, then it could be worthwhile using an extender.
[attachment=19268:full_scene.jpg] [attachment=19269:F8_v_F11_at_560mm.jpg] [attachment=19270:20D_F11_...at_560mm.jpg]