a bought a 50D in hopes of getting more detail from the 100-400, but got no improvement over the 40D (after micro-adjusting the 50D to +8 which is about what it required on all of my lenses) and returned it. the 50D was significantly better than the 40D only with the 100 macro (and i assume it would have been with the 50 1.4). the 50D was also not as sharp as my 5D
It's always been the case that good lenses are appreciated whatever the film type or sensor pixel density.
In olden days of film, if you wanted sharpest results in circumstances where a slow shutter speed was not a handicap, you needed to use a fine-grain film (equivalent to a high-pixel-density sensor).
Whether or not the film or sensor outresolves the lens is merely academic. Image resolution is always a product of lens resolution and sensor resolution. Increase either one and image resolution is inevitably increased (assuming perfect technique, of course).
The question that needs to be answered is, in practical terms just how significant is any increase in sensor pixel count, or lens resolution?
When using a teleconverter, you are basically taking one step forward and one step backwards. One step forward in terms of extended focal length, and one step backwards in terms of lens quality.
Even with the Canon D60, I rarely saw any improvement in detail using the 1.4x extender (version II) with the 100-400 zoom at 400mm. And that's with extreme pixel peeping at equivalent print sizes on the monitor that are far beyond the maximum print size of my Epson 7600.
Since we have few details of sensor and lens performance (separately) at MTF 50, it's difficult to demonstrate any results by way of calculation without making inaccurate assumptions, but one can see from the Photozone charts below that the 100-400 is a pretty poor performer compared with a top class zoom such as the Canon 70-200 F4 IS.
The reason the 100-400 is so popular is that it is still as good or better than an expensive prime with 2x extender.
However, looking at those Photozone charts, I wonder if the 70-200 F4 IS with 2x extender might get as close as matters to the 100-400. I still think it would be marginally worse (at F4 with 2x extender) than the 100-400 at 400 at F8, but perhaps only at a pixel-peeping level which may not matter even for large prints.
If anyone reading this happens to own a Canon 100-400, 70-200/4 IS, 2x extender and a 50D, don't be shy. Post some comparisons.
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