Because of the relatively large aperture (f/2.8) for a 400mm, the DOF is
extremely shallow when used wide open and hence accurate focusing is
essential for obtaining sharp images. The lens definitely
needs to be stopped down to achieve sharp image when used with either
the EF1.4x or EF2x converters. At least that is my personal experience.
In fact this is further confirmed if you take a look at Canon's
published MTF charts in their EF Lens Work II.
> Date: Tue, 19 Dec 2000 18:22:12 -0500
> From: "Dave Buyens" <[EMAIL PROTECTED]>
> Subject: RE: EOS EF 400/2.8L II
>
> K K Hui wrote:
> > Would like to hear direct experience from users on the optical
> > performance of the EF400/2.8L II. I have one and find it a little bit
> > soft when use wide open compared with my EF300/2.8L which is superb.
> > With EF1.4x, the lens really needs be stopped down by one to two stops
> > to get sharp images.
>
> Sharpness is a matter for personal tastes, but I have never heard this
> comment from anyone before and have not experienced it myself. In fact,
> this very lens is reputed to be the world's very best at low f-stops due to
> sharpness and bokeh (forgetting the new IS lens for a moment). Opinions may
> vary.
>
> I don't have a 1.4x, but I've never heard that the 1.4x reduces sharpness.
> I do have the 2x converter and I can notice a definite reduction in
> sharpness on both my 400 2.8 II and 70-200L. I've often wondered if I have
> a sub-standard 2x (it's a recent Canon), but have also seen others noticing
> the difference also.
>
> I don't have a 300/2.8 for comparison--just never had a need. However, with
> digital bodies coming on, that may change.
>
> Hope this helps.
>
> Dave B.
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