70-200mm IS USM v 100-400mm

[Fallacy]

I've read a similar thread on this topic before, but my question is a bit different.

I'm trying to decide between the two, however, I wouldn't be using a 2x teleconverter.

I have a 50D now (1.6x) and am planning on getting the 1.4x teleconverter. Now, correct me if I'm mistaken but that would yield the same results as if I was using a full-framed camera with a 2x converter, right?

Anyway, I haven't seen any comparisons with the 1.4x converter, and I'm not sure how much sharpness it will lose... Does anyone have any insight on this?

[Geoff F]

In many respects, it depends what you will be doing with the lens.

The 70-200 is an excellent general purpose zoom while the 100-400 is favoured with wildlife photographers who frequently use it with a converter for more reach.

Both lenses should work reasonably well with a 1.4x converter; subject to the usual warnings about slight loss of light etc.

[Fallacy]

Do you happen to know by chance how significant the loss of sharpness is with the 1.4x converter?

Now I'm having trouble deciding which lens to get; both seem to be great. The 70-200 for sports / walk around zoom, the 100-400 for wildlife.

[Colin Southern]

Do you happen to know by chance how significant the loss of sharpness is with the 1.4x converter?

Now I'm having trouble deciding which lens to get; both seem to be great. The 70-200 for sports / walk around zoom, the 100-400 for wildlife.

Wide open the 100-400 is marginally sharper than the 70-200 with a 1.4x TC, but once you stop them both down into the F5.6 / F8 range there's nothing between them.

Also, don't let a minor drop-off in sharpness concern you too much - in my experience around 90% of lack of sharpness that I see in images is due to incorrect (and often non-existant) sharpening, and only around 10% due to the lens. In practice you have to be a little more precise when sharpening images with TCs attached, but it's no big deal.

We discussed it recently - have a read of this thread if you have time.

Also, you can read a bit about sharpening here.

[Colin Southern]

Both lenses should work reasonably well with a 1.4x converter; subject to the usual warnings about slight loss of light etc.

Keep in mind though that you'll probably lose autofucus with the 100-400 + 1.4xTC at the 400mm (F5.6) end on a 50D. (F5.6 + 1 Stop loss for 1.4x TC = F8 which will only AF on 1D series cameras).