Brad...
When you say the 40 CFE IF is "markedly different" would you please describe what you think the difference is? I have my eye on this lens but the price difference between th IF and others is substantial.
The price difference is substantial--it's a good question.
Objectively, what's 'markedly different'? The lenses themselves, for starters. The 40/4 CF and 40/4 CFE FLE have the same optical formula, MTF curves, etc. The CFE FLE has better internal baffling, electronic databus capability, new mechanical construction and a third ring to adjust the floating elements. The CFE IF has these improvements over the CF as well, minus the third ring. Instead the IF has a mechanism which automatically drives the floating elements in concert with the focus distance (optimized not just for three particular distances like the FLE, but continually throughout the focusing range). Perhaps most importantly of all, a completely new optical formula:
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Secondly, the MTF curves also tell an interesting story. These MTF charts are measured at infinity and there is quite a difference, particularly with the high frequency (detail rendition) curve. (Speculation: As one focuses closer, one would expect to see the IF distance itself further due to its improved floating element design.) Of note to me is that the IF has significantly flatter MTF curves, but slightly more astigmatism.
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One final objective difference (this time in favor of the FLE) is the distortion. The IF's distortion is huge. There's no other way to say it. It's double that of the previous design. You won't be able to tell if you're shooting the great outdoors, but anything requiring a straight line will be a problem. It *can* be corrected in post, but if you don't/won't/can't do such things to your images, the amount of distortion this lens produces could be a deal-breaker in and of itself.
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On the subjective side, I do think the lens is incredible. Worth the extra cash? It depends on your individual needs, of course. The FLE is no slouch. But just to tempt you a bit, here's what Kornelius J. Mueller (Fleischer), of Carl Zeiss at the time, had to say about the IF version:
"Within Zeiss, I was the driving head behind coming up with an improved 40 mm lens for the SLR viewfinder (even though I like the Biogon 38 very much). Compared to the previous 40 FLE, I wanted significantly more sharpness at infinity for landscape and "citiyscape" shooters, sharpness on the level of the 50 FLE - at least. I wanted to get rid of the FLE mechanics with its confusing two focus rings. I wanted a single one. And I wanted it to operate smoothly, similar to the great focusing mechanism in the Tele-Superachromat 350, which I consinder the best focusing mechanism anywhere in the photo industry. On top of that I wanted a good offering for professionals who use digital backs and enjoy the Makro-Planar 120 for their commercial photography, but needed something significantly shorter in focal length. All of this has materialised in the new Distagon 4/40 IF - the first super wide angle lens for the SLR which can sucessfully challenge the Biogon 38.
If you have the opportunity to use it with your best tripod on landscapes with very fine detail at a far horizon, the result will stunn [sic]
you."
I use it for exactly this purpose, and have to concur.
I hope that helps your choice, AlaBill,
-Brad