Sorry, the post didn't appear in the right place. I was refferring ot Alan's comment.
>Nicely said. I just want to share what I've learned the hard >way. The regular distortion of wider angle when focused >close will still be there despite of the multiplier. Using a >50mm as a 75mm for head and shoulder portraits will give you >the 50mm distortion within the 75mm crop. It sounds obvious >but one can easily overlook this and end up with big headed >portraits.