Richard,
I have to disagree with you on this one. Get the 250mm and a "Komura" 2X! Just kidding of course since they could both be made by the same company for all I know.. I bought a near mint 250mm chrome from Hunt's Camera for $105.00 if I remember right, plus shipping. It was/is a great picture taker. I have always wanted the 500mm Tessar, but just couldn't justify buying it for the few times I would use it. Then I spotted a like new Komura 2X on eBay for next to nothing. Well, I'm very happy I did. I thought the Komura 2X converter would degrade the image quality of the 250mm, but it really didn't. To test the combo I setup my tripod and shot a picture of a brick building with T-max 100 and a deep-yellow filter. I then moved back to try and keep the very same angle of view/perspective and took the shot with the 2X Komura/250mm. Both were so darn good I now have no desire for the 500mm. Unless of course it lands in my lap for almost nothing, which I don't see happening anytime soon. The 2X and 250mm combo might just have a very, very slight touch less contrast due to a few more elements(glass surfaces), but sharpness is still very high. I would really like to put it up against the 500mm Tessar just to see how close they were, but will probably never get the chance. Just my 2 cents. John W