Hi all, and thanks again for the input. I shall certainly go after those step-down rings now that I have seen the result of the internal diaphragm and its effect.
Well, I managed to rent the 70-200mm f4L lens for this weekend, and got out with it tonight. This is only an assessment, so the image is short on exposure, high on noise etc. But I am pleased with the lens.
The image
here is 11 x 3 minutes, shot at f6.3, FL was 100mm, ISO800.
The image is 50% of its original size, and the lens had an UV filter on the front of it.
Instead of dark subtracting, I used a high dither rate, and used the power of CCDSTACK to remove the dark noise (it did an okay job).
I accept its noisy, and needs more exposure, but I like the lens. There is almost no cropping on this image except for minor alignment areas, and the stars are pretty good.
I have some more questions for you DSLR experts later.
Comments on this image very welcome.
cheers
Martin