Whoops, that's what comes with having two large breeds, your perception is slightly skewed What MP is the D90? I would be suprised if you lost quality by cropping at 200mm for the vast majority of ring sizes (possibly barring the BIS ring). Obviously it would depend on the size of the dog, a pug would need far more cropping than an Elkhound from distance. My 28 - 135 was fine for large breeds.

Would you not consider two lenses (since the breeds will be judged separately unless you regularly shoot AVNSC or AV classes). You could then use an 18 - 200 for medium to large dogs and a 70 - 300 for table dogs?

I do feel I am being awkward but most of the shooting is for the BIS ring and it is sometimes 20 mtr square, it is mostly hounds so you have dachshund one moment and deerhound the next. You may be right and cropping to fill the image area will be fine.

Think I am beginning to get things clear in my mind now, do I buy another 28 - 300mm Sigma and live with any aberration or do I buy better glass with less scope and hope it will take the hefty cropping. Well at least with your help I have managed to sort things that far so now I need opinions of whether or not I should stick to the Sigma (means buying another with Nikon fit) or do I buy better and go for something at the closer end - don't know what is available but a 24 - 150 or 200mm may do the job.

Poor Snapper, you do sound confused sorry can't help

Have you given any thought to renting a lens or two to try them out? That way you can pick the one that works best for your needs.