It is hard to make specifications like what is good in 7x10 -
the best 500mm mirror I have tried (the older version Nikkor F8) has limitations (and most others are little, if any, better than the
Spiratone), and all are so hard to use that rarely is one seeing the
optimum image quality in a print that they are capable of. You get
a LONG lens image with a relatively small, light, and inexpensive
lens - high quality is a different issue, and is best had with the
VERY big and VERY expensive ED Nikkors. The mirrors, under optimum
conditions, will produce a decent image at any size (since one views
larger prints at greater distances, smaller prints at smaller
distances, print size does not relate very well to lens quality).
Hope this helps.