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If I get it right, major agencies will not accept the files from my 10D ( I know they do accept 1Ds)
Hi Erik,
Why ? Did they tell it to you ? What if you just upsize them to the size of the 1Ds ? Who will see it ? This would mean a upsizing of about 30% which is absolutely possible nowadays without quality loss.
On the other side with 3072 pixels width at 300dpi for printing you still get a very decent print of about 10" width. That 300dpi at a quality factor of 1.5 would mean that they go for printing at 200 lines per inch. Overkill.
I have seen high quality books at 175 lines per inch that means at a quality factor of 1.5 you can deliver prints up to a size of 11.7" nearly 12".
Where is the problem ?
Get a 1Ds or a used 1Ds instead of the 1D MarkII I would say if it is absolutely necessary or wait a moment and get the 1Ds MarkII.
IMO and that is at least what I would do.