I have not taken many shots with my Opticron HR 66ED and 41110 Telephoto adapter but I must say the results are very good to my eye. The problems are carrying that lot around, finding the bird and then camera shake in a hide with people moving and subject movement. It appears I am getting 1600mm at f15 as the Canon factor is 1.6. I have no idea about suitability of Nikon 40 or the Pentax.
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